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{{short description|Standard for computer data connections}}
{{About| |the portable USB storage device|USB flash drive|other uses|USB (disambiguation)}}
{{about|the computer bus standard|other uses}}
{{Merge from|Device Firmware Update|discuss=Talk:USB#Merger proposal for DFU|date=August 2014}}
{{use American English|date=May 2023}}
{{use dmy dates|date=May 2023}}
{{cs1 config|mode=cs1|name-list-style=none|display-authors=all}}
{{infobox connector {{infobox connector
| name = Universal Serial Bus (USB) | name = USB<br />Universal Serial Bus
| type = ] | type = ]
| image = | image = ]
| logo = ] | logo = Certified USB.svg
| caption = Certified USB logo | caption = The current connector for USB, Thunderbolt, and other protocols, USB-C (plug and receptacle shown)
| designer = Compaq, Digital Equipment Corporation, IBM, Intel, Microsoft, NEC and Nortel | designer = {{plainlist|
* ]
| design_date = 1996
| manufacturer = Intel, Compaq, Microsoft, NEC, Digital Equipment Corporation, IBM, Nortel * ]
* ]
| key_people = Ajay Bhatt, Bala Cadambi<ref>{{Cite news|url= http://articles.cnn.com/2010-02-04/tech/ajay.bhatt.usb.inventor_1_usb-plugs-rock?_s=PM:TECH | title= USB inventor is tech's unlikely 'rock star' | publisher= CNN|date=2010-02-04 |accessdate= 2011-12-12}}</ref>
* ]
| production_date = 1997&ndash;present
* ]
| superseded = ], ], ], ], ]
* ]
| superseded_by =
* ]
}}
| design_date = {{start date and age |1996|1}}
| production_date = Since May 1996<ref>{{cite web |url=http://download.intel.com/design/intarch/datashts/29055002.pdf |publisher=Intel |title=82371FB (PIIX) and 82371SB (PIIX3) PCI ISA IDE Xcelerator |date=May 1996 |access-date=12 March 2016 |url-status=dead |archive-url=https://web.archive.org/web/20160313120109/http://download.intel.com/design/intarch/datashts/29055002.pdf |archive-date=13 March 2016}}</ref>
| superseded = ], ], ], ], ], and ] (IEEE 1394)
| superseded_by =
| superseded_by_date = | superseded_by_date =
}}
| external = Yes
| hotplug = Yes
| length = {{convert|2|-|5|m|abbr=on}} (by category)
| width = 12&nbsp;mm (A-plug),<ref name=a-plug-size>{{cite web | title = USB ‘A’ Plug Form Factor Revision 1.0 | url = http://www.usb.org/developers/devclass_docs/CCWG__A__Plug_Form_Factor_Guideline__Revision_1.0_.pdf | publisher = USB Implementers Forum | page = 1 | format = PDF | date = 23 March 2005 | accessdate = 4 April 2012 | quote = Body length is fully 12mm in width by 4.5mm in height with no deviations}}</ref> 8.45&nbsp;mm (B-plug); 7&nbsp;mm (Mini / Micro-USB)
| height = 4.5&nbsp;mm (A-plug),<ref name=a-plug-size/> 7.78&nbsp;mm (B-plug, pre-v3.0); 1.5–3&nbsp;mm (Mini/Micro-USB)
| electrical = 5 volt ]
| ground = Dedicated pin and shield
| maximum_voltage = 5.00±0.25&nbsp;V (pre-3.0); 5.00+0.25-0.55&nbsp;V (USB 3.0)
| maximum_current = 0.5–0.9 A (general);<br />
5 A (charging devices)
| data_signal = Packet data, defined by specifications
| data_bit_width = 1 bit
| data_bandwidth = 1.5/12/480/5,000/10,000&nbsp;] (depending on mode)
| data_devices = 127
| data_style = ]
| cable = 4 wires plus shield (pre-3.0); 9 wires plus shield (USB 3.0)
| physical_connector = Unique
| num_pins = 4: 1 supply, 2 data, 1 ground (pre-3.0); 9 (USB 3.0); 11 (powered USB 3.0); 5 (pre-3.0 Micro-USB)
| pinout_col1_name =
| pinout_col2_name =
| pinout_image = ]
| pinout_caption = The standard USB A plug (left) and B plug (right)
| pin1 = V<sub>CC</sub> (+5&nbsp;V, red wire)
| pin1_name =
| pin2 = Data− (white wire)
| pin2_name =
| pin3 = Data+ (green wire)
| pin3_name =
| pin4 = Ground (black wire)
| pin4_name =
| pinout_notes =}}
'''Universal Serial Bus''' ('''USB''') is an ] developed in the mid-1990s that defines the cables, connectors and ]s used in a ] for connection, communication, and power supply between ]s and electronic devices.<ref>{{Citation |url= http://simson.net/clips/1999/99.Globe.05-20.USB_deserves_more_support+.shtml | newspaper = Boston Globe Online | title = Business | contribution = USB deserves more support |publisher=Simson |date = 1995-12-31 |accessdate=2011-12-12}}</ref>


'''Universal Serial Bus''' ('''USB''') is an ], developed by ] (USB-IF), that allows data exchange and delivery of power between many types of electronics. It specifies its architecture, in particular its physical ], and ]s for data transfer and power delivery to and from ''hosts'', such as ]s, to and from ] ''devices'', e.g. displays, keyboards, and mass storage devices, and to and from intermediate ''hubs'', which multiply the number of a host's ports.<ref name="USB4Spec">{{cite web |url=https://www.usb.org/document-library/usb4r-specification-v20 |title=USB4 Specification v2.0 |edition=Version 2.0 |date=June 30, 2023 |format=ZIP |publisher=USB |access-date=23 October 2023}}</ref>
USB was designed to standardize the connection of ]s (including keyboards, ], digital cameras, printers, ]s, ]s and ]) to ]s, both to communicate and to supply ]. It has become commonplace on other devices, such as ]s, ]s and ]s.<ref>{{Citation | contribution = Sony Playstation 3 60 GB | title = Consoles | type = review | newspaper = CNet | url = http://reviews.cnet.com/consoles/sony-playstation-3-60gb/4505-10109_7-31355103.html}}</ref> USB has effectively replaced a variety of earlier interfaces, such as ] and ]s, as well as separate ]s for portable devices.

Introduced in 1996, USB was originally designed to standardize the connection of peripherals to computers, replacing various interfaces such as ]s, ]s, ]s, and ] ports.<ref>{{cite web |url=https://www.usb.org/about |title=About USB-IF |publisher=USB Implementers Forum |access-date=27 April 2023}}</ref> Early versions of USB became commonplace on a wide range of devices, such as keyboards, mice, cameras, printers, scanners, flash drives, smartphones, game consoles, and power banks.<ref>{{Cite news |url = http://simson.net/clips/1999/99.Globe.05-20.USB_deserves_more_support+.shtml |newspaper = Boston Globe Online |department = Business |title = USB deserves more support |publisher = Simson |date = 31 December 1995 |access-date = 12 December 2011 |url-status=live |archive-url = https://web.archive.org/web/20120406080011/http://simson.net/clips/1999/99.Globe.05-20.USB_deserves_more_support+.shtml |archive-date = 6 April 2012}}</ref> USB has since evolved into a standard to replace virtually all common ports on computers, mobile devices, peripherals, power supplies, and manifold other small electronics.

In the current standard, the ] connector replaces the many various connectors for power (up to 240&nbsp;W), displays (e.g. DisplayPort, HDMI), and many other uses, as well as all previous USB connectors.

{{As of|2024|post=,}} USB consists of four generations of specifications: ], ], ], and ]. USB4 enhances the data transfer and power delivery functionality with

{{blockquote|... a connection-oriented, tunneling architecture designed to combine multiple protocols onto a single physical interface so that the total speed and performance of the USB4 Fabric can be dynamically shared.<ref name="USB4Spec"/>}}

USB4 particularly supports the tunneling of the ] protocols, namely ] (PCIe, load/store interface) and ] (display interface). USB4 also adds host-to-host interfaces.<ref name="USB4Spec"/>

Each specification sub-version supports different ]s from 1.5 and 12&nbsp;Mbit/s ] in USB&nbsp;1.0/1.1 to 80&nbsp;Gbit/s ] in USB4&nbsp;2.0.<ref name=USB1Spec>{{cite web |date= |title=Universal Serial Bus 3.1 Specification |url=https://www.usb.org/sites/default/files/documents/usb_3_1_1_0.zip |access-date=27 April 2023 |publisher=USB Implementers Forum |format=ZIP |edition= }}{{Dead link|date=November 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref name=USB2Spec>{{cite web |url=https://www.usb.org/sites/default/files/documents/usb_2_0.zip |title=Universal Serial Bus 2.0 Specification |edition=Revision 2.0 |date=27 April 2000 |format=ZIP |publisher=USB Implementers Forum |access-date=27 April 2023 }}{{Dead link|date=November 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref name=USB3Spec>{{cite web |url=https://www.usb.org/document-library/usb-32-revision-11-june-2022 |title=USB 3.2 Revision 1.1 - June 2022 |edition=Revision 1.01 |date=Oct 2023 |format=HTML |access-date=14 April 2024}}</ref><ref name="USB4Spec"/> USB also provides power to peripheral devices; the latest versions of the standard extend the power delivery limits for battery charging and devices requiring up to 240 watts as defined in ] Rev. V3.1.<ref name=PDSpec>{{cite web |url=https://www.usb.org/sites/default/files/documents/pd_specification.zip |title=Universal Serial Bus Power Delivery Specification Revision 3.0 Version 2.0a (Released) |format=ZIP |publisher=USB Implementers Forum |access-date=27 April 2023 }}{{Dead link|date=November 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Over the years, USB(-PD) has been adopted as the standard power supply and charging format for many mobile devices, such as mobile phones, reducing the need for proprietary chargers.<ref>{{cite web |url=http://www.gsmworld.com/newsroom/press-releases/2009/3582.htm |title=Universal Charging Solution |publisher=GSMA |date=17 February 2009 |access-date=12 December 2011 |url-status=live |archive-url=https://web.archive.org/web/20111130092204/http://www.gsmworld.com/newsroom/press-releases/2009/3582.htm |archive-date=30 November 2011}}</ref>


== Overview == == Overview ==
USB was designed to standardize the connection of ]s to personal computers, both to exchange data and to supply electric power. It has largely replaced interfaces such as ]s and ]s and has become commonplace on various devices. Peripherals connected via USB include computer keyboards and mice, video cameras, printers, portable media players, mobile (portable) digital telephones, disk drives, and network adapters.
In general, there are four basic kinds or sizes related to the USB connectors and types of established connection:
* the older "standard" size, in its USB 1.1/2.0 and USB 3.0 variants (for example, on USB ])
* the "mini" size (primarily for the B connector end, such as on many cameras)
* the "micro" size, in its USB 1.1/2.0 and USB 3.0 variants (for example, on most modern cellphones)
* the versatile "]" scheme, in both mini and micro sizes.


USB connectors have been increasingly replacing other types of charging cables for portable devices.
Unlike other data cables (Ethernet, HDMI etc.), each end of a USB cable uses a different ''kind'' of connector; an A-type or a B-type. This kind of design was chosen to prevent electrical overloads and damaged equipment, as only the A-type socket provides power. There are cables with A-type connectors on both ends, but they should be used carefully.<ref name="cablestogo">{{cite web|title=USB connector guide|url=http://www.cablestogo.com/support/connector-guides/usb|publisher=C2G|accessdate=2 December 2013}}</ref> Therefore in general, each of the different "sizes" requires four different connectors; USB cables have the A-type and B-type connectors, and the corresponding sockets are on the computer or electronic device. In common practice, the A-type connector is usually the full size, and the B-type side can vary as needed.


USB connector interfaces are classified into three types: the many various ''legacy'' Type-A (upstream) and Type-B (downstream) connectors found on ''hosts'', ''hubs'', and ''peripheral devices'', and the modern Type-C (]) connector, which replaces the many legacy connectors as the only applicable connector for USB4.
Counter-intuitively, the "micro" size is the most durable from the point of designed insertion lifetime, as the result of latching mechanism (parts providing gripping force) being moved into plugs on the cable side.<ref name="stackexchange-miniusb">{{cite web
| url = http://electronics.stackexchange.com/questions/18552/why-was-mini-usb-deprecated-in-favor-of-micro-usb
| title = Why was Mini USB deprecated in favor of Micro USB?
| year = 2011 | accessdate = 2013-12-03
| publisher = stackexchange.com
}}</ref>


The Type-A and Type-B connectors came in Standard, Mini, and Micro sizes. The standard format was the largest and was mainly used for desktop and larger peripheral equipment. The Mini-USB connectors (Mini-A, Mini-B, Mini-AB) were introduced for mobile devices. Still, they were quickly replaced by the thinner Micro-USB connectors (Micro-A, Micro-B, Micro-AB). The Type-C connector, also known as USB-C, is not exclusive to USB, is the only current standard for USB, is required for USB4, and is required by other standards, including modern DisplayPort and Thunderbolt. It is reversible and can support various functionalities and protocols, including USB; some are mandatory, and many are optional, depending on the type of hardware: host, peripheral device, or hub.<ref>{{cite web |url=https://www.usb.org/sites/default/files/documents/cabconn20.pdf |title=Universal Serial Bus Cables and Connectors Class Document Revision 2.0 |publisher=USB Implementers Forum |access-date=27 April 2023 }}{{Dead link|date=November 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{cite web |url=https://www.usb.org/sites/default/files/documents/usb_type-c_specification_1_0.pdf |title=Universal Serial Bus Type-C Cable and Connector Specification Revision 1.0 |publisher=USB Implementers Forum |access-date=27 April 2023 }}{{Dead link|date=November 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
USB connections also come in four data transfer speeds: Low Speed, Full Speed, High Speed and ]. High Speed is only supported by specifically designed USB 2.0 High Speed interfaces (that is, USB 2.0 controllers without the High Speed designation do not support it), as well as by USB 3.0 interfaces. SuperSpeed is supported only by USB 3.0 interfaces.


USB specifications provide backward compatibility, usually resulting in decreased signaling rates, maximal power offered, and other capabilities. The USB 1.1 specification replaces USB 1.0. The USB 2.0 specification is backward-compatible with USB 1.0/1.1. The USB 3.2 specification replaces USB 3.1 (and USB 3.0) while including the USB 2.0 specification. USB4 "functionally replaces" USB 3.2 while retaining the USB 2.0 bus operating in parallel.<ref name="USB1Spec"/><ref name="USB2Spec"/><ref name="USB3Spec"/><ref name="USB4Spec"/>
== History ==
]
]


The USB&nbsp;3.0 specification defined a new architecture and protocol named ''SuperSpeed'' (aka ''SuperSpeed USB'', marketed as ''SS''), which included a new lane for a new signal coding scheme (8b/10b symbols, 5&nbsp;Gbit/s; later also known as ''Gen 1'') providing full-duplex data transfers that physically required five additional wires and pins, while preserving the USB&nbsp;2.0 architecture and protocols and therefore keeping the original four pins/wires for the USB&nbsp;2.0 backward-compatibility resulting in 9 wires (with 9 or 10 pins at connector interfaces; ID-pin is not wired) in total.
A group of seven companies began the development of USB in 1994: ], ], ], ], ], ], and ].<ref>{{cite web|last=Janssen |first=Cory |url=http://www.techopedia.com/definition/2320/universal-serial-bus-usb |title=What is a Universal Serial Bus (USB)? |publisher=Techopedia |date= |accessdate=2014-02-12}}</ref> The goal was to make it fundamentally easier to connect external devices to PCs by replacing the multitude of connectors at the back of PCs, addressing the usability issues of existing interfaces, and simplifying software configuration of all devices connected to USB, as well as permitting greater data rates for external devices. A team including ] worked on the standard at Intel;<ref>{{Citation | url = http://www.intel.com/pressroom/kits/bios/abhatt.htm | title = Ajay Bhatt: Fellow | publisher = ] | type = biography}}</ref><ref>{{cite web | url = http://www.oregonlive.com/business/index.ssf/2009/05/intel_ad_campaign_remakes_rese.html|title=Intel ad campaign remakes researchers into rock stars |first=Mark Graves|publisher=]|date=May 9, 2009 | accessdate =September 23, 2009}}</ref> the first ]s supporting USB were produced by Intel in 1995.<ref name="1394_2_4">{{cite book|editor-first =Hui | editor1-last = Pan | editor2-first = Paul | editor2-last =Polishuk|title= 1394 Monthly Newsletter|url = http://books.google.com/books?id=fRvbxgH4wmsC&pg=PA7 | accessdate=23 October 2012|publisher= Information Gatekeepers |pages= 7–9|id = GGKEY:H5S2XNXNH99}}</ref>


The USB&nbsp;3.1 specification introduced an ''Enhanced SuperSpeed System'' – while preserving the ''SuperSpeed'' architecture and protocol (''SuperSpeed USB'') – with an additional ''SuperSpeedPlus'' architecture and protocol (aka ''SuperSpeedPlus USB'') adding a new coding schema (128b/132b symbols, 10&nbsp;Gbit/s; also known as ''Gen 2''); for some time marketed as ''SuperSpeed+'' (''SS+'').
The original USB 1.0 specification, which was introduced in January 1996, defined data transfer rates of 1.5&nbsp;]/s "Low Speed" and 12&nbsp;Mbit/s "Full Speed".<ref name="1394_2_4" /> The first widely used version of USB was 1.1, which was released in September 1998. The 12&nbsp;Mbit/s data rate was intended for higher-speed devices such as disk drives, and the lower 1.5&nbsp;Mbit/s rate for low data rate devices such as ]s.<ref>{{cite web| first=Peter | last=Seebach | date=April 26, 2005 | title=Standards and specs: The ins and outs of USB | publisher=IBM| url=http://www.ibm.com/developerworks/power/library/pa-spec7.html | archiveurl =http://web.archive.org/web/20100110094907/http://www.ibm.com/developerworks/power/library/pa-spec7.html | archivedate=2010-01-10 | accessdate=8 September 2012}}</ref>


The USB&nbsp;3.2 specification<ref name="USB3Spec"/> added a second lane to the ''Enhanced SuperSpeed System'' besides other enhancements so that the ''SuperSpeedPlus USB'' system part implements the ''Gen 1×2'', ''Gen 2×1,'' and ''Gen 2×2'' operation modes. However, the ''SuperSpeed USB'' part of the system still implements the one-lane ''Gen 1×1'' operation mode. Therefore, two-lane operations, namely ''USB&nbsp;3.2 Gen&nbsp;1×'''2''' ''(10&nbsp;Gbit/s) and ''Gen&nbsp;2×'''2''' ''(20&nbsp;Gbit/s), are only possible with Full-Featured USB-C. As of 2023, they are somewhat rarely implemented; Intel, however, started to include them in its 11th-generation SoC processor models, but Apple never provided them. On the other hand, ''USB&nbsp;3.2 Gen&nbsp;1(×1)'' (5&nbsp;Gbit/s) and ''Gen&nbsp;2(×1)'' (10&nbsp;Gbit/s) have been quite common for some years.
]]]
The USB 2.0 specification was released in April 2000 and was ratified by the ] (USB-IF) at the end of 2001. ], Intel, ] (now Alcatel-Lucent), NEC and ] jointly led the initiative to develop a higher data transfer rate, with the resulting specification achieving 480&nbsp;Mbit/s, a 40-times increase over the original USB 1.1 specification.


===Connector type quick reference===
The USB 3.0 specification was published on 12 November 2008. Its main goals were to increase the data transfer rate (up to 5&nbsp;Gbit/s), decrease power consumption, increase power output, and be backwards-compatible with USB 2.0.<ref>{{cite book | title=Universal Serial Bus Specification Revision 3.0: 3.1 | page=41 (3–1) | url = http://www.usb.org/developers/docs/usb_30_spec_081312.zip | date=9 September 2011 | format = Zip | accessdate = 14 October 2011}}<sup> 08-Sep-2012</sup></ref> USB 3.0 includes a new, higher speed bus called SuperSpeed in parallel with the USB 2.0 bus.<ref>{{cite book | title=Universal Serial Bus Specification Revision 3.0: 1.6 | page =31 (1–3) | url= http://www.usb.org/developers/docs/usb_30_spec_081312.zip | date=9 September 2011 | format = Zip | accessdate =14 October 2011 }}</ref> For this reason, the new version is also called SuperSpeed.<ref name="usb_3_article">{{cite web| date= January 9, 2010 | title= USB 3.0 SuperSpeed gone wild at CES 2010, trumps even your new SSD | url= http://www.engadget.com/2010/01/09/usb-3-0-superspeed-gone-wild-at-ces-2010-trumps-even-your-new-s/ | accessdate= 2011-02-20 }}</ref> The first USB 3.0 equipped devices were presented in January 2010.<ref name="usb_3_article" /><ref name="usb_3_article2">{{cite web| date= January 11, 2010 | title = USB 3.0 Finally Arrives | url = http://www.pcworld.com/article/186566/usb_30_finally_arrives.html | accessdate =2011-02-20}}</ref>
{{Main|USB hardware#Connectors}}
Each USB connection is made using two connectors: a ''receptacle'' and a ''plug''. Pictures show only receptacles:
{{mw-datatable}}
{| class="wikitable mw-datatable" style="text-align:center; margin-left:auto; margin-right:auto;"
|+ Available connectors by USB standard
|-
! colspan=2|Standard
! ]<br />1996
! ]<br />1998
! ]<br />2000
! USB 2.0<br />Revised
! ]<br />2008
! ]<br />2013
! ]<br />2017
! ]<br />2019
! ]<br />2022
|-
! rowspan="4" | Max Speed
! Current marketing name
| colspan="2" | ''Basic-Speed''
| rowspan="3" colspan="2" | ''High-Speed''
| ''USB 5Gbps''
| ''USB 10Gbps''
| ''USB 20Gbps''
| rowspan="2" |''USB 40Gbps''
| rowspan="2" | ''USB 80Gbps''
|-
! Original label
| rowspan="2" colspan="2" | ''Low-Speed & Full-Speed''
| ''SuperSpeed'', or ''SS''
| ''SuperSpeed+'', or ''SS+''
| ''SuperSpeed USB 20Gbps''
|-
! Operation mode
| USB&nbsp;3.2 Gen&nbsp;1×1
| USB&nbsp;3.2 Gen&nbsp;2×1
| USB&nbsp;3.2 Gen&nbsp;2×2
| USB4 Gen&nbsp;3×2
| USB4 Gen&nbsp;4×2
|-
! Signaling rate
| colspan="2" | 1.5 Mbit/s & 12&nbsp;Mbit/s
| colspan="2" | 480 Mbit/s
| 5 Gbit/s
| 10 Gbit/s
| 20 Gbit/s
| 40 Gbit/s
| 80 Gbit/s
|-
! rowspan="9" | Connector
! {{nowrap|Standard-A}}
| colspan="2" | ]
| colspan="2" | ]
| colspan="2" | ]
| style="background:#FEE4BA" | ]<ref group="rem" name="OLOp">Limited to max speed at 10&nbsp;Gbit/s, since only one-lane (''×1'') operation mode is possible.</ref>
| colspan="2" rowspan="2" {{N/A}}
|-
! {{nowrap|Standard-B}}
| colspan="4" | ]
| colspan="2" | ]
| style="background:#FEE4BA" | ]<ref group="rem" name="OLOp"/>
|-
! {{nowrap|Mini-A}}
| rowspan="3" |<ref group="rem" name="bc1.1">] given.</ref>
| colspan="3" | ]
| colspan="5" rowspan="3" {{N/A}}
|-
! {{nowrap|Mini-AB}}<ref group="rem" name="RecOnly">Only as receptacle.</ref><ref group="rem">Accepts both Mini-A and Mini-B plugs.</ref>
| colspan="3" | ]
|-
! {{nowrap|Mini-B}}
| colspan="3" | ]
|-
! {{nowrap|Micro-A}}<ref group="rem" name="PlugOnly">Only as plug.</ref>
| colspan="3" rowspan="3" |&nbsp;<ref group="rem" name="bc1.1"/><ref group="rem" name="bc2"/>
| ]
| colspan="2" | ]
| style="background:#FEE4BA" | ]<ref group="rem" name="OLOp"/>
| colspan="2" rowspan="3" {{N/A}}
|-
! {{nowrap|Micro-AB}}<ref group="rem" name="RecOnly"/><ref group="rem">Accepts both Micro-A and Micro-B plugs.</ref>
| ]
| colspan="2" | ]
| style="background:#FEE4BA" | ]<ref group="rem" name="OLOp"/>
|-
! {{nowrap|Micro-B}}
| ]
| colspan="2" | ]
| style="background:#FEE4BA" | ]<ref group="rem" name="OLOp"/>
|-
! {{nowrap|Type-C}} {{nowrap|(USB-C)}}
| colspan="3" |<ref group="rem" name="bc2">Backward compatibility given by USB 2.0 implementation.</ref>
| colspan="6" style="font-size:60%"| ]<br/>(Enlarged to show detail)
|- style="background:#FEE4BA"
| ''Remarks:''
| scope=col colspan=10 | {{reflist|group="rem"}}
|}


=== Objectives ===
{{As of|2008}}, approximately six billion USB ports and interfaces were in the global marketplace, and about two billion were being sold each year.<ref>{{cite web| work = PC world | url=http://www.pcworld.com/article/156494/superspeed_usb_30_more_details_emerge.html |title=SuperSpeed USB 3.0: More Details Emerge |date=6 Jan 2009}}</ref>
The Universal Serial Bus was developed to simplify and improve the interface between personal computers and peripheral devices, such as cell phones, computer accessories, and monitors, when compared with previously existing standard or ''ad hoc'' proprietary interfaces.<ref name="JA2015">Axelson, Jan (2015). ''USB Complete: The Developer's Guide, Fifth Edition'', Lakeview Research LLC, {{ISBN|1931448280}}, pp. 1-7.</ref>


From the computer user's perspective, the USB interface improves ease of use in several ways:
=== Version history ===
* The USB interface is self-configuring, eliminating the need for the user to adjust the device's settings for speed or data format, or configure ]s, input/output addresses, or direct memory access channels.<ref>{{cite web |website=PC |url=https://www.pcmag.com/encyclopedia/term/44434/how-to-install-a-pc-peripheral |title=Definition of: how to install a PC peripheral |publisher=] |access-date=17 February 2018 |archive-date=22 March 2018 |archive-url=https://web.archive.org/web/20180322020256/https://www.pcmag.com/encyclopedia/term/44434/how-to-install-a-pc-peripheral |url-status=live}}</ref>
] USB 2.0 card for a computer motherboard]]
* USB connectors are standardized at the host, so any peripheral can use most available receptacles.
* USB takes full advantage of the additional processing power that can be economically put into peripheral devices so that they can manage themselves. As such, USB devices often do not have user-adjustable interface settings.
* The USB interface is ] (devices can be exchanged without shutting the host computer down).
* Small devices can be powered directly from the USB interface, eliminating the need for additional power supply cables.
* Because use of the USB logo is only permitted after ], the user can have confidence that a USB device will work as expected without extensive interaction with settings and configuration.
* The USB interface defines protocols for recovery from common errors, improving reliability over previous interfaces.<ref name="JA2015"/>
* Installing a device that relies on the USB standard requires minimal operator action. When a user plugs a device into a port on a running computer, it either entirely automatically configures using existing ]s, or the system prompts the user to locate a driver, which it then installs and configures automatically.


The USB standard also provides multiple benefits for hardware manufacturers and software developers, specifically in the relative ease of implementation:
==== {{anchor|0.7|0.8|0.9|0.99|1.0RC}}Prereleases ====
The USB standard evolved through several versions before its official release in 1996: * The USB standard eliminates the requirement to develop proprietary interfaces to new peripherals.
* The wide range of transfer speeds available from a USB interface suits devices ranging from keyboards and mice up to streaming video interfaces.
* ''USB 0.7''{{snd}} released in November 1994
* A USB interface can be designed to provide the best available ] for time-critical functions or can be set up to do background transfers of bulk data with little impact on system resources.
* ''USB 0.8''{{snd}} released in December 1994
* The USB interface is generalized with no signal lines dedicated to only one function of one device.<ref name="JA2015"/>
* ''USB 0.9''{{snd}} released in April 1995
* ''USB 0.99''{{snd}} released in August 1995
* ''USB 1.0 Release Candidate''{{snd}} released in November 1995


===Limitations===
==== {{anchor|1.0|1.1|1.x|LS|FS|}}USB 1.x ====
As with all standards, USB possesses multiple limitations to its design:
Released in January 1996, USB 1.0 specified data rates of ''1.5&nbsp;Mbit/s'' (''Low-Bandwidth'') and ''12&nbsp;Mbit/s'' (''Full-Bandwidth''). It did not allow for extension cables or pass-through monitors, due to timing and power limitations. Few USB devices made it to the market until USB 1.1 was released in August 1998, fixing problems identified in 1.0, mostly related to using hubs. USB 1.1 was the earliest revision that was widely adopted.
* USB cables are limited in length, as the standard was intended for peripherals on the same tabletop, not between rooms or buildings. However, a USB port can be connected to a ] that accesses distant devices.
* USB data transfer rates are slower than those of other interconnects such as ].
* USB has a strict ] topology and ] protocol for addressing peripheral devices; slave devices cannot interact with one another except via the host, and two hosts cannot communicate over their USB ports directly. Some extension to this limitation is possible through ], Dual-Role-Devices<ref>{{cite web |url=https://blogs.synopsys.com/tousbornottousb/2018/05/03/usb-dual-role-replaces-usb-on-the-go/ |title=To USB or Not to USB: USB Dual Role replaces USB On-The-Go |last=Huang |first=Eric |work=synopsys.com |date=3 May 2018 |access-date=21 July 2021 |archive-date=25 July 2021 |archive-url=https://web.archive.org/web/20210725064610/https://blogs.synopsys.com/tousbornottousb/2018/05/03/usb-dual-role-replaces-usb-on-the-go/ |url-status=live}}</ref> and ].
* A host cannot broadcast signals to all peripherals at once; each must be addressed individually.
* While converters exist between certain ] and USB, they might not provide a full implementation of the legacy hardware. For example, a USB-to-parallel-port converter might work well with a printer, but not with a scanner that requires bidirectional use of the data pins.


For a product developer, using USB requires the implementation of a complex protocol and implies an "intelligent" controller in the peripheral device. Developers of USB devices intended for public sale generally must obtain a USB ID, which requires that they pay a fee to the ] (USB-IF). Developers of products that use the USB specification must sign an agreement with the USB-IF. Use of the USB logos on the product requires annual fees and membership in the organization.<ref name="JA2015"/>
==== {{anchor|2.0|2.0HS|HS|62680-1}}USB 2.0 ====
]


== History ==
''USB 2.0'' was released in April 2000 (now called ''"Hi-Speed"''), adding higher maximum signaling rate of ''480&nbsp;Mbit/s'' (due to bus access constraints the effective throughput is limited to 35&nbsp;MB/s or 280&nbsp;Mbit/s), in addition to the "USB 1.x Full Speed" signaling rate of 12&nbsp;Mbit/s.<ref>{{cite techreport |title=Universal Serial Bus Specification | number=v2.0 | year= 2000 | section=5.5.4 | page=40 | url=http://sdphca.ucsd.edu/Lab_Equip_Manuals/usb_20.pdf}}</ref><ref>{{cite web | url = http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00022531&lc=en&cc=th& | title = Difference Between USB 2.0 Full Speed and USB 2.0 High Speed Devices}}</ref>
]


A group of seven companies began the development of USB in 1995:<ref>{{cite web |url=https://www.usb.org/members |title=Members |access-date=7 November 2021 |archive-date=7 November 2021 |archive-url=https://web.archive.org/web/20211107220855/https://www.usb.org/members |url-status=live}}</ref> ], ], ], ], ], ], and ]. The goal was to make it fundamentally easier to connect external devices to PCs by replacing the multitude of connectors at the back of PCs, addressing the usability issues of existing interfaces, and simplifying software configuration of all devices connected to USB, as well as permitting greater data transfer rates for external devices and ] features.<ref>{{cite web|url=https://www.intel.com/content/www/us/en/standards/usb-two-decades-of-plug-and-play-article.html|title=Two decades of "plug and play": How USB became the most successful interface in the history of computing|access-date=14 June 2021|archive-date=15 June 2021|archive-url=https://web.archive.org/web/20210615025638/https://www.intel.com/content/www/us/en/standards/usb-two-decades-of-plug-and-play-article.html|url-status=live}}</ref> ] and his team worked on the standard at Intel;<ref>{{cite web | url = http://www.intel.com/pressroom/kits/bios/abhatt.htm | title = Intel Fellow: Ajay V. Bhatt | publisher = ] | url-status=dead | archive-url = https://web.archive.org/web/20091104041719/http://www.intel.com/pressroom/kits/bios/abhatt.htm | archive-date = 4 November 2009}}</ref><ref>{{cite web |url= http://www.oregonlive.com/business/index.ssf/2009/05/intel_ad_campaign_remakes_rese.html |title= Intel ad campaign remakes researchers into rock stars |first= Mark |last= Rogoway |work= ] |date= 9 May 2009 |access-date= 23 September 2009 |url-status=live |archive-url= https://web.archive.org/web/20090826081315/http://www.oregonlive.com/business/index.ssf/2009/05/intel_ad_campaign_remakes_rese.html |archive-date= 26 August 2009}}</ref> the first ]s supporting USB were produced by Intel in 1995.<ref name="1394_2_4">{{cite book |editor-first = Hui |editor1-last = Pan |editor2-first = Paul |editor2-last = Polishuk |title = 1394 Monthly Newsletter |url = https://books.google.com/books?id=fRvbxgH4wmsC&pg=PA7 |access-date = 23 October 2012 |publisher = Information Gatekeepers |pages = 7–9 |id = GGKEY:H5S2XNXNH99 |url-status=live |archive-url = https://web.archive.org/web/20121112184629/http://books.google.com/books?id=fRvbxgH4wmsC&pg=PA7 |archive-date = 12 November 2012}}</ref>
Further modifications to the USB specification have been done via ]s (ECN). The most important of these ECNs are included into the USB 2.0 specification package available from USB.org:<ref>{{cite web | url = http://www.USB.org/ | title = USB Implementers Forum}}</ref>
* ''Mini-A and Mini-B Connector ECN'': Released in October 2000.<br />Specifications for Mini-A and B plug and receptacle. Also receptacle that accepts both plugs for On-The-Go. These should not be confused with Micro-B plug and receptacle.
* ''Errata as of December 2000'': Released in December 2000
* ''Pull-up/Pull-down Resistors ECN'': Released in May 2002
* ''Errata as of May 2002'': Released in May 2002
* ''Interface Associations ECN'': Released in May 2003.<br />New standard descriptor was added that allows associating multiple interfaces with a single device function.
* ''Rounded Chamfer ECN'': Released in October 2003.<br />A recommended, compatible change to Mini-B plugs that results in longer lasting connectors.
* ''Unicode ECN'': Released in February 2005.<br />This ECN specifies that strings are encoded using ]. USB 2.0 specified ], but did not specify the encoding.
* ''Inter-Chip USB Supplement'': Released in March 2006
* ''On-The-Go Supplement 1.3'': Released in December 2006.<br />] makes it possible for two USB devices to communicate with each other without requiring a separate USB host. In practice, one of the USB devices acts as a host for the other device.
* ''Battery Charging Specification 1.1'': Released in March 2007 (Updated 15 Apr 2009).<br />Adds support for dedicated chargers (power supplies with USB connectors), host chargers (USB hosts that can act as chargers) and the No Dead Battery provision, which allows devices to temporarily draw 100&nbsp;mA current after they have been attached. If a USB device is connected to dedicated charger, maximum current drawn by the device may be as high as 1.8&nbsp;A. (Note that this document is not distributed with USB 2.0 specification package only USB 3.0 and USB On-The-Go.)
* ''Micro-USB Cables and Connectors Specification 1.01'': Released in April 2007.
* ''Link Power Management Addendum ECN'': Released in July 2007.<br />This adds "sleep", a new power state between enabled and suspended states. Device in this state is not required to reduce its power consumption. However, switching between enabled and sleep states is much faster than switching between enabled and suspended states, which allows devices to sleep while idle.
* ''Battery Charging Specification 1.2'':<ref name= battchargespec1.2>{{cite web|url= http://www.usb.org/developers/devclass_docs/BCv1.2_011912.zip|title = Battery Charging v1.2 Spec and Adopters Agreement|date= 7 December 2010|accessdate=13 January 2012|publisher= USB Implementers Forum | format = Zip}}</ref> Released in December 2010.<br />Several changes and increasing limits including allowing 1.5&nbsp;A on charging ports for unconfigured devices, allowing High Speed communication while having a current up to 1.5&nbsp;A and allowing a maximum current of 5&nbsp;A.


=== USB 1.''x'' <span class="anchor" id="1.0"></span><span class="anchor" id="1.1"></span><span class="anchor" id="1.x"></span><span class="anchor" id="LS"></span><span class="anchor" id="FS"></span> ===
==== {{anchor|3.0|SS|3.x}}USB 3.0 ====
]
{{Main |USB 3.0}}
Released in January 1996, USB&nbsp;1.0 specified signaling rates of 1.5&nbsp;Mbit/s (''Low Bandwidth'' or ''Low Speed'') and 12&nbsp;Mbit/s (''Full Speed'').<ref>{{cite tech report | title=Universal Serial Bus Specification | number=v1.0 | year=1996 | section=4.2.1 | page=29 | url=https://fl.hw.cz/docs/usb/usb10doc.pdf | url-status=live | archive-url=https://web.archive.org/web/20180130144424/https://fl.hw.cz/docs/usb/usb10doc.pdf | archive-date=30 January 2018}}</ref> It did not allow for extension cables, due to timing and power limitations. Few USB devices made it to the market until USB&nbsp;1.1 was released in August 1998. USB&nbsp;1.1 was the earliest revision that was widely adopted and led to what Microsoft designated the "]".<ref name="Macworld iMac">{{cite web |url=http://www.macworld.com/article/135017/2008/08/imacanniversary.html |title=Eight ways the iMac changed computing |work=Macworld |date=15 August 2008 |access-date=5 September 2017 |url-status=live |archive-url=https://web.archive.org/web/20111222091746/http://www.macworld.com/article/135017/2008/08/imacanniversary.html |archive-date=22 December 2011 }}</ref><ref name="BusinessWeek iMac">{{cite web | work = Business week | year = 1999 | url = http://www.businessweek.com/1999/99_50/c3659057.htm | title = The PC Follows iMac's Lead | url-status=dead | archive-url = https://web.archive.org/web/20150923221417/http://www.businessweek.com/1999/99_50/c3659057.htm | archive-date = 23 September 2015}}</ref><ref name="Popular Mechanics iMac">{{cite journal|title=Popular Mechanics: Making Connections|journal = Popular Mechanics Magazine|url=https://books.google.com/books?id=R9MDAAAAMBAJ&pg=PA59|date=February 2001|publisher=Hearst Magazines|page=59|issn=0032-4558|url-status=live|archive-url=https://web.archive.org/web/20170215084550/https://books.google.com/books?id=R9MDAAAAMBAJ&pg=PA59|archive-date=15 February 2017}}</ref>
]


Neither USB 1.0 nor 1.1 specified a design for any connector smaller than the standard type A or type B. Though many designs for a miniaturized type B connector appeared on many peripherals, conformity to the USB&nbsp;1.''x'' standard was hampered by treating peripherals that had miniature connectors as though they had a tethered connection (that is: no plug or receptacle at the peripheral end). There was no known miniature type A connector until USB&nbsp;2.0 (revision 1.01) introduced one.
''USB&nbsp;3.0'' was released in November 2008. The standard defines a new ''SuperSpeed'' mode with a signaling speed of 5&nbsp;Gbit/s and, due to encoding overhead, usable data rate of up to 4&nbsp;Gbit/s (500&nbsp;MB/s). A USB&nbsp;3.0 port is usually colored blue, and is backwards compatible with USB&nbsp;2.0.


=== USB 2.0 <span class="anchor" id="2.0"></span><span class="anchor" id="2.0HS"></span><span class="anchor" id="HS"></span><span class="anchor" id="62680-1"></span>===
The USB&nbsp;3.0 Promoter Group announced on 17 November 2008 that the specification of version 3.0 had been completed and had made the transition to the USB Implementers Forum (USB-IF), the managing body of USB specifications.<ref name="ReferenceA">{{Cite press release | url= http://www.usb.org/press/USB-IF_Press_Releases/2008_11_17_USB_IF.pdf |title= USB |format= PDF | publisher = Implementers Forum | date= 2008-11-17 | accessdate= 2010-06-22}}</ref> This move effectively opened the specification to hardware developers for implementation in products.
]


USB&nbsp;2.0 was released in April 2000, adding a higher maximum signaling rate of 480&nbsp;Mbit/s (maximum theoretical data throughput 53&nbsp;MByte/s<ref name="throughput2.0">{{cite web |url= https://microchipdeveloper.com/usb:high-speed |title= High Speed USB Maximum Theoretical Throughput |date= 23 March 2021 | publisher= Microchip Technology Incorporated |url-status=live |archive-url= https://web.archive.org/web/20210326115716/https://microchipdeveloper.com/usb:high-speed |archive-date= 26 March 2021 | access-date=23 March 2021}}</ref>) named ''High Speed'' or ''High Bandwidth'', in addition to the USB&nbsp;1.''x'' ''Full Speed'' signaling rate of 12&nbsp;Mbit/s (maximum theoretical data throughput 1.2&nbsp;MByte/s).<ref name="throughput1.1">{{cite web |url= https://microchipdeveloper.com/usb:full-speed |title= Full Speed USB Maximum Theoretical Throughput |date= 23 March 2021 | publisher= Microchip Technology Incorporated |url-status=live |archive-url= https://web.archive.org/web/20210326115653/https://microchipdeveloper.com/usb:full-speed |archive-date= 26 March 2021 | access-date=23 March 2021}}</ref>
The new ''SuperSpeed'' bus provides a fourth transfer mode at 5.0&nbsp;Gbit/s (raw data rate), in addition to the modes supported by earlier versions. The payload throughput is 4&nbsp;Gbit/s (using ]), and the specification considers it reasonable to achieve around 3.2&nbsp;Gbit/s (0.4&nbsp;GB/s or 400&nbsp;MB/s), which should increase with future hardware advances. Communication is ] in SuperSpeed transfer mode; in the modes supported previously, by 1.x and 2.0, communication is half-duplex, with direction controlled by the host.<ref>{{cite web

| url = http://h20195.www2.hp.com/v2/GetPDF.aspx%2F4AA4-2724ENW.pdf
Modifications to the USB specification have been made via ] (ECNs). The most important of these ECNs are included into the USB&nbsp;2.0 specification package available from USB.org:<ref>{{cite web |url= http://www.usb.org/developers/docs/usb20_docs/ |title= USB 2.0 Specification |publisher= USB Implementers Forum |url-status=dead |archive-url= https://web.archive.org/web/20171203144114/http://www.usb.org/developers/docs/usb20_docs/ |archive-date= 3 December 2017 |access-date=28 April 2019 }}</ref>
| title = USB 3.0 Technology

| year = 2012 | accessdate = 2014-01-02
* ''Mini-A and Mini-B Connector''
| publisher = hp.com | format = PDF
* ''Micro-USB Cables and Connectors Specification 1.01''
* ''] Supplement''
* ''On-The-Go Supplement 1.3'' ] makes it possible for two USB devices to communicate with each other without requiring a separate USB host
* ''] Specification 1.1'' Added support for dedicated chargers, host chargers behavior for devices with dead batteries
* ''Battery Charging Specification 1.2'':<ref name="battchargespec1.2">{{cite web |url=http://www.usb.org/developers/docs/devclass_docs/BCv1.2_070312.zip |title=Battery Charging v1.2 Spec and Adopters Agreement |date=7 March 2012 |publisher=USB Implementers Forum |format=ZIP |url-status=live |archive-url=https://web.archive.org/web/20141006113700/http://www.usb.org/developers/docs/devclass_docs/BCv1.2_070312.zip |archive-date=6 October 2014 |access-date=13 May 2021 }}</ref> with increased current of 1.5&nbsp;A on charging ports for unconfigured devices, allowing high-speed communication while having a current up to 1.5&nbsp;A
* ''Link Power Management Addendum ECN'', which adds a ''sleep'' power state

=== USB 3.''x'' <span class="anchor" id="3.0"></span><span class="anchor" id="SS"></span><span class="anchor" id="3.x"></span>===
{{Main|USB 3.0}}
]

The USB&nbsp;3.0 specification was released on 12 November 2008, with its management transferring from USB&nbsp;3.0 Promoter Group to the USB Implementers Forum (USB-IF) and announced on 17 November 2008 at the SuperSpeed USB Developers Conference.<ref name="ReferenceA">{{Cite press release | url= http://www.usb.org/press/USB-IF_Press_Releases/2008_11_17_USB_IF.pdf | title= USB 3.0 Specification Now Available | location= San Jose, Calif. | via= usb.org | date= 17 November 2008 | access-date= 22 June 2010 | archive-url= https://web.archive.org/web/20100331035202/http://www.usb.org/press/USB-IF_Press_Releases/2008_11_17_USB_IF.pdf | archive-date= 31 March 2010}}</ref>

USB&nbsp;3.0 adds a new architecture and protocol named ''SuperSpeed'', with associated ] plugs, receptacles, and cables. SuperSpeed plugs and receptacles are identified with a distinct logo and blue inserts in standard format receptacles.

The SuperSpeed architecture provides for an operation mode at a rate of 5.0&nbsp;Gbit/s, in addition to the three existing operation modes. Its efficiency is dependent on a number of factors including physical symbol encoding and link-level overhead. At a 5&nbsp;Gbit/s signaling rate with ], each byte needs 10 bits to transmit, so the raw throughput is 500&nbsp;MB/s. When flow control, packet framing and protocol overhead are considered, it is realistic for about two thirds of the raw throughput, or 330&nbsp;MB/s to transmit to an application.<ref name="spec_3.0"/>{{rp|at=4–19}} SuperSpeed's architecture is ]; all earlier implementations, USB 1.0-2.0, are all half-duplex, arbitrated by the host.<ref>{{cite web
|url = http://www8.hp.com/h20195/v2/GetDocument.aspx?docname=4AA4-2724ENW
|title = USB 3.0 Technology
|year = 2012
|access-date = 2 January 2014
|publisher = ]
|format = PDF
|url-status = live
|archive-url = https://web.archive.org/web/20150219151039/http://www8.hp.com/h20195/v2/GetDocument.aspx?docname=4AA4-2724ENW
|archive-date = 19 February 2015
}}</ref> }}</ref>


Low-power and high-power devices remain operational with this standard, but devices implementing SuperSpeed can provide increased current of between 150&nbsp;mA and 900&nbsp;mA, by discrete steps of 150&nbsp;mA.<ref name="spec_3.0"/>{{rp|at=9–9}}
As with previous USB versions, USB&nbsp;3.0 ports come in low-power and high-power variants, providing 150&nbsp;mA and 900&nbsp;mA respectively while simultaneously transmitting data at SuperSpeed rates.<ref name="USB3.0 SPec">{{cite web | url= http://www.gaw.ru/pdf/interface/usb/USB%203%200_english.pdf |title= Universal Serial Bus 3.0 Specification | publisher = USB Implementers Forum | date= 2008-11-12 | accessdate= 2012-12-29}}</ref> Additionally, there is a Battery Charging Specification (Version 1.2 – December 2010), which increases the power handling capability to 1.5&nbsp;A but does ''not'' allow concurrent data transmission.<ref name="battchargespec1.2" /> The Battery Charging Specification requires that the physical ports themselves be capable of handling 5&nbsp;A of current{{citation needed|date=December 2012}} but the specification limits the maximum current drawn to 1.5&nbsp;A.


USB 3.0 also introduced the ], which provides generally faster transfer speeds than the BOT (Bulk-Only-Transfer) protocol.
===== {{anchor|3.1|SS+}}USB 3.1 =====
<!---]-->
<!--The above image link commented out until the creator/uploader/inserter of it fixes the terribly confused mess of units (combining Mbits and Mbytes on the same set of bars) and corrects the actually incorrect figures (USB 2.0 does not transmit at 60MByte/s - its signal rate is 480Mbit/s, with bytes encoded using 10 bits each...) - Please see the notes I've put on the image's talk page for further detail on this, as well as the article text itself.-->


A January 2013 press release from the USB group revealed plans to update USB&nbsp;3.0 to 10&nbsp;Gbit/s, effectively putting it on par with ] by mid-2013.<ref>{{Cite press release |url=http://www.usb.org/press/USB-IF_Press_Releases/SuperSpeed_10Gbps_USBIF_Final.pdf |title=USB |format=PDF |publisher=Implementers Forum |date=2013-01-06 | accessdate=2013-01-06}}</ref> The USB&nbsp;3.1 specification was released on 31 July 2013,<ref>http://www.usb.org/press/USB-IF_Press_Releases/SuperSpeedUSB_10Gbps_Available_20130731.pdf</ref> introducing a faster transfer mode called "SuperSpeed USB 10&nbsp;Gbps"; its logo features a ''Superspeed+'' (stylized as ''SUPER'''SPEED+''''') caption. The USB&nbsp;3.1 standard increases the signalling rate to 10&nbsp;Gbit/s, double that of USB&nbsp;3.0, and reduces line encoding overhead to just 3% by changing the ] to ].<ref>{{cite web ], released in July 2013 has two variants. The first one preserves USB&nbsp;3.0's ''SuperSpeed'' architecture and protocol and its operation mode is newly named ''USB&nbsp;3.1 Gen 1'',<ref name="usb.org 3.1"/><ref>{{cite web |url=https://www.msi.com/blog/usb-3-1-gen1-gen2-explained |title=USB 3.1 Gen 1 & Gen 2 explained |author=Silvia |website=www.msi.org |date=5 August 2015 |access-date=5 April 2018 |archive-date=8 July 2018 |archive-url=https://web.archive.org/web/20180708020201/https://www.msi.com/blog/usb-3-1-gen1-gen2-explained |url-status=live }}</ref> and the second version introduces a distinctively new ''SuperSpeedPlus'' architecture and protocol with a second operation mode named as ''USB&nbsp;3.1 Gen 2'' (marketed as ''SuperSpeed+ USB''). SuperSpeed+ doubles the maximum signaling rate to 10&nbsp;Gbit/s (later marketed as ''SuperSpeed USB 10&nbsp;Gbps'' by the USB&nbsp;3.2 specification), while reducing line encoding overhead to just 3% by changing the ] to ].<ref name="usb.org 3.1"/><ref>{{cite book
|url=http://www.usb.org/developers/docs/usb_31_102214.zip
|url= http://www.heraldonline.com/2013/07/31/5071745/superspeed-usb-10-gbps-ready-for.html |title= SuperSpeed USB 10&nbsp;Gbps - Ready for Development |publisher=Rock Hill Herald |accessdate =2013-07-31}}</ref> Though, some initial tests demonstrated usable transfer speeds of only 7.2&nbsp;Gbit/s, suggesting a 30% overall overhead.<ref>
|archive-url=https://web.archive.org/web/20141121225502/http://www.usb.org/developers/docs/usb_31_102214.zip
{{cite press release
|title=Universal Serial Bus 3.1 Specification
| author = <!--Staff writer(s); no by-line.-->
|publisher=] ] ] ] ] ]
| title = Synopsys Demonstrates Industry's First SuperSpeed USB 10 Gbps Platform-to-Platform Host-Device IP Data Transfer
|date=26 July 2013 |access-date=19 November 2014 |archive-date=21 November 2014 |via=Usb.org |format=ZIP
| url = http://news.synopsys.com/2013-12-10-Synopsys-Demonstrates-Industrys-First-SuperSpeed-USB-10-Gbps-Platform-to-Platform-Host-Device-IP-Data-Transfer
}}</ref>
| location = ]
| publisher = ]
| date = 2013-12-10
| accessdate = 2013-12-23
}}
</ref>


], released in September 2017,<ref>{{Cite web|url=https://www.usb.org/document-library/usb-32-specification-released-september-22-2017-and-ecns|title=The USB 3.2 Specification released on September 22, 2017 and ECNs|date=22 September 2017|website=usb.org|access-date=4 September 2019|archive-date=6 July 2019|archive-url=https://web.archive.org/web/20190706231129/https://www.usb.org/document-library/usb-32-specification-released-september-22-2017-and-ecns|url-status=live}}</ref> preserves existing USB&nbsp;3.1 ''SuperSpeed'' and ''SuperSpeedPlus'' architectures and protocols and their respective operation modes, but introduces two additional ''SuperSpeedPlus'' operation modes (''USB&nbsp;3.2 Gen&nbsp;1×2'' and ''USB&nbsp;3.2 Gen&nbsp;2×2'') with the new ] Fabric with signaling rates of 10 and 20&nbsp;Gbit/s (raw data rates of 1212 and 2424&nbsp;MB/s). The increase in bandwidth is a result of two-lane operation over existing wires that were originally intended for flip-flop capabilities of the USB-C connector.<ref>{{Cite press release |url=http://www.usb.org/press/USB_3.2_PR_USB-IF_Final.pdf |title=USB 3.0 Promoter Group Announces USB 3.2 Update |date=25 July 2017 |via=www.usb.org |location=Beaverton, Oregon, US |access-date=27 July 2017 |archive-date=21 September 2017 |archive-url=https://web.archive.org/web/20170921191940/http://www.usb.org/press/USB_3.2_PR_USB-IF_Final.pdf |url-status=live}}</ref>
The USB&nbsp;3.1 standard is ] with USB&nbsp;3.0 and&nbsp;USB 2.0. Using three power profiles of those defined in the ], it lets devices with larger energy demands request higher currents and supply voltages from compliant hosts{{snd}} up to 2&nbsp;A at 5&nbsp;V (for a power consumption of up to 10&nbsp;W), and optionally up to 5&nbsp;A at either 12&nbsp;V (60&nbsp;W) or 20&nbsp;V (100&nbsp;W).<ref>{{cite web |last=Burgess |first=Rick |title=USB 3.0 SuperSpeed update to eliminate need for chargers |url=http://www.techspot.com/news/52321-usb-30-superspeed-update-to-eliminate-need-for-chargers.html|publisher=TechSpot}}</ref>


==== Naming scheme ====
== {{Anchor|HOST}}System design ==
Starting with the USB 3.2 specification, USB-IF introduced a new naming scheme.<ref>{{Cite web|url=https://www.usb.org/sites/default/files/usb_3_2_language_product_and_packaging_guidelines_final.pdf |title=USB 3.2 Specification Language Usage Guidelines from USB-IF|date=26 February 2019 |website=usb.org |access-date=4 September 2019 |archive-date=3 November 2021|archive-url=https://web.archive.org/web/20211103022718/https://www.usb.org/sites/default/files/usb_3_2_language_product_and_packaging_guidelines_final.pdf|url-status=live}}</ref> To help companies with the branding of the different operation modes, USB-IF recommended branding the 5, 10, and 20&nbsp;Gbit/s capabilities as ''SuperSpeed USB 5Gbps'', ''SuperSpeed USB 10&nbsp;Gbps'', and ''SuperSpeed USB 20&nbsp;Gbps'', respectively.<ref>{{Cite web|last=Ravencraft|first=Jeff|url=https://www.usb.org/sites/default/files/D1T2-1%20-%20USB%20Branding%20Session.pdf|title=USB DevDays 2019 – Branding Session|date=19 November 2019|website=USB Implementers Forum|pages=16|type=Presentation|access-date=22 March 2020|archive-url=https://web.archive.org/web/20200322121822/https://www.usb.org/sites/default/files/D1T2-1%20-%20USB%20Branding%20Session.pdf|archive-date=22 March 2020}}</ref>
The design architecture of USB is ] in its topology, consisting of a ], a multitude of downstream USB ports, and multiple ]s connected in a tiered-]. Additional ]s may be included in the tiers, allowing branching into a tree structure with up to five tier levels.<!-- or should it be five tiers? --> A USB host may implement multiple host controllers and each host controller may provide one or more USB ports. Up to 127 devices, including hub devices if present, may be connected to a single host controller.<ref>{{cite book | title = Universal Serial Bus Specification Revision 2.0 | pages =13; 30; 256 | format = ] | url=http://www.usb.org/developers/docs/usb_20_101111.zip | date=11 October 2011 | accessdate=8 September 2012}}</ref><ref>{{cite book | title= Universal Serial Bus Specification Revision 3.0: 8.8 | pages=8–25 | url=http://www.usb.org/developers/docs/usb_30_spec_092911.zip | date=9 September 2011 | archiveurl= http://web.archive.org/web/20111104201259/http://www.usb.org/developers/docs/usb_30_spec_092911.zip | archivedate= 2011-11-04 | accessdate=14 October 2011 | format = Zip}}<sup> 08-Sep-2012</sup></ref> USB devices are linked in series through hubs. One hub—built into the host controller—is the root hub.


In 2023, they were replaced again,<ref></ref> removing ''"SuperSpeed"'', with ''USB 5Gbps'', ''USB 10Gbps'', and ''USB 20Gbps''. With new ''Packaging'' and ''Port'' logos.<ref></ref>
A physical USB device may consist of several logical sub-devices that are referred to as ''device functions''. A single device may provide several functions, for example, a ] (video device function) with a built-in microphone (audio device function). This kind of device is called a ''composite device''. An alternative to this is ''compound device,'' in which the host assigns each logical device a distinctive address and all logical devices connect to a built-in hub that connects to the physical USB cable.
<span class="anchor" id="USB4"></span>


=== USB4===
]


{{Update|section|date=August 2024|reason=Incomplete, erroneous and not up-to-date; e.g. lacks differences between USB4 first version and 2.0. Applies also to main article.}}
USB device communication is based on ''pipes'' (logical channels). A pipe is a connection from the host controller to a logical entity, found on a device, and named an '']''. Because pipes correspond 1-to-1 to endpoints, the terms are sometimes used interchangeably. A USB device could have up to 32 endpoints (16 IN, 16 OUT), though it's rare to have so many. An endpoint is defined and numbered by the device during initialization (the period after physical connection called "enumeration") and so is relatively permanent, whereas a pipe may be opened and closed.


{{main|USB4}}
There are two types of pipe: stream and message. A message pipe is bi-directional and is used for ''control'' transfers. Message pipes are typically used for short, simple commands to the device, and a status response, used, for example, by the bus control pipe number 0. A stream pipe is a uni-directional pipe connected to a uni-directional endpoint that transfers data using an ''isochronous'', ''interrupt'', or ''bulk'' transfer:
* ''isochronous transfers'': at some guaranteed data rate (often, but not necessarily, as fast as possible) but with possible data loss (e.g., realtime audio or video).
* ''interrupt transfers'': devices that need guaranteed quick responses (bounded latency) (e.g., pointing devices and keyboards).
* ''bulk transfers'': large sporadic transfers using all remaining available bandwidth, but with no guarantees on bandwidth or latency (e.g., file transfers).


]
An endpoint of a pipe is addressable with a ] ''(device_address, endpoint_number)'' as specified in a TOKEN packet that the host sends when it wants to start a data transfer session. If the direction of the data transfer is from the host to the endpoint, an OUT packet (a specialization of a TOKEN packet) having the desired device address and endpoint number is sent by the host. If the direction of the data transfer is from the device to the host, the host sends an IN packet instead. If the destination endpoint is a uni-directional endpoint whose manufacturer's designated direction does not match the TOKEN packet (e.g., the manufacturer's designated direction is IN while the TOKEN packet is an OUT packet), the TOKEN packet is ignored. Otherwise, it is accepted and the data transaction can start. A bi-directional endpoint, on the other hand, accepts both IN and OUT packets.


The USB4 specification was released on 29 August 2019 by the USB Implementers Forum.<ref name="usb.org">{{Cite web|url=https://www.usb.org/sites/default/files/USB4%20Specification.zip|title=USB Promoter Group USB4 Specification|date=29 August 2019|website=usb.org|access-date=30 August 2019|archive-date=13 February 2021|archive-url=https://web.archive.org/web/20210213071426/https://www.usb.org/sites/default/files/USB4%20Specification.zip|url-status=live}}</ref>
]

The USB4 2.0 specification was released on 1 September 2022 by the USB Implementers Forum.<ref>{{Cite web |title=USB Promoter Group Announces USB4 Version 2.0 Specification defines delivering up to 80&nbsp;Gbps over USB Type-C |url=https://www.usb.org/sites/default/files/2022-09/USB%20PG%20USB4%20Version%202.0%2080Gbps%20Announcement_FINAL.pdf}}</ref>

USB4 is based on the ] protocol.<ref>{{Cite web|url=https://arstechnica.com/gadgets/2019/03/thunderbolt-3-becomes-usb4-as-intels-interconnect-goes-royalty-free/|title=Thunderbolt 3 becomes USB4, as Intel's interconnect goes royalty-free|last=Bright|first=Peter|date=4 March 2019|website=Ars Technica|access-date=4 March 2019|archive-date=13 February 2021|archive-url=https://web.archive.org/web/20210213071427/https://arstechnica.com/gadgets/2019/03/thunderbolt-3-becomes-usb4-as-intels-interconnect-goes-royalty-free/|url-status=live}}</ref> It supports 40&nbsp;Gbit/s throughput, is compatible with Thunderbolt 3, and backward compatible with USB&nbsp;3.2 and USB&nbsp;2.0.<ref>{{Cite web|url=https://www.cnet.com/news/usb4-marries-thunderbolt-3-for-faster-speeds-and-smarter-transfers/|title=USB4 Marries Thunderbolt 3 for Faster Speeds and Smarter Transfers |last=Grunin|first=Lori|website=CNET|date=4 March 2019|access-date=4 March 2019|archive-date=4 March 2019|archive-url=https://web.archive.org/web/20190304232454/https://www.cnet.com/news/usb4-marries-thunderbolt-3-for-faster-speeds-and-smarter-transfers/|url-status=live}}</ref><ref>{{Cite web|url=https://www.pcmag.com/news/366931/thunderbolt-3-merges-with-usb-to-become-usb4|title=Thunderbolt 3 Merges With USB to Become USB4|last=Brant|first=Tom|date=4 March 2019|website=PC Magazine|access-date=4 March 2019|archive-date=5 March 2019|archive-url=https://web.archive.org/web/20190305024642/https://www.pcmag.com/news/366931/thunderbolt-3-merges-with-usb-to-become-usb4|url-status=live}}</ref> The architecture defines a method to share a single high-speed link with multiple end device types dynamically that best serves the transfer of data by type and application.

During ], USB-IF and Intel stated their intention to allow USB4 products that support all the optional functionality as ] products.

USB4&nbsp;2.0 with 80&nbsp;Gbit/s speeds was to be revealed in November 2022.<ref>{{Cite web|url=https://www.pcgamer.com/usb4-version-20-with-80gbps-speeds-is-coming/|title=USB4 Version 2.0 with 80Gbps speeds is coming|last=Szewczyk|first=Chris|date=September 5, 2022|website=PC Gamer|access-date=September 20, 2024}}</ref><ref>{{Cite press release|url=https://www.usb.org/sites/default/files/2022-09/USB%20PG%20USB4%20Version%202.0%2080Gbps%20Announcement_FINAL.pdf|last=Saunders|first=Brad|last2=Balich|first2=Joe|title=USB Promoter Group Announces USB4® Version 2.0|date=September 1, 2022|publisher=USB Implementers Forum|location=Beaverton, OR, USA|access-date=September 20, 2024}}</ref> Further technical details were to be released at two USB developer days scheduled for November 2022.<ref>{{Cite press release|url=https://usb.org/sites/default/files/2022-10/USB-IF%20USB%2080Gbps%20Announcement_FINAL_v2.pdf|last=Balich|first=Joe|title=USB-IF Announces Publication of New USB4® Specification to Enable USB 80Gbps Performance|date=October 18, 2022|publisher=USB Implementers Forum|location=Beaverton, OR, USA|access-date=September 20, 2024}}</ref>{{update-inline|date=October 2024}}

The USB4 specification states that the following technologies shall be supported by USB4:<ref name="usb.org"/>
{{clear}}
{| class="wikitable"
|-
! rowspan="2" style="width:240px" | Connection
! colspan="3" | Mandatory for
! rowspan="2" | Remarks
|-
! style="width:50px; text-align:center"| host !! style="width:50px; text-align:center"| hub !! style="width:50px; text-align:center"| device
|-
| '''USB 2.0''' (480&nbsp;Mbit/s)|| {{yes}} || {{yes}} || {{yes}} || Contrary to other functions – which use the multiplexing of high-speed links – USB 2.0 over USB-C utilizes its own differential pair of wires.
|-
| '''Tunneled USB 3.2 Gen 2×1''' (10&nbsp;Gbit/s)|| {{yes}} || {{yes}} || {{no}} ||
|-
| '''Tunneled USB 3.2 Gen 2×2''' (20&nbsp;Gbit/s)|| {{no}} || {{no}} || {{no}} ||
|-
| '''Tunneled USB 3 Gen T''' (5–80&nbsp;Gbit/s)|| {{no}} || {{no}} || {{no}} || A type of USB 3 Tunneling architecture where the Enhanced SuperSpeed System is extended to allow operation at the maximum bandwidth available on the USB4 Link.
|-
| '''USB4 Gen 2''' (10 or 20&nbsp;Gbit/s) || {{yes}} || {{yes}} || {{yes}} || rowspan="2"| Either one or two lanes
|-
| '''USB4 Gen 3''' (20 or 40&nbsp;Gbit/s) || {{no}} || {{yes}} || {{no}}
|-
| '''Tunneled DisplayPort 1.4a''' || {{yes}} || {{yes}} || {{no}} ||The specification requires that hosts and hubs support the DisplayPort Alternate Mode.
|-
| '''Tunneled PCI Express 3.0''' || {{no}} || {{yes}} || {{no}} || The PCI Express function of USB4 replicates the functionality of previous versions of the ] specification.
|-

| '''Host-to-Host communications''' || {{yes}} || {{yes}} || {{n/a}} || A LAN-like connection between two peers.

|-

| '''Thunderbolt 3 Alternate Mode''' || {{no}} || {{yes}} || {{no}} || Thunderbolt 3 uses USB-C cables; the USB4 specification allows hosts and devices and requires hubs to support interoperability with the standard using the Thunderbolt 3 Alternate Mode (namely DisplayPort and PCIe).
|-
| '''Other Alternate Modes'''|| {{no}} || {{no}} || {{no}}
| USB4 products may optionally offer interoperability with the ], ], and ] Alternate Modes.
|}

==== September 2022 naming scheme ====
[[File:USB 2022 September naming scheme.svg|thumb|upright=2.8|An overview of USB naming scheme that was put in place in September 2022
<br />(A mix of USB specifications and their marketing names are being displayed<br />because specifications are sometimes wrongly used as marketing names.){{Disputed inline|File:USB 2022 September naming scheme.svg|for=USB4 20&nbsp;Gbit/s does not exist; USB4 2×2 is not interchangeable with USB 3.2 2×2 as<br />indicated by the logo; logos for USB 3.x and USB4 are different.|date=July 2023}}]]
Because of the previous confusing naming schemes, USB-IF decided to change it once again. As of 2 September 2022, marketing names follow the syntax "USB&nbsp;''x''Gbps", where ''x'' is the speed of transfer in Gbit/s.<ref>{{Cite web|title=USB Data Performance, Language Usage Guidelines from USB-IF|url=https://usb.org/sites/default/files/usb_data_performance_language_usage_guidelines_september_2022.pdf|access-date=2 September 2022|archive-date=1 October 2022|archive-url=https://web.archive.org/web/20221001115816/https://www.usb.org/sites/default/files/usb_data_performance_language_usage_guidelines_september_2022.pdf|url-status=dead}}</ref> Overview of the updated names and logos can be seen in the adjacent table.

The operation modes USB&nbsp;3.2&nbsp;Gen&nbsp;2×2 and USB4&nbsp;Gen&nbsp;2×2 – or: USB&nbsp;3.2&nbsp;Gen&nbsp;2×1 and USB4&nbsp;Gen&nbsp;2×1 – are not interchangeable or compatible; all participating controllers must operate with the same mode.

=== Version history <span class="anchor" id="0.6e"></span><span class="anchor" id="0.7"></span><span class="anchor" id="0.8"></span><span class="anchor" id="0.9"></span><span class="anchor" id="0.99"></span><span class="anchor" id="1.0RC"></span> ===

==== Release versions ====
{| class="wikitable sortable"
|-
! scope="row" | Name
! scope="row" | Release date
! scope="row" | Maximum signaling rate
! scope="row" | Note
|-
! scope="row" | {{Nowrap|USB 0.7}}
| {{Dts|November 1994}}
| {{dunno}}
| rowspan="4" | Pre-release.
|-
! scope="row" | {{Nowrap|USB 0.8}}
| {{Dts|December 1994}}
| {{dunno}}
|-
! scope="row" | {{Nowrap|USB 0.9}}
| {{Dts|April 1995}}
| {{Nowrap|12&nbsp;Mbit/s: Full Speed (FS)}}
|-
! scope="row" | {{Nowrap|USB 0.99}}
| {{Dts|August 1995}}
| {{dunno}}
|-
! scope="row" | {{Nowrap|USB 1.0-RC}}
| {{Dts|November 1995}}
| {{dunno}}
| Release Candidate.
|-
! scope="row" | {{Nowrap|USB 1.0}}
| {{Dts|January 1996}}
| rowspan=2|{{Nowrap|1.5&nbsp;Mbit/s: Low Speed (LS)}}<br />{{Nowrap|12&nbsp;Mbit/s: Full Speed (FS)}}
| rowspan="2" | Renamed to ''Basic-Speed''.
|-
! scope="row" | {{Nowrap|USB 1.1}}
| {{Dts|September 1998}}
|-
! scope="row" | {{Nowrap|USB 2.0}}
| {{Dts|April 2000}}
| {{Nowrap|480&nbsp;Mbit/s: High Speed (HS)}}
|
|-
! scope="row" | {{Nowrap|USB 3.0}}
| {{Dts|November 2008}}
| {{Nowrap|5&nbsp;Gbit/s: SuperSpeed (SS)}}
| Renamed to ''USB&nbsp;3.1 Gen&nbsp;1'',<ref name="usb.org 3.1"/> and later to ''USB&nbsp;3.2 Gen&nbsp;1×1''.
|-
! scope="row" | {{Nowrap|USB 3.1}}
| {{Dts|July 2013}}
| {{Nowrap|10&nbsp;Gbit/s: SuperSpeed+ (SS+)}}
| Renamed to ''USB&nbsp;3.1 Gen&nbsp;2'',<ref name="usb.org 3.1"/> and later to ''USB&nbsp;3.2 Gen&nbsp;2×1''.
|-
! scope="row" | {{Nowrap|USB 3.2}}
| {{Dts|August 2017}}
| {{Nowrap|20&nbsp;Gbit/s: SuperSpeed+ two-lane}}
| Includes new ''USB&nbsp;3.2 Gen&nbsp;1×2'' and ''Gen&nbsp;2×2'' two-lane modes.<ref>{{cite web |title=USB 3.2 explained: Making sense of current and confusing USB standards |url=https://www.cnet.com/tech/mobile/usb-3-2-explained-making-sense-of-current-and-confusing-usb-standard/ |author=Matt Elliot |date=11 March 2019|website=CNET |access-date=26 July 2022 |url-status=live |archive-url=https://web.archive.org/web/20210707230329/https://www.cnet.com/tech/mobile/usb-3-2-explained-making-sense-of-current-and-confusing-usb-standard/|archive-date=7 July 2021 }}</ref> Requires Full-Featured ].
|-
! scope="row" | {{Nowrap|USB4}}
| {{Dts|August 2019}}
| {{Nowrap|40&nbsp;Gbit/s: two-lane}}
| Includes new ''USB4 Gen&nbsp;2×2'' (64b/66b encoding) and ''Gen&nbsp;3×2'' (128b/132b encoding) modes and introduces USB4 routing for tunneling of USB&nbsp;3.2, DisplayPort&nbsp;1.4a and PCI Express traffic and host-to-host transfers, based on the Thunderbolt&nbsp;3 protocol; requires USB4 Fabric.
|-
! scope="row" | {{Nowrap|USB4 2.0}}
| {{Dts|September 2022}}
| {{Nowrap|120&nbsp;⇄ 40&nbsp;Gbit/s: asymmetric}}
| Includes new ''USB4 Gen&nbsp;4×2'' (PAM-3 encoding) mode to get 80 and 120&nbsp;Gbit/s over Type-C connector.<ref>{{cite web | url=https://www.usb.org/document-library/usb4r-specification-v20 | title=USB4 Specification v2.0 &#124; USB-IF }}</ref> Requires USB4 Fabric.
|}

==== Power-related standards <span class="anchor" id="PD"></span>====
{| class="wikitable sortable"
|-
! style="width:16em;" | Release name
! Release date
! style="width:8em;" | Max. power
! Note
|-
| ] Rev. 1.0
| 2007-03-08
| 7.5&nbsp;W (5&nbsp;V, 1.5&nbsp;A)
|
|-
| USB Battery Charging Rev. 1.1
| 2009-04-15
| 7.5&nbsp;W (5&nbsp;V, 1.5&nbsp;A)
|Page 28, Table 5–2, but with limitation on paragraph 3.5. In ordinary USB 2.0's standard-A port, 1.5&nbsp;A only.<ref>{{Cite web|url=https://usb.org/document-library/battery-charging-v11-spec-and-adopters-agreement|title=Battery Charging v1.1 Spec and Adopters Agreement|website=USB.org|access-date=31 July 2019|archive-date=11 January 2021|archive-url=https://web.archive.org/web/20210111005750/https://usb.org/document-library/battery-charging-v11-spec-and-adopters-agreement|url-status=dead}}</ref>
|-
| USB Battery Charging Rev. 1.2
| 2010-12-07
| 7.5&nbsp;W (5&nbsp;V, 1.5&nbsp;A)
|<ref>{{Cite web|url=https://usb.org/document-library/battery-charging-v12-spec-and-adopters-agreement|title=Battery Charging v1.2 Spec and Adopters Agreement|website=USB.org|access-date=31 July 2019|archive-date=31 July 2019|archive-url=https://web.archive.org/web/20190731150826/https://usb.org/document-library/battery-charging-v12-spec-and-adopters-agreement|url-status=live}}</ref>
|-
| ] Rev. 1.0 (V. 1.0)
| 2012-07-05
| 100&nbsp;W (20&nbsp;V, 5&nbsp;A)
| Using FSK protocol over bus power (V{{sub|BUS}})
|-
| USB Power Delivery Rev. 1.0 (V. 1.3)
| 2014-03-11
| 100&nbsp;W (20&nbsp;V, 5&nbsp;A)
|
|-
| USB Type-C Rev. 1.0
| 2014-08-11
| 15&nbsp;W (5&nbsp;V, 3&nbsp;A)
| New connector and cable specification
|-
| USB Power Delivery Rev. 2.0 (V. 1.0)
| 2014-08-11
| 100&nbsp;W (20&nbsp;V, 5&nbsp;A)
| Using BMC protocol over communication channel (CC) on USB-C cables.
|-
| USB Type-C Rev. 1.1
| 2015-04-03
| 15&nbsp;W (5&nbsp;V, 3&nbsp;A)
|
|-
| USB Power Delivery Rev. 2.0 (V. 1.1)
| 2015-05-07
| 100&nbsp;W (20&nbsp;V, 5&nbsp;A)
|
|-
| USB Type-C Rev. 1.2
| 2016-03-25
| 15&nbsp;W (5&nbsp;V, 3&nbsp;A)
|
|-
| USB Power Delivery Rev. 2.0 (V. 1.2)
| 2016-03-25
| 100&nbsp;W (20&nbsp;V, 5&nbsp;A)
|
|-
| USB Power Delivery Rev. 2.0 (V. 1.3)
| 2017-01-12
| 100&nbsp;W (20&nbsp;V, 5&nbsp;A)
|
|-
| USB Power Delivery Rev. 3.0 (V. 1.1)
| 2017-01-12
| 100&nbsp;W (20&nbsp;V, 5&nbsp;A)
|
|-
| USB Type-C Rev. 1.3
| 2017-07-14
| 15&nbsp;W (5&nbsp;V, 3&nbsp;A)
|
|-
| USB Power Delivery Rev. 3.0 (V. 1.2)
| 2018-06-21
| 100&nbsp;W (20&nbsp;V, 5&nbsp;A)
|
|-
| USB Type-C Rev. 1.4
| 2019-03-29
| 15&nbsp;W (5&nbsp;V, 3&nbsp;A)
|
|-
| USB Type-C Rev. 2.0
| 2019-08-29
| 15&nbsp;W (5&nbsp;V, 3&nbsp;A)
| Enabling USB4 over USB Type-C connectors and cables.
|-
| USB Power Delivery Rev. 3.0 (V. 2.0)
| 2019-08-29
| 100&nbsp;W (20&nbsp;V, 5&nbsp;A)
|<ref>{{Cite web|url=https://www.usb.org/document-library/usb-power-delivery-0|title=USB Power Delivery|website=USB.org|access-date=3 September 2019|archive-date=3 September 2019|archive-url=https://web.archive.org/web/20190903163400/https://www.usb.org/document-library/usb-power-delivery-0|url-status=live}}</ref>
|-
| USB Power Delivery Rev. 3.1 (V. 1.0)
| 2021-05-24
| 240&nbsp;W (48&nbsp;V, 5&nbsp;A)
|
|-
| USB Type-C Rev. 2.1
| 2021-05-25
| 15&nbsp;W (5&nbsp;V, 3&nbsp;A)
|<ref>{{Cite web|url=https://usb.org/document-library/usb-type-cr-cable-and-connector-specification-revision-21|title=USB Type-C Cable and Connector Specification Revision 2.1|website=USB.org|access-date=27 May 2021|archive-date=27 May 2021|archive-url=https://web.archive.org/web/20210527135133/https://usb.org/document-library/usb-type-cr-cable-and-connector-specification-revision-21|url-status=live}}</ref>
|-
| USB Power Delivery Rev. 3.1 (V. 1.1)
| 2021-07-06
| 240&nbsp;W (48&nbsp;V, 5&nbsp;A)
|<ref name="USB Power Delivery">{{Cite web|url=https://usb.org/document-library/usb-power-delivery|title=USB Power Delivery|website=USB.org|access-date=27 May 2021|archive-date=27 May 2021|archive-url=https://web.archive.org/web/20210527151756/https://usb.org/document-library/usb-power-delivery|url-status=live}}</ref>
|-
| USB Power Delivery Rev. 3.1 (V. 1.2)
| 2021-10-26
| 240&nbsp;W (48&nbsp;V, 5&nbsp;A)
| Including errata through October 2021<ref name="USB Power Delivery"/>
This version incorporates the following ECNs:
* Clarify use of Retries
* Battery Capabilities
* FRS timing problem
* PPS power rule clarifications
* Peak current support for EPR AVS APDO
|}

== System design <span class="anchor" id="HOST"></span>==
A USB system consists of a host with one or more downstream facing ports (DFP),<ref>{{cite web |title=Type-C CC and VCONN Signals |publisher=Microchip Technology, Inc. |url=https://microchipdeveloper.com/usb:tc-pins |access-date=August 18, 2023}}</ref> and multiple peripherals, forming a tiered-]. Additional ]s may be included, allowing up to five tiers. A USB host may have multiple controllers, each with one or more ports. Up to 127 devices may be connected to a single host controller.<ref>{{cite web | title = Universal Serial Bus Specification Revision 2.0 | pages = 13; 30; 256 | format = ] | url = http://www.usb.org/developers/docs/usb_20_101111.zip |website=USB.org | date = 11 October 2011 | access-date = 8 September 2012 | archive-url = https://web.archive.org/web/20120528075527/http://www.usb.org/developers/docs/usb_20_101111.zip | archive-date = 28 May 2012}}</ref><ref name="spec_3.0"/>{{rp|at=8–29}} USB devices are linked in series through hubs. The hub built into the host controller is called the ''root hub''.

A USB device may consist of several logical sub-devices that are referred to as ''device functions''. A ''composite device'' may provide several functions, for example, a ] (video device function) with a built-in microphone (audio device function). An alternative to this is a ''],'' in which the host assigns each logical device a distinct address and all logical devices connect to a built-in hub that connects to the physical USB cable.

]

USB device communication is based on ''pipes'' (logical channels). A pipe connects the host controller to a logical entity within a device, called an '']''. Because pipes correspond to endpoints, the terms are sometimes used interchangeably. Each USB device can have up to 32 endpoints (16 ''in'' and 16 ''out''), though it is rare to have so many. Endpoints are defined and numbered by the device during initialization (the period after physical connection called "enumeration") and so are relatively permanent, whereas pipes may be opened and closed.

There are two types of pipe: stream and message.
* A ''message'' pipe is bi-directional and is used for ''control'' transfers. Message pipes are typically used for short, simple commands to the device, and for status responses from the device, used, for example, by the bus control pipe number 0.
* A ''stream'' pipe is a uni-directional pipe connected to a uni-directional endpoint that transfers data using an '']'',<ref>{{cite web
|url = http://www.usb.org/developers/presentations/SuperSpeed_USB_DevCon_Isochronous_Froelich.pdf
|title = Isochronous Protocol
|date = 20 May 2009
|access-date = 21 November 2014
|author = Dan Froelich
|website = USB.org
|url-status=dead
|archive-url = https://web.archive.org/web/20140817061140/http://www.usb.org/developers/presentations/SuperSpeed_USB_DevCon_Isochronous_Froelich.pdf
|archive-date = 17 August 2014
}}</ref> ''interrupt'', or ''bulk'' transfer:
*;Isochronous transfers: At some guaranteed data rate (for fixed-bandwidth streaming data) but with possible data loss (e.g., realtime audio or video)
*;Interrupt transfers: Devices that need guaranteed quick responses (bounded latency) such as pointing devices, ], and keyboards
*;Bulk transfers: Large sporadic transfers using all remaining available bandwidth, but with no guarantees on bandwidth or latency (e.g., file transfers)

When a host starts a data transfer, it sends a TOKEN packet containing an endpoint specified with a ] of ''(device_address, endpoint_number)''. If the transfer is from the host to the endpoint, the host sends an OUT packet (a specialization of a TOKEN packet) with the desired device address and endpoint number. If the data transfer is from the device to the host, the host sends an IN packet instead. If the destination endpoint is a uni-directional endpoint whose manufacturer's designated direction does not match the TOKEN packet (e.g. the manufacturer's designated direction is IN while the TOKEN packet is an OUT packet), the TOKEN packet is ignored. Otherwise, it is accepted and the data transaction can start. A bi-directional endpoint, on the other hand, accepts both IN and OUT packets.

]


Endpoints are grouped into ''interfaces'' and each interface is associated with a single device function. An exception to this is endpoint zero, which is used for device configuration and is not associated with any interface. A single device function composed of independently controlled interfaces is called a ''composite device''. A composite device only has a single device address because the host only assigns a device address to a function. Endpoints are grouped into ''interfaces'' and each interface is associated with a single device function. An exception to this is endpoint zero, which is used for device configuration and is not associated with any interface. A single device function composed of independently controlled interfaces is called a ''composite device''. A composite device only has a single device address because the host only assigns a device address to a function.


When a USB device is first connected to a USB host, the USB device enumeration process is started. The enumeration starts by sending a reset signal to the USB device. The data rate of the USB device is determined during the reset signaling. After reset, the USB device's information is read by the host and the device is assigned a unique 7-bit address. If the device is supported by the host, the ]s needed for communicating with the device are loaded and the device is set to a configured state. If the USB host is restarted, the enumeration process is repeated for all connected devices. When a USB device is first connected to a USB host, the USB device enumeration process is started. The enumeration starts by sending a reset signal to the USB device. The signaling rate of the USB device is determined during the reset signaling. After reset, the USB device's information is read by the host and the device is assigned a unique 7-bit address. If the device is supported by the host, the ]s needed for communicating with the device are loaded and the device is set to a configured state. If the USB host is restarted, the enumeration process is repeated for all connected devices.


The host controller directs traffic flow to devices, so no USB device can transfer any data on the bus without an explicit request from the host controller. In USB&nbsp;2.0, the host controller ] the bus for traffic, usually in a ] fashion. The throughput of each USB port is determined by the slower speed of either the USB port or the USB device connected to the port. The host controller directs traffic flow to devices, so no USB device can transfer any data on the bus without an explicit request from the host controller. In USB&nbsp;2.0, the host controller ] the bus for traffic, usually in a ] fashion. The throughput of each USB port is determined by the slower speed of either the USB port or the USB device connected to the port.


High-speed USB&nbsp;2.0 hubs contain devices called transaction translators that convert between high-speed USB&nbsp;2.0 buses and full and low speed buses. When a high-speed USB&nbsp;2.0 hub is plugged into a high-speed USB host or hub, it operates in high-speed mode. The USB hub then uses either one transaction translator per hub to create a full/low-speed bus routed to all full and low speed devices on the hub, or uses one transaction translator per port to create an isolated full/low-speed bus per port on the hub. High-speed USB&nbsp;2.0 hubs contain devices called transaction translators that convert between high-speed USB&nbsp;2.0 buses and full and low speed buses. There may be one translator per hub or per port.


Because there are two separate controllers in each USB&nbsp;3.0 host, USB&nbsp;3.0 devices transmit and receive at USB&nbsp;3.0 data rates regardless of USB&nbsp;2.0 or earlier devices connected to that host. Operating data rates for earlier devices are set in the legacy manner. Because there are two separate controllers in each USB&nbsp;3.0 host, USB&nbsp;3.0 devices transmit and receive at USB&nbsp;3.0 signaling rates regardless of USB&nbsp;2.0 or earlier devices connected to that host. Operating signaling rates for earlier devices are set in the legacy manner.


== Device classes == == Device classes <span class="anchor" id="PHDC"></span>==
The functionality of USB devices is defined by class codes, communicated to the USB host to affect the loading of suitable software driver modules for each connected device. This provides for adaptability and device independence of the host to support new devices from different manufacturers. The functionality of a USB device is defined by a class code sent to a USB host. This allows the host to load software modules for the device and to support new devices from different manufacturers.


Device classes include:<ref>{{Cite journal | url = http://www.usb.org/developers/defined_class | title = Class Codes | publisher = USB Implementers Forum | postscript = <!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}.</ref> Device classes include:<ref>{{Cite web | url = https://www.usb.org/defined-class-codes | title = USB Class Codes | date = 22 September 2018 | via=www.usb.org | url-status=live | archive-url = https://web.archive.org/web/20180922111633/https://www.usb.org/defined-class-codes | archive-date = 22 September 2018}}</ref>


{| class="wikitable" {| class="wikitable"
|- |-
! Class ! Class<br />(])
! Usage ! Usage
! Description ! Description
! Examples, or exception ! Examples, or exception
|- |-
| 00
| 00]
| Device | Device
| Unspecified<ref>Use class information in the interface descriptors. This base class is defined to use in device descriptors to indicate that class information should be determined from the Interface Descriptors in the device.</ref> | Unspecified<ref>Use class information in the interface descriptors. This base class is defined to use in device descriptors to indicate that class information should be determined from the Interface Descriptors in the device.</ref>
| Device class is unspecified, interface descriptors are used to determine needed drivers | Device class is unspecified, interface descriptors are used to determine needed drivers
|- |-
| 01h | 01
| Interface | Interface
| Audio | Audio
| ], ], ], ] | ], ], ], ]
|- |-
| 02h | 02
| Both | Both
| ] | ]
| ], ], ] adapter | ] and ] ], ], ] adapter, ] adapter. Used together with class 0Ah ''(CDC-Data'') below
|- |-
| 03h | 03
| Interface | Interface
| ] | ]
| ], ], joystick | ], ], joystick
|- |-
| 05h | 05
| Interface | Interface
| Physical Interface Device (PID) | Physical interface device (PID)
| Force feedback joystick | Force feedback joystick
|- |-
| 06h | 06
| Interface | Interface
| Media (]/])
| Image
| ], ] | ], ]
|- |-
| 07h | 07
| Interface | Interface
| ] | ]
| ], ], ] | ], ], ]
|- |-
| 08h | 08
| Interface | Interface
| ] (MSC or UMS) | ], ]
| ], ] ], ], ], external drive | ], ], ], ], external drive
|- |-
| 09h | 09
| Device | Device
| ] | ]
| Full bandwidth hub | High speed USB hub
|- |-
| 0Ah | 0A
| Interface | Interface
| CDC-Data | CDC-Data
| Used together with class 02h: communications and CDC control | Used together with class 02h ''(Communications and CDC Control'') above
|- |-
| 0Bh | 0B
| Interface | Interface
| ] | ]
| USB smart card reader | USB smart card reader
|- |-
| 0Dh | 0D
| Interface | Interface
| Content security | Content security
| Fingerprint reader | Fingerprint reader
|- |-
| 0Eh | 0E
| Interface | Interface
| ] | ]
| ] | ]
|- |-
| 0Fh | 0F
| Interface | Interface
| Personal Healthcare | Personal healthcare device class (PHDC)
| Pulse monitor (watch) | Pulse monitor (watch)
|- |-
| 10h | 10
| Interface | Interface
| Audio/Video (AV) | Audio/Video (AV)
| ], TV | ], TV
|- |-
| 11h | 11
| Device | Device
| Billboard | Billboard
| Describes USB-C alternate modes supported by device
| Programmable LED sign
|- |-
| DCh | DC
| Both | Both
| Diagnostic Device | Diagnostic device
| USB compliance testing device | USB compliance testing device
|- |-
| E0h | E0
| Interface | Interface
| ] Controller | ] Controller
| ] adapter, Microsoft ] | ] adapter
|- |-
| EFh | EF
| Both | Both
| Miscellaneous | Miscellaneous
| ] device | ] device
|- |-
| FEh | FE
| Interface | Interface
| Application-specific | Application-specific
| ] Bridge, Test & Measurement Class (USBTMC),<ref>{{Cite journal | title = Universal Serial Bus Test and Measurement Class Specification (USBTMC) Revision 1.0 | date = April 14, 2003 | publisher = USB Implementers Forum}}</ref> USB DFU (Direct Firmware Update)<ref>{{Cite journal |url = http://www.usb.org/developers/devclass_docs/DFU_1.1.pdf |title= DFU 1.1 | publisher = USB Implementers Forum | format =PDF |accessdate=2010-06-22}}</ref> | ] Bridge, ], Test & Measurement Class (USBTMC),<ref>{{Cite web |title= Universal Serial Bus Test and Measurement Class Specification (USBTMC) Revision 1.0 |url= http://sdpha2.ucsd.edu/Lab_Equip_Manuals/USBTMC_1_00.pdf |date= 14 April 2003 |publisher= USB Implementers Forum |via= sdpha2.ucsd.edu |access-date= 10 May 2018 |archive-date= 23 December 2018 |archive-url= https://web.archive.org/web/20181223041215/http://sdpha2.ucsd.edu/Lab_Equip_Manuals/USBTMC_1_00.pdf |url-status=live}}</ref> USB DFU (Device Firmware Upgrade)<ref name="dfu-1.1">{{cite web
|url = https://www.usb.org/document-library/device-firmware-upgrade-11-new-version-31-aug-2004
|title = Universal Serial Bus Device Class Specification for Device Firmware Upgrade, Version 1.1
|date = 15 October 2004
|access-date = 8 September 2014
|publisher = USB Implementers Forum
|pages = 8–9
|url-status = live
|archive-url = https://web.archive.org/web/20141011015811/http://www.usb.org/developers/docs/devclass_docs/DFU_1.1.pdf
|archive-date = 11 October 2014
}}</ref>
|- |-
| FFh | FFh
Line 299: Line 677:
|} |}


=== {{anchor|MASSSTORAGE}}USB mass storage / USB drive === === USB mass storage / USB drive <span class="anchor" id="MASSSTORAGE"></span>===
], a typical USB mass-storage device]]
]
{{See also|USB mass storage device class|Disk enclosure|External hard disk drive}} {{See also|USB mass storage device class|Disk enclosure|External hard disk drive}}
], a typical USB mass-storage device]]
] (2242) solid-state-drive (]) connected into USB 3.0 adapter and connected to computer]]


USB implements connections to storage devices using a set of standards called the ] (MSC or UMS). This was at first intended for traditional magnetic and optical drives and has been extended to support ]. It has also been extended to support a wide variety of novel devices as many systems can be controlled with the familiar metaphor of file manipulation within directories. The process of making a novel device look like a familiar device is also known as extension.{{citation needed|date=December 2013}} The ability to boot a write-locked ] with a USB adapter is particularly advantageous for maintaining the integrity and non-corruptible, pristine state of the booting medium. The ] (MSC or UMS) standardizes connections to storage devices. At first intended for magnetic and optical drives, it has been extended to support ] and ] readers. The ability to boot a write-locked ] with a USB adapter is particularly advantageous for maintaining the integrity and non-corruptible, pristine state of the booting medium.


Though most post-Summer 2004 computers can boot from USB mass storage devices, USB is not intended as a primary bus for a computer's internal storage. Buses such as ] (PATA or IDE), ] (SATA), or ] fulfill that role in PC class computers. However, USB has one important advantage, in that it is possible to install and remove devices without rebooting the computer (]), making it useful for mobile peripherals, including drives of various kinds. Though most personal computers since early 2005 can boot from USB mass storage devices, USB is not intended as a primary bus for a computer's internal storage. However, USB has the advantage of allowing ], making it useful for mobile peripherals, including drives of various kinds.


Firstly conceived and still used today for optical storage devices (] drives, ] drives, etc.), several manufacturers offer external portable USB ]s, or empty enclosures for disk drives. These offer performance comparable to internal drives, limited by the current number and types of attached USB devices, and by the upper limit of the USB interface (in practice about 30&nbsp;MB/s for USB 2.0 and potentially 400&nbsp;MB/s or more<ref>''Universal Serial Bus 3.0 Specification'',4.4.11 "Efficiency"</ref> for USB 3.0). These external drives typically include a "translating device" that bridges between a drive's interface to a USB interface port. Functionally, the drive appears to the user much like an internal drive. Other competing standards for external drive connectivity include ], ] (now at version 2.0), ] (IEEE 1394), and most recently ]. Several manufacturers offer external portable USB ]s, or empty enclosures for disk drives. These offer performance comparable to internal drives, limited by the number and types of attached USB devices, and by the upper limit of the USB interface. Other competing standards for external drive connectivity include ], ], ] (IEEE 1394), and most recently ].


Another use for USB mass storage devices is the portable execution of software applications (such as web browsers and VoIP clients) with no need to install them on the host computer.<ref>{{cite web| url = http://www.makeuseof.com/tag/portable-software-usb/ | title = 100 Portable Apps for your USB Stick (both for Mac and Win) | accessdate = 2008-10-30}}</ref><ref>{{cite web| url = http://www.VoIP-Download.com/Skype.htm#USB/ | title =Skype VoIP USB Installation Guide|accessdate = 2008-10-30}}</ref> Another use for USB mass storage devices is the portable execution of software applications (such as web browsers and VoIP clients) with no need to install them on the host computer.<ref>{{cite web | url = http://www.makeuseof.com/tag/portable-software-usb/ | title = 100 Portable Apps for your USB Stick (both for Mac and Win) | access-date = 30 October 2008 | url-status=live | archive-url = https://web.archive.org/web/20081202121455/http://www.makeuseof.com/tag/portable-software-usb/ | archive-date = 2 December 2008}}</ref><ref>{{cite web | url = http://www.VoIP-Download.com/Skype.htm#USB/ | title = Skype VoIP USB Installation Guide | access-date = 30 October 2008 | url-status=dead | archive-url = https://web.archive.org/web/20140706153501/http://www.voip-download.com/Skype.htm#USB/ | archive-date = 6 July 2014}}</ref>


=== Media Transfer Protocol === === Media Transfer Protocol ===
{{See also|Picture Transfer Protocol}}
] (MTP) was designed by ] to give higher-level access to a device's filesystem than USB mass storage, at the level of files rather than disk blocks. It also has optional ] features. MTP was designed for use with ]s, but it has since been adopted as the primary storage access protocol of the ] from the version 4.1 Jelly Bean as well as Windows Phone 8 (Windows Phone 7 devices had used the Zune protocol which was an evolution of MTP). The primary reason for this is that MTP does not require exclusive access to the storage device the way UMS does, alleviating potential problems should an Android program request the storage while it is attached to a computer. The main drawback is that MTP is not as well supported outside of Windows operating systems.

] (MTP) was designed by ] to give higher-level access to a device's filesystem than USB mass storage, at the level of files rather than disk blocks. It also has optional ] features. MTP was designed for use with ]s, but it has since been adopted as the primary storage access protocol of the ] from the version 4.1 Jelly Bean as well as Windows Phone 8 (Windows Phone 7 devices had used the Zune protocol—an evolution of MTP). The primary reason for this is that MTP does not require exclusive access to the storage device the way UMS does, alleviating potential problems should an Android program request the storage while it is attached to a computer. The main drawback is that MTP is not as well supported outside of Windows operating systems.


=== Human interface devices === === Human interface devices ===
{{Main |USB human interface device class}} {{Main|USB human interface device class}}
Joysticks, keypads, tablets and other human-interface devices (HIDs) are also progressively migrating from MIDI, and PC ] connectors to USB.{{Citation needed |date=November 2011}}


USB mice and keyboards can usually be used with older computers that have ]s with the aid of a small USB-to-PS/2 adapter. For mice and keyboards with dual-protocol support, an adaptor that contains no ] may be used: the hardware in the USB keyboard or mouse is designed to detect whether it is connected to a USB or PS/2 port, and communicate using the appropriate protocol. Converters also exist that connect PS/2 keyboards and mice (usually one of each) to a USB port.<ref>{{cite web | url = http://www.startech.com/Server-Management/KVM-Switches/PS-2-to-USB-Keyboard-and-Mouse-Adapter~PS22USB | title = PS/2 to USB Keyboard and Mouse Adapter}}</ref> These devices present two HID endpoints to the system and use a ] to perform bidirectional data translation between the two standards. A USB mouse or keyboard can usually be used with older computers that have ]s with the aid of a small USB-to-PS/2 adapter. For mice and keyboards with dual-protocol support, a passive adapter that contains no ] may be used: the ] in the keyboard or mouse is designed to detect whether it is connected to a USB or PS/2 port, and communicate using the appropriate protocol.{{Citation needed|date=May 2023}} Active converters that connect USB keyboards and mice (usually one of each) to PS/2 ports also exist.<ref>{{cite web |url=http://www.startech.com/Server-Management/KVM-Switches/PS-2-to-USB-Keyboard-and-Mouse-Adapter~PS22USB |title=PS/2 to USB Keyboard and Mouse Adapter |archive-url=https://web.archive.org/web/20141112214808/http://www.startech.com/Server-Management/KVM-Switches/PS-2-to-USB-Keyboard-and-Mouse-Adapter~PS22USB |website=StarTech.com |archive-date=12 November 2014 |access-date=21 May 2023}}</ref>


=== Device Firmware Upgrade mechanism <span class="anchor" id="DFU"></span>===
== {{anchor|CONNECTORS}}Connectors and plugs ==
''Device Firmware Upgrade'' (DFU) is a generic mechanism for upgrading the ] of USB devices with improved versions provided by their manufacturers, offering (for example) a way to deploy firmware bug fixes. During the firmware upgrade operation, USB devices change their operating mode effectively becoming a ] programmer. Any class of USB device can implement this capability by following the official DFU specifications. Doing so allows use of DFU-compatible host tools to update the device.<ref name="dfu-1.1"/><ref name="dfu-1.0">{{cite web
{{repetition|section|date=December 2013}}
| url = http://www.usb.org/developers/devclass_docs/usbdfu10.pdf
| archive-url = https://web.archive.org/web/20140824054756/http://www.usb.org/developers/devclass_docs/usbdfu10.pdf
| title = Universal Serial Bus Device Class Specification for Device Firmware Upgrade, Version 1.0
| date = 13 May 1999 | access-date = 8 September 2014 | archive-date = 24 August 2014
| publisher = USB Implementers Forum
| pages = 7–8
}}</ref><ref>{{cite web
| url = https://admin.fedoraproject.org/pkgdb/package/dfu-util/
| title = rpms/dfu-util: USB Device Firmware Upgrade tool
| date = 14 May 2014
| access-date = 8 September 2014
| website = fedoraproject.org
| archive-date = 8 September 2014
| archive-url = https://web.archive.org/web/20140908112041/https://admin.fedoraproject.org/pkgdb/package/dfu-util/
| url-status = live
}}</ref>


DFU is sometimes used as a flash memory programming protocol in microcontrollers with built-in USB bootloader functionality.
=== Connectors properties ===
<ref>{{cite web
]
| url = https://www.st.com/resource/en/application_note/cd00264379-usb-dfu-protocol-used-in-the-stm32-bootloader-stmicroelectronics.pdf
| title = AN3156: USB DFU protocol used in the STM32 bootloader
| date = 7 February 2023
| access-date = 28 January 2024
| website = st.com
}}</ref>


=== Audio streaming ===
The connectors the USB committee specifies support a number of USB's underlying goals, and reflect lessons learned from the many connectors the computer industry has used. The connector mounted on the host or device is called the ''receptacle'', and the connector attached to the cable is called the ''plug''.<ref name=USB_3_0_spec>{{Cite book |authors=Hewlett-Packard, Intel, Microsoft, NEC, ST-Ericsson, Texas Instruments | title=Universal Serial Bus 3.0 Specification: Revision 1.0 |page=531 |date = June 6, 2011 |accessdate= 2011-07-26}}</ref> The standard purposely defines this to prevent the use of extension cables.{{Citation needed|date=September 2012}} The official USB specification documents also periodically define the term ''male'' to represent the plug, and ''female'' to represent the receptacle.{{Citation needed|date=September 2012}}
The USB Device Working Group has laid out specifications for audio streaming, and specific standards have been developed and implemented for audio class uses, such as microphones, speakers, headsets, telephones, musical instruments, etc. The working group has published three versions of audio device specifications:<ref>{{Cite press release |date=27 September 2016 |title=USB-IF Announces USB Audio Device Class 3.0 Specification |location=Houston, Texas & Beaverton, Oregon |url=https://www.businesswire.com/news/home/20160927006252/en/USB-IF-Announces-USB-Audio-Device-Class-3.0 |website=Business Wire |access-date=4 May 2018 |archive-date=4 May 2018 |archive-url=https://web.archive.org/web/20180504155618/https://www.businesswire.com/news/home/20160927006252/en/USB-IF-Announces-USB-Audio-Device-Class-3.0 |url-status=live }}</ref><ref>{{Cite web |url=http://www.usb.org/developers/docs/devclass_docs/ |title=USB Device Class Specifications |website=www.usb.org |access-date=4 May 2018 |archive-date=13 August 2014 |archive-url=https://web.archive.org/web/20140813051139/http://www.usb.org/developers/docs/devclass_docs/ |url-status=live }}</ref> USB Audio&nbsp;1.0, 2.0, and 3.0, referred to as "UAC"<ref name="xmos2015"/> or "ADC".<ref>{{Cite web |url=https://docs.microsoft.com/en-us/windows-hardware/drivers/audio/usb-2-0-audio-drivers |title=USB Audio 2.0 Drivers |website=Microsoft Hardware Dev Center |access-date=4 May 2018 |quote=ADC-2 refers to the USB Device Class Definition for Audio Devices, Release 2.0. |archive-date=4 May 2018 |archive-url=https://web.archive.org/web/20180504155514/https://docs.microsoft.com/en-us/windows-hardware/drivers/audio/usb-2-0-audio-drivers |url-status=live }}</ref>


UAC 3.0 primarily introduces improvements for portable devices, such as reduced power usage by bursting the data and staying in low power mode more often, and power domains for different components of the device, allowing them to be shut down when not in use.<ref>{{Cite web|url=https://www.synopsys.com/designware-ip/technical-bulletin/usb-audio-dwtb-q117.html|title=New USB Audio Class for USB Type-C Digital Headsets|website=Synopsys.com|access-date=7 May 2018|archive-date=7 May 2018|archive-url=https://web.archive.org/web/20180507221645/https://www.synopsys.com/designware-ip/technical-bulletin/usb-audio-dwtb-q117.html|url-status=live}}</ref>
==== {{anchor|TYPE-C}}Usability and orientation ====
]


UAC 2.0 introduced support for High Speed USB (in addition to Full Speed), allowing greater bandwidth for multi-channel interfaces, higher sample rates,<ref name=":2">{{Cite web |url=http://thewelltemperedcomputer.com/KB/USB.html |title=USB |website=The Well-Tempered Computer |last=Kars |first=Vincent |date=May 2011 |access-date=7 May 2018 |quote=All operating systems (Win, OSX, and Linux) support USB Audio Class 1 natively. This means you don't need to install drivers, it is plug&play. |archive-date=7 May 2018 |archive-url=https://web.archive.org/web/20180507153825/http://thewelltemperedcomputer.com/KB/USB.html |url-status=live }}</ref> lower inherent latency,<ref>{{Cite web |url=https://www.xmos.ai/file/fundamentals-of-usb-audio?version=latest |title=Fundamentals of USB Audio |publisher=XMOS Ltd. |website=www.xmos.com |format=PDF |date=2015 |access-date=10 December 2020 |quote=Note that Full Speed USB has a much higher intrinsic latency of 2ms}}</ref><ref name="xmos2015"/> and 8× improvement in timing resolution in synchronous and adaptive modes.<ref name="xmos2015"/> UAC2 also introduced the concept of clock domains, which provides information to the host about which input and output terminals derive their clocks from the same source, as well as improved support for audio encodings like ], audio effects, channel clustering, user controls, and device descriptions.<ref name="xmos2015"/><ref name=":4"/>
By design, it is difficult to insert a USB plug into its receptacle incorrectly. The USB specification states that the required USB icon must be embossed on the "topside" of the USB plug, which "...provides easy user recognition and facilitates alignment during the mating process." The specification also shows that the "recommended" "Manufacturer's logo" ("engraved" on the diagram but not specified in the text) is on the opposite side of the USB icon. The specification further states, "The USB Icon is also located adjacent to each receptacle. Receptacles should be oriented to allow the icon on the plug to be visible during the mating process." However, the specification does not consider the height of the device compared to the eye level height of the user, so the side of the cable that is "visible" when mated to a computer on a desk can depend on whether the user is standing or kneeling.<ref name=USB_3_0_spec />


UAC 1.0 devices are still common, however, due to their cross-platform driverless compatibility,<ref name=":2"/> and also partly due to ]'s failure to implement UAC 2.0 for over a decade after its publication, having finally added support to ] through the Creators Update on 20 March 2017.<ref>{{Cite web |url=https://blogs.windows.com/windowsexperience/2016/09/21/announcing-windows-10-insider-preview-build-14931-for-pc/ |title=Announcing Windows 10 Insider Preview Build 14931 for PC |website=Windows Experience Blog |date=21 September 2016 |access-date=7 May 2018 |quote=We now have native support for USB Audio 2.0 devices with an inbox class driver! This is an early version of the driver that does not have all features enabled |archive-date=23 September 2016 |archive-url=https://web.archive.org/web/20160923032703/https://blogs.windows.com/windowsexperience/2016/09/21/announcing-windows-10-insider-preview-build-14931-for-pc/ |url-status=live }}</ref><ref>{{Cite web |url=http://amplioaudio.blogspot.com/2017/09/usb-audio-class-20-support-in-windows.html |title=Ampliozone: USB Audio Class 2.0 Support in Windows 10, FINALLY!!!! |last=Plummer |first=Gregg |date=20 September 2017 |website=Ampliozone |access-date=7 May 2018 |archive-date=7 May 2018 |archive-url=https://web.archive.org/web/20180507154036/http://amplioaudio.blogspot.com/2017/09/usb-audio-class-20-support-in-windows.html |url-status=live }}</ref><ref name=":4">{{Cite web|url=https://www.computeraudiophile.com/ca/bits-and-bytes/this-just-in-microsoft-launches-native-class-2-usb-audio-support-wait-what-r647/|title=This Just In: Microsoft Launches Native Class 2 USB Audio Support. Wait, What?|website=Computer Audiophile|date=2 May 2017 |access-date=7 May 2018|quote=Class 2 support enables much higher sample rates such as PCM 24 bit / 384 kHz and DSD (DoP) up through DSD256.|archive-date=2 September 2018|archive-url=https://web.archive.org/web/20180902023557/https://www.computeraudiophile.com/ca/bits-and-bytes/this-just-in-microsoft-launches-native-class-2-usb-audio-support-wait-what-r647/|url-status=live}}</ref> UAC 2.0 is also supported by ], ], and ],<ref name="xmos2015"/> however ] only implements a subset of the UAC 1.0 specification.<ref name=":5">{{Cite web |url=https://source.android.com/docs/core/audio/usb#hostAudio |title=USB Digital Audio |website=Android Open Source Project |access-date=16 February 2023 |quote=Synchronous sub-mode is not commonly used with audio because both host and peripheral are at the mercy of the USB clock. }}</ref>
While it would have been better for usability if the cable could be plugged in with either side up, the original design left this out to make manufacturing as inexpensive as possible. ], who was involved in the original USB design team, is working on a new design to make the cable insertable either side up.<ref name="usb-power-delivery">{{cite news
| url = http://www.economist.com/news/international/21588104-humble-usb-cable-part-electrical-revolution-it-will-make-power-supplies?fsrc=scn/fb/wl/pe/edisonsrevenge
| title = Edison’s revenge
| publisher = The Economist
| date = 2013-10-19 | accessdate = 2013-10-23
}}</ref> The new reversible plug is also much smaller than the current ] connector. It is called ''Type-C,'' and should be introduced as an addition to the existing ] specification.<ref>{{cite web
| url = http://www.theverge.com/2013/12/4/5173686/usb-type-c-connector-specification-announced
| title = Next USB plug will finally be reversible
| date = 2013-12-04 | accessdate = 2013-12-04
| author = Sam Byford | publisher = The Verge
}}</ref>


USB provides three isochronous (fixed-bandwidth) synchronization types,<ref>{{cite web|url=http://www.atmel.com/Images/doc32139.pdf|title=32-bit Atmel Microcontroller Application Note|date=2011|publisher=Atmel Corporation|access-date=13 April 2016|url-status=live|archive-url=https://web.archive.org/web/20160506204128/http://www.atmel.com/Images/doc32139.pdf|archive-date=6 May 2016}}</ref> all of which are used by audio devices:<ref>{{Cite web|url=http://www.ti.com/lit/ds/symlink/pcm2906c.pdf|title=PCM2906C datasheet|date=November 2011|website=Texas Instruments|quote=The PCM2906C employs SpAct architecture, TI's unique system that recovers the audio clock from USB packet data.|access-date=4 May 2018|archive-date=4 May 2018|archive-url=https://web.archive.org/web/20180504225235/http://www.ti.com/lit/ds/symlink/pcm2906c.pdf|url-status=live}}</ref>
Only moderate force is needed to insert or remove a USB cable. USB cables and small USB devices are held in place by the gripping force from the receptacle (without need of the screws, clips, or thumb-turns other connectors have required).


* Asynchronous&nbsp;— The ADC or DAC are not synced to the host computer's clock at all, operating off a free-running clock local to the device.
==== Power-use topology ====
* Synchronous&nbsp;— The device's clock is synced to the USB start-of-frame (SOF) or Bus Interval signals. For instance, this can require syncing an 11.2896&nbsp;MHz clock to a 1&nbsp;kHz SOF signal, a large frequency multiplication.<ref>{{Cite web|url=http://www.cypress.com/file/102921/download|title=Designing Modern USB Audio Systems|last=Castor-Perry|first=Kendall|date=October 2010|website=Cypress Semiconductor|access-date=4 May 2018|archive-date=5 May 2018|archive-url=https://web.archive.org/web/20180505172950/http://www.cypress.com/file/102921/download|url-status=live}}</ref><ref name=":1">{{Cite web|url=http://www.cypress.com/file/122521/download|title=Programmable Clock Generation and Synchronization for USB Audio Systems|last=Castor-Perry|first=Kendall|date=2011|website=Cypress Semiconductor|quote=Early USB replay interfaces used synchronous mode but acquired a reputation for poor quality of the recovered clock (and resultant poor replay quality). This was primarily due to deficiencies of clocking implementation rather than inherent shortcomings of the approach.|access-date=4 May 2018|archive-date=4 May 2018|archive-url=https://web.archive.org/web/20180504181023/http://www.cypress.com/file/122521/download|url-status=live}}</ref>
The standard connectors were deliberately intended to enforce the directed ] of a USB network: type A connectors on host devices that supply power and type B connectors on target devices that draw power. This is intended to prevent users from accidentally connecting two USB power supplies to each other, which could lead to ]s and dangerously high currents, circuit failures, or even fire. USB does not support cyclic networks and the standard connectors from incompatible USB devices are themselves incompatible.<ref name="cablestogo" />
* Adaptive&nbsp;— The device's clock is synced to the amount of data sent per frame by the host<ref>{{Cite web|url=http://www.thewelltemperedcomputer.com/Lib/Hitoshi%20Kondoh%20story.pdf|title=The D/A diaries: A personal memoir of engineering heartache and triumph|last=Kondoh|first=Hitoshi|date=20 February 2002|quote=The fact that there is no clock line within the USB cable leads to a thinner cable, which is an advantage. But, no matter how good the crystal oscillators are at the send and receive ends, there will always be some difference between the two...|access-date=4 May 2018|archive-date=12 December 2019|archive-url=https://web.archive.org/web/20191212230749/http://www.thewelltemperedcomputer.com/Lib/Hitoshi%20Kondoh%20story.pdf|url-status=live}}</ref>


While the USB spec originally described asynchronous mode being used in "low cost speakers" and adaptive mode in "high-end digital speakers",<ref>{{Cite web |url=http://www.usb.org/developers/docs/usb20_docs/ |title=USB 2.0 Documents |website=www.usb.org |access-date=7 May 2018 |archive-date=3 December 2017 |archive-url=https://web.archive.org/web/20171203144114/http://www.usb.org/developers/docs/usb20_docs/ |url-status=live }}</ref> the opposite perception exists in the ] world, where asynchronous mode is advertised as a feature, and adaptive/synchronous modes have a bad reputation.<ref>{{Cite web |url=https://www.cambridgeaudio.com/usa/en/blog/our-guide-usb-audio-why-should-i-use-it |title=Our Guide to USB Audio - Why Should I Use it? |website=Cambridge Audio |date=9 May 2016 |access-date=7 May 2018 |quote=Synchronous USB DAC is the lowest quality of the three ... Adaptive ... means that there is no continuous, accurate master clock in the DAC, which causes jitter in the audio stream. ... Asynchronous – this is the most complex to implement but it is a huge improvement on the other types. |archive-date=7 May 2018 |archive-url=https://web.archive.org/web/20180507153701/https://www.cambridgeaudio.com/usa/en/blog/our-guide-usb-audio-why-should-i-use-it |url-status=live }}</ref><ref>{{Cite web |url=http://thewelltemperedcomputer.com/Intro/SQ/USB_USB.htm |title=USB versus USB |website=The Well-Tempered Computer |last=Kars |first=Vincent |date=July 2012 |access-date=7 May 2018 |quote=Synchronous is not used in a quality DAC as it is very jittery. ... asynchronous is the better of these modes. |archive-date=22 April 2018 |archive-url=https://web.archive.org/web/20180422204100/http://thewelltemperedcomputer.com/Intro/SQ/USB_USB.htm |url-status=live }}</ref><ref name=":5"/> In reality, all types can be high-quality or low-quality, depending on the quality of their engineering and the application.<ref name=":1"/><ref name="xmos2015"/><ref>{{Cite news |url=https://www.head-fi.org/threads/low-jitter-usb-dan-lavry-michael-goodman-adaptive-asynchronous.493152/#post-6661517 |title=Low-Jitter USB: Dan Lavry, Michael Goodman, Adaptive, Asynchronous |work=Headphone Reviews and Discussion - Head-Fi.org |access-date=7 May 2018 |quote=Some manufacturers may lead you to believe that Asynchronous USB transfers are superior to Adaptive USB transfers and that therefore you must believe in the asynchronous solution. This no more true than saying that you "must" hold the fork in your left hand. In fact, if you know what you are doing, you will feed yourself with either hand. The issue is really about good engineering practices. |archive-date=7 May 2018 |archive-url=https://web.archive.org/web/20180507153738/https://www.head-fi.org/threads/low-jitter-usb-dan-lavry-michael-goodman-adaptive-asynchronous.493152/#post-6661517 |url-status=live }}</ref> Asynchronous has the benefit of being untied from the computer's clock, but the disadvantage of requiring ] when combining multiple sources.
However, some of this directed topology is lost with the advent of multi-purpose USB connections (such as ] in smartphones, and USB-powered Wi-Fi routers), which require A-to-A, B-to-B, and sometimes Y/splitter cables. See the ] section below, for a more detailed summary description.


== Connectors <span class="anchor" id="CONNECTORS"></span>==
==== Durability ====
{{Main|USB hardware#Connectors}}
The standard connectors were designed to be robust. Because USB is ], the connectors would be used more frequently, and perhaps with less care, than other connectors. Many previous connector designs were fragile, specifying embedded component pins or other delicate parts that were vulnerable to bending or breaking. The electrical contacts in a USB connector are protected by an adjacent plastic tongue, and the entire connecting assembly is usually protected by an enclosing metal sheath.<ref name="usb.org-cabconn">{{cite web
| url = http://www.usb.org/developers/devclass_docs/CabConn20.pdf
| title = Universal Serial Bus Cables and Connectors Class Document Revision 2.0
| date = August 2007 | accessdate = 2013-12-03
| format = PDF | publisher = usb.org
}}</ref>


The connectors the USB committee specifies support a number of USB's underlying goals, and reflect lessons learned from the many connectors the computer industry has used. The female connector mounted on the host or device is called the ''receptacle'', and the male connector attached to the cable is called the ''plug''.<ref name="spec_3.0"/>{{rp|pages=2-5–2-6}} The official USB specification documents also periodically define the term ''male'' to represent the plug, and ''female'' to represent the receptacle.<ref>{{cite web |title=USB 2.0 Specification Engineering Change Notice (ECN) #1: Mini-B connector |via=www.usb.org |url=http://www.usb.org/developers/docs/ecn1.pdf |date=20 October 2000 |url-status=live |archive-url=https://web.archive.org/web/20150412121600/http://www.usb.org/developers/docs/ecn1.pdf |archive-date=12 April 2015 |access-date=29 December 2014}}</ref>
The connector construction always ensures that the external sheath on the plug makes contact with its counterpart in the receptacle before any of the four connectors within make electrical contact. The external metallic sheath is typically connected to system ground, thus dissipating damaging static charges. This enclosure design also provides a degree of protection from electromagnetic interference to the USB signal while it travels through the mated connector pair (the only location when the otherwise ] travels in parallel). In addition, because of the required sizes of the power and common connections, they are made after the system ground but before the data connections. This type of staged make-break timing allows for electrically safe hot-swapping.<ref name="usb.org-cabconn" />
].]]
The design is intended to make it difficult to insert a USB plug into its receptacle incorrectly. The USB specification requires that the cable plug and receptacle be marked so the user can recognize the proper orientation.<ref name="spec_3.0"/> The USB-C plug however is reversible. USB cables and small USB devices are held in place by the gripping force from the receptacle, with no screws, clips, or thumb-turns as some connectors use.


The different A and B plugs prevent accidentally connecting two power sources. However, some of this directed topology is lost with the advent of multi-purpose USB connections (such as ] in smartphones, and USB-powered Wi-Fi routers), which require A-to-A, B-to-B, and sometimes Y/splitter cables.
The newer Micro-USB receptacles are designed for up to 10,000 cycles of insertion and removal between the receptacle and plug, compared to 1,500 for the standard USB and 5,000 for the Mini-USB receptacle. This is accomplished by adding a locking device and by moving the leaf-spring connector from the jack to the plug, so that the most-stressed part is on the cable side of the connection. This change was made so that the connector on the less expensive cable would bear the most wear instead of the more expensive micro-USB device.<ref name="stackexchange-miniusb" /><ref name="usb.org-cabconn" />


USB connector types multiplied as the specification progressed. The original USB specification detailed standard-A and standard-B plugs and receptacles. The connectors were different so that users could not connect one computer receptacle to another. The data pins in the standard plugs are recessed compared to the power pins, so that the device can power up before establishing a data connection. Some devices operate in different modes depending on whether the data connection is made. Charging docks supply power, and do not include a host device or data pins, allowing any capable USB device to charge or operate from a standard USB cable. Charging cables provide power connections but not data. In a charge-only cable, the data wires are shorted at the device end; otherwise, the device may reject the charger as unsuitable.
==== Compatibility ====
The USB standard specifies relatively loose tolerances for compliant USB connectors to minimize physical incompatibilities in connectors from different vendors. To address a weakness present in some other connector standards, the USB specification also defines limits to the size of a connecting device in the area around its plug. This was done to prevent a device from blocking adjacent ports due to the size of the cable strain relief mechanism (usually molding integral with the cable outer insulation) at the connector. Compliant devices must either fit within the size restrictions or support a compliant extension cable that does.


== Cabling <span class="anchor" id="CABLING"></span>==
In general, cables have only plugs, and hosts and devices have only receptacles. Hosts almost universally have type-A receptacles, and devices one or another type-B variety. Type-A plugs mate only with type-A receptacles, and type-B with type-B; they are deliberately physically incompatible. However, an extension to USB standard specification called ] allows a single port to act as either a host or a device—chosen by which end of the cable plugs into the receptacle on the unit. Even after the cable is hooked up and the units are communicating, the two units may "swap" ends under program control. This capability is meant for units such as ] in which the USB link might connect to a PC's host port as a device in one instance, yet connect as a host itself to a keyboard and mouse device in another instance.
{{Main|USB hardware#Cabling}}
]]]


The USB&nbsp;1.1 standard specifies that a standard cable can have a maximum length of {{convert|5|m|ftin|sp=us}} with devices operating at full speed (12&nbsp;Mbit/s), and a maximum length of {{convert|3|m|ftin|sp=us}} with devices operating at low speed (1.5&nbsp;Mbit/s).<ref>{{cite web |url=http://www.cablesplususa.com/pdf/USB_Cable_Length_Limitations.pdf |title=USB Cable Length Limitations |website=CablesPlusUSA.com |date=3 November 2010 |access-date=2 February 2014 |archive-url=https://web.archive.org/web/20141011015850/http://www.cablesplususa.com/pdf/USB_Cable_Length_Limitations.pdf |archive-date=11 October 2014}}</ref><ref>{{cite web |url=https://www.techwalla.com/articles/what-is-the-maximum-length-of-a-usb-cable |title=What Is the Maximum Length of a USB Cable? |website=Techwalla.com |access-date=18 November 2017 |url-status=live |archive-url=https://web.archive.org/web/20171201043247/https://www.techwalla.com/articles/what-is-the-maximum-length-of-a-usb-cable |archive-date=1 December 2017}}</ref><ref name="faq"/>
===== USB 3.0 connectors =====
{{Main|USB 3.0 backward compatibility}}


USB&nbsp;2.0 provides for a maximum cable length of {{convert|5|m|ftin|sp=us}} for devices running at high speed (480&nbsp;Mbit/s).<ref name="faq">{{Cite web | url = http://www.usb.org/developers/usbfaq/#cab1 | title = Cables and Long-Haul Solutions | work = USB 2.0 Frequently Asked Questions | publisher = USB Implementers Forum | access-date = 28 April 2019 | url-status=dead | archive-url = https://web.archive.org/web/20110118225750/http://www.usb.org/developers/usbfaq/#cab1 | archive-date = 18 January 2011}}</ref>
Type A plugs and receptacles from both USB&nbsp;3.0 and USB&nbsp;2.0 are designed to interoperate. Type B plugs and receptacles in USB&nbsp;3.0 are somewhat larger than those in USB&nbsp;2.0; thus, USB&nbsp;2.0 Type B plugs can fit into USB&nbsp;3.0 Type B receptacles, while the opposite is not possible.


The USB&nbsp;3.0 standard does not directly specify a maximum cable length, requiring only that all cables meet an electrical specification: for copper cabling with ]&nbsp;26 wires the maximum practical length is {{convert|3|m|ftin|sp=us}}.<ref>{{cite web |title=USB 3.0 Developers FAQ |url=http://janaxelson.com/usb3faq.htm#ca_maximum |access-date=20 October 2016 |last=Axelson |first=Jan |url-status=live |archive-url=https://web.archive.org/web/20161220073858/http://www.janaxelson.com/usb3faq.htm#ca_maximum |archive-date=20 December 2016 }}</ref>
=== Connector types ===
]


=== USB bridge cables ===
There are several types of USB connector, including some that have been added while the specification progressed. The original USB specification detailed Standard-A and Standard-B plugs and receptacles; the B connector was necessary so that cabling could be plug ended at both ends and still prevent users from connecting one computer receptacle to another. The first engineering change notice to the USB 2.0 specification added Mini-B plugs and receptacles.
USB bridge cables, or data transfer cables can be found within the market, offering direct PC to PC connections. A bridge cable is a special cable with a chip and active electronics in the middle of the cable. The chip in the middle of the cable acts as a peripheral to both computers and allows for peer-to-peer communication between the computers. The USB bridge cables are used to transfer files between two computers via their USB ports.


Popularized by Microsoft as ], the Microsoft utility used a special USB bridge cable to transfer personal files and settings from a computer running an earlier version of Windows to a computer running a newer version. In the context of the use of ''Windows Easy Transfer'' software, the bridge cable can sometimes be referenced as ''Easy Transfer cable''.
The data connectors in the Standard-A plug are actually recessed in the plug as compared to the outside power connectors. This permits the power to connect first, which prevents data errors by allowing the device to power up first and then transfer the data. Some devices operate in different modes depending on whether the data connection is made. This difference in connection can be exploited by inserting the connector only partially. For example, some battery-powered MP3 players switch into file transfer mode and cannot play MP3 files while a USB plug is fully inserted, but can be operated in MP3 playback mode using USB power by inserting the plug only part way so that the power slots make contact while the data slots do not. This enables those devices to be operated in MP3 playback mode while getting power from the cable.{{or|date=November 2011}}


Many USB bridge / data transfer cables are still USB 2.0, but there are also a number of USB 3.0 transfer cables. Despite USB 3.0 being 10 times faster than USB 2.0, USB 3.0 transfer cables are only 2 to 3 times faster given their design.{{clarify|reason="given their design" suggests there are reasons for this, what are they?|date=October 2022}}
To reliably enable a charge-only feature, modern USB accessory peripherals now include charging cables that provide power connections to the host port but no data connections, and both home and vehicle charging docks are available that supply power from a converter device and do not include a host device and data pins, allowing any capable USB device to charge or operate from a standard USB cable.


The USB 3.0 specification introduced an A-to-A cross-over cable without power for connecting two PCs. These are not meant for data transfer but are aimed at diagnostic uses.
==== Standard connectors ====
]
The USB 2.0 Standard-A type of USB plug is a flattened rectangle that inserts into a "downstream-port" receptacle on the USB host, or a hub, and carries both power and data. This plug is frequently seen on cables that are permanently attached to a device, such as one connecting a keyboard or mouse to the computer via USB connection.


==== Dual-role USB connections ====
USB connections eventually wear out as the connection loosens through repeated plugging and unplugging. The lifetime of a USB-A male connector is approximately 1,500 connect/disconnect cycles.<ref>{{cite web|url=http://www.getusb.info/what-is-the-life-cycle-of-a-usb-flash-drive/ |title= What is the Life Cycle of a USB Flash Drive? |publisher=Get USB |date=2007-03-08 |accessdate=2011-12-12}}</ref>
USB bridge cables have become less important with USB dual-role-device capabilities introduced with the USB 3.1 specification. Under the most recent specifications, USB supports most scenarios connecting systems directly with a Type-C cable. For the capability to work, however, connected systems must support role-switching. Dual-role capabilities requires there be ''two'' controllers within the system, as well as a ''role controller''. While this can be expected in a mobile platform such as a tablet or a phone, desktop PCs and laptops often will not support dual roles.<ref>{{cite web |url=https://superuser.com/questions/1080002/usb-3-1-type-c-host-to-host |title=USB 3.1 - Type-C Host to Host |website=superuser.com |access-date=21 July 2021 |archive-date=14 October 2021 |archive-url=https://web.archive.org/web/20211014022330/https://superuser.com/questions/1080002/usb-3-1-type-c-host-to-host |url-status=live }}</ref>


== Power <span class="anchor" id="POWER"></span>==
A Standard-B plug—which has a square shape with beveled exterior corners—typically plugs into an "upstream receptacle" on a device that uses a removable cable, e.g., a printer. On some devices, the Type B receptacle has no data connections, being used solely for accepting power from the upstream device. This two-connector-type scheme (A/B) prevents a user from accidentally creating an ].<ref>{{cite journal | last = Quinnell| first = Richard A| title =USB: a neat package with a few loose ends| work = EDN Magazine | publisher = Reed | year = 1996| url = http://www.edn.com/design/systems-design/4351549/USB-a-neat-package-with-a-few-loose-end| accessdate = 2013-02-18}}</ref>
{{Main|USB hardware#Power}}


Upstream USB connectors supply power at a nominal 5&nbsp;V DC via the V_BUS pin to downstream USB devices.
==== {{anchor|MINI|MICRO}}Mini and Micro connectors ====
]
]
]


=== Low-power and high-power devices <span class="anchor" id="HOST"></span>===
Various connectors have been used for smaller devices such as ]s, ]s, and ]s. These include the now-deprecated<ref name=depmini>{{cite press release | publisher=USB Implementers Forum | title=Deprecation of the Mini-A and Mini-AB Connectors | date=2007-05-27 | accessdate=2009-01-13 | url = http://www.usb.org/developers/Deprecation_Announcement_052507.pdf | format=PDF}}</ref> (i.e. de-certified but standardized) Mini-A and Mini-AB connectors (Mini-B connectors are still supported but not OTG (On The Go, i.e. mobile) compliant).<ref>{{cite web| publisher=USB IF Compliance Updates | title= ID Pin Resistance on Mini B-plugs and Micro B-plugs Increased to 1 Mohm |date = December 2009| accessdate=2010-03-01 | url = http://compliance.usb.org/index.asp?UpdateFile=Cables%20and%20Connectors&Format=Standard#63}}</ref> The Mini-B USB connector was standard for transferring data to and from the early data phones and PDAs, such as Blackberry models.


This section describes the power distribution model of USB that existed before ] (USB-PD). On devices that do not use PD, USB provides up to 4.5&nbsp;W through Type-A and Type-B connectors, and up to 15&nbsp;W through USB-C. All pre-PD USB power is provided at 5&nbsp;V.
The Mini-A and Mini-B plugs are approximately 3 by 7&nbsp;mm. The micro-USB plugs have a similar ] and approximately half the thickness, enabling their integration into thinner portable devices. The micro-A connector is 6.85 by 1.8&nbsp;] with a maximum overmold size of 11.7 by 8.5&nbsp;mm. The micro-B connector is 6.85 by 1.8&nbsp;mm with a maximum overmold size of 10.6 by 8.5&nbsp;mm.<ref name = microspec>{{cite web | publisher =USB Implementers Forum | title=Universal Serial Bus Micro-USB Cables and Connectors Specification | date =2007-04-04 | accessdate=2010-09-03 | url= http://193.219.66.80/datasheets/usb_20/Micro-USB_final/Micro-USB_1_01.pdf | format= PDF}}</ref>


For a host providing power to devices, USB has a concept of the ''unit load''. Any device may draw power of one unit, and devices may request more power in these discrete steps. It is not required that the host provide requested power, and a device may not draw more power than negotiated.
The Micro-USB connector was announced by the ] on 4 January 2007.<ref>{{cite press release | publisher=USB Implementers Forum | title= Mobile phones to adopt new, smaller USB connector | date = 2007-01-04 | accessdate= 2007-01-08 | url= http://www.usb.org/press/pressroom/2007_01_04_usbif.pdf | format = PDF}}</ref> The Micro B USB connector has a maximum current rating of either 1&nbsp;A per pin, or 1.8&nbsp;A for pins 1 and 5, and 0.5&nbsp;A for pins 2, 3, and 4.<ref name="hirose-e24200011"></ref> The Mini-A connector and the Mini-AB receptacle connector were deprecated on 23 May 2007.<ref>{{cite press release | publisher = USB Implementers Forum | title= Deprecation of the Mini-A and Mini-AB Connectors | date=2007-05-23 | accessdate=2010-12-23 | url= http://www.usb.org/developers/Deprecation_Announcement_052507.pdf | format= PDF}}</ref> While many currently available devices and cables still use Mini plugs, the newer Micro connectors are being widely adopted and as of December 2010, they are the most widely used.{{citation needed| date= December 2012}} The thinner micro connectors are intended to replace the Mini plugs in new devices including ]s, ]s, and cameras.<ref></ref> The Micro plug design is rated for at least 10,000 connect&ndash;disconnect cycles{{mdashb}}significantly more than the Mini plug design.<ref name="hirose-e24200011" /><ref name="MicroUSB 1.01">{{cite web|url=http://www.usb.org/developers/docs/usb_20_101111.zip |title = Universal Serial Bus Micro-USB Cables and Connectors Specification to the USB 2.0 Specification, Revision 1.01 | date =2007-04-07|accessdate=2010-11-18|publisher=USB Implementers Forum | format = Zip | quote=Section 1.3: Additional requirements for a more rugged connector that is durable past 10,000 cycles and still meets the USB 2.0 specification for mechanical and electrical performance was also a consideration. The Mini-USB could not be modified and remain backward compatible to the existing connector as defined in the USB OTG specification. | archiveurl = http://193.219.66.80/datasheets/usb_20/Micro-USB_final/Micro-USB_1_01.pdf | archivedate = 2007-04-08}}</ref> It is also designed to reduce the mechanical wear on the device; instead the easier-to-replace cable is designed to bear the mechanical wear of connection and disconnection. The ''Universal Serial Bus Micro-USB Cables and Connectors Specification'' details the mechanical characteristics of Micro-A ]s, Micro-AB receptacles (which accept both Micro-A and Micro-B plugs), and Micro-B plugs and receptacles,<ref name="MicroUSB 1.01" /> along with a Standard-A receptacle to Micro-A plug adapter.


Devices that draw no more than one unit are said to be ''low-power'' devices. All devices must act as low-power devices when starting out as unconfigured. For USB devices up to USB&nbsp;2.0 a unit load is 100&nbsp;mA (or 500&nbsp;mW), while USB&nbsp;3.0 defines a unit load as 150&nbsp;mA (750&nbsp;mW). Full-featured USB-C can support low-power devices with a unit load of 250&nbsp;mA (or 1250&nbsp;mW).
The cellular phone carrier group, ] (OMTP) in 2007 endorsed Micro-USB as the standard connector for data and power on mobile devices<ref>{{cite web| url=http://www.omtp.org/Publications/Display.aspx?Id=08d2e4e3-ebee-407c-a51b-94057e7f7b19 | title=OMTP Local Connectivity: Data Connectivity | publisher =] | date=17 September 2007 | accessdate= 2009-02-11}}{{Dead link | date = March 2011}}</ref> In addition, on 22 October 2009 the ] (ITU) has also announced that it had embraced micro-USB as the ''Universal Charging Solution'' its "energy-efficient one-charger-fits-all new mobile phone solution", and added: "Based on the Micro-USB interface, UCS chargers also include a 4-star or higher efficiency rating{{mdashb}}up to three times more energy-efficient than an unrated charger."<ref>{{cite web| publisher = Pressinfo |url=http://www.itu.int/newsroom/press_releases/2009/49.html | type = press release | title = Universal phone charger standard approved—One-size-fits-all solution will dramatically cut waste and GHG emissions | work =ITU |date=2009-10-22 |accessdate= 2009-11-04}}</ref>


Devices that draw more than one unit are ''high-power'' devices (such as typical 2.5-inch hard disk drives). USB up to 2.0 allows a host or hub to provide up to 2.5&nbsp;W to each device, in five discrete steps of 100&nbsp;mA, and SuperSpeed devices (USB 3.x) allows a host or a hub to provide up to 4.5&nbsp;W in six steps of 150&nbsp;mA.
The European Standardisation Bodies ], ] and ] (independent of the OMTP/GSMA proposal) defined a ] (EPS) for use with smartphones sold in the EU based on micro-USB.<ref>{{cite web | url= http://europa.eu/rapid/pressReleasesAction.do?reference=IP/10/1776&format=HTML&aged=0&language=EN&guiLanguage=en |title= Commission welcomes new EU standards for common mobile phone charger | work = Press Releases |publisher=Europa |date=2010-12-29 |accessdate=2011-05-22}}</ref> 14 of the world's largest mobile phone manufacturers signed the EU's common EPS Memorandum of Understanding (MoU).<ref>{{Citation | url = http://europa.eu/rapid/pressReleasesAction.do?reference=IP/10/1776 | title = New EU standards for common mobile phone charger | publisher = Europa | type = press release}}</ref><ref>{{Citation | url = http://europa.eu/rapid/pressReleasesAction.do?reference=MEMO/09/301; | title = The following 10 biggest mobile phone companies have signed the MoU: Apple, LG, Motorola, NEC, Nokia, Qualcomm, Research In Motion, Samsung, Sony Ericsson, Texas Instruments | publisher = Europa | type = press release}}</ref> ], one of the original MoU signers, makes micro-USB adapters available – as permitted in the Common EPS MoU – for its ]s equipped with Apple's proprietary ] or (later) "]" connector.<ref>{{Citation | url = http://gigaom.com/apple/nice-micro-usb-adapter-apple-now-sell-it-everywhere/ | title = Nice micro-usb adapter Apple, now sell it everywhere | date = Oct 5, 2011 | newspaper = Giga om}}</ref><ref>{{Citation | url = http://www.engadget.com/2012/11/03/apples-lightning-to-microusb-adapter-in-us/ | title = Apple's Lightning to micro-USB adapter now available in US, not just Europe anymore | date = 2012-11-03 | newspaper = Engadget}}</ref>
USB-C allows for dual-lane operation of USB 3.x with larger unit load (250&nbsp;mA; up to 7.5&nbsp;W)<ref>{{Cite web |title=USB 3.2 Revision 1.1 - June 2022 |url=https://usb.org/document-library/usb-32-revision-11-june-2022 |access-date=2024-12-31 |website=usb.org |publisher=USB-IF |at=p470, sec. 11.4.5 Vbus Electrical Characteristics}}</ref>. USB-C also allows for Type-C Current as a replacement for USB BC, signaling power availability in a simple way, without needing any data connection.<ref>{{Cite web |title=USB Type-C® Cable and Connector Specification Release 2.4 {{!}} USB-IF |url=https://usb.org/document-library/usb-type-cr-cable-and-connector-specification-release-24 |access-date=2024-12-31 |website=usb.org |publisher=USB-IF |at=p41, sec. 2.4 Vbus}}</ref>


{| class="wikitable sortable" style="margin:0 0 1em 1em;"
===={{anchor|USB-ON-THE-GO}}USB On-The-Go connectors====
|+ USB power standards
{{Main|USB On-The-Go}}

All current ] (OTG) devices are required to have one, and only one, USB connector{{snd}} a Micro-AB receptacle. Non-OTG compliant devices are not allowed to use the micro-AB receptacle, due to power supply shorting hazards on the V<sub>BUS</sub> line. The micro-AB receptacle is capable of accepting both Micro-A and Micro-B plugs, attached to any of the legal cables and adapters as defined in revision 1.01 of the Micro-USB specification. Prior to the development of Micro-USB, USB On-The-Go devices were required to use Mini-AB receptacles to perform the equivalent job.

The OTG device with the A-plug inserted is called the A-device and is responsible for powering the USB interface when required and by default assumes the role of host. The OTG device with the B-plug inserted is called the B-device and by default assumes the role of peripheral. An OTG device with no plug inserted defaults to acting as a B-device. If an application on the B-device requires the role of host, then the Host Negotiation Protocol (HNP) is used to temporarily transfer the host role to the B-device.

OTG devices attached either to a peripheral-only B-device or a standard/embedded host have their role fixed by the cable, since in these scenarios it is only possible to attach the cable one way.{{Citation needed|date=October 2011}}

==== Host and Device interface receptacles ====
Connectors (receptacles and plugs) mating matrix is displayed below. As a note, all pin numbers depicted below are individually numbered for each size type of connectors (standard, mini and micro). In other words, the same pin numbers can have differently assigned functions (V<sub>CC</sub>, D+, D−, GND or ID) between different sizes of connectors, while their assignments stay the same between plugs and receptacles of the same size type. For example, pin numbered below as "4" on the Mini-B receptacle is used for On-The-Go host/client identification, while pin numbered below as "4" on the Type-A plug is connected to GND.<ref name="usbpinout">{{cite web
| url = http://www.usbpinout.net/
| title = USB Pinout
| accessdate = 2014-06-23
| publisher = usbpinout.net
}}</ref>

{| class="wikitable"
|+ USB connectors mating matrix
|- |-
! Specification
! rowspan="2" | Receptacle<br/><small>(images not to scale)</small>
! max current
! colspan="6" | Plug <small>(images not to scale)</small>
! Voltage
! max power
|- |-
| Low-power device
! ]
| {{right|100&nbsp;mA}} || {{right|5&nbsp;V}}{{Efn |name="Vdrop" |The V{{sub |BUS}} supply from a low-powered hub port may drop to 4.40&nbsp;V.}} || {{right|0.50&nbsp;W}}
! ]
! ]
! ]
! ]
! ]
|- |-
| Low-power SuperSpeed / USB 3.x device
! ]
| {{right|150&nbsp;mA}} || {{right|5&nbsp;V}}{{Efn |name="Vdrop"}} || {{right|0.75&nbsp;W}}
| {{Yes}}
| {{No}}
| {{No}}
| {{No}}
| {{No}}
| {{No}}
|- |-
| High-power device
! ]
| {{right|500&nbsp;mA}}{{Efn|Up to five unit loads; with non-SuperSpeed devices, one unit load is 100&nbsp;mA.}} || {{right|5&nbsp;V}} || {{right|2.5&nbsp;W}}
| {{No}}
| {{Yes}}
| {{No}}
| {{No}}
| {{No}}
| {{No}}
|- |-
| High-power SuperSpeed / USB 3.x single-lane device
! ]
| {{right|900&nbsp;mA}}{{Efn|Up to six unit loads; with SuperSpeed devices, one unit load is 150&nbsp;mA.}} || {{right|5&nbsp;V}} || {{right|4.5&nbsp;W}}
| {{No}}
| {{No}}
| {{Partial|Deprecated}}
| {{No}}
| {{No}}
| {{No}}
|- |-
| High-power SuperSpeed / USB 3.x dual-lane device{{Efn|name="usbcOnly"|for USB-C only}}
! ]
| {{right|1.5&nbsp;A}}{{Efn|name="Vml"|Up to six unit loads; with multi-lane devices, one unit load is 250&nbsp;mA.}} || {{right|5&nbsp;V}} || {{right|7.5&nbsp;W}}
| {{No}}
| {{No}}
| {{Partial|Deprecated}}
| {{Partial|Deprecated}}
| {{No}}
| {{No}}
|- |-
| Battery Charging (BC)
! ]
| {{right|1.5&nbsp;A}} || {{right|5&nbsp;V}} || {{right|7.5&nbsp;W}}
| {{No}}
| {{No}}
| {{No}}
| {{Yes}}
| {{No}}
| {{No}}
|- |-
| USB4{{Efn|Not Type-C current, only available after starting USB4 connection. Can be combined with Type-C current.}}
! ]
| {{right|1.5&nbsp;A}} || {{right|5&nbsp;V}} || {{right|7.5&nbsp;W}}
| {{No}}
| {{No}}
| {{No}}
| {{No}}
| {{Yes}}
| {{Yes}}
|- |-
| Type-C current 1.5&nbsp;A{{Efn|optional for every USB-C host port. Mandatory for USB-C ports with USB-BC or for even higher PD output.}}
! ]
| {{right|1.5&nbsp;A}} || {{right|5&nbsp;V}} || {{right|7.5&nbsp;W}}
| {{No}}
| {{No}}
| {{No}}
| {{No}}
| {{No}}
| {{Yes}}
|}

==== Cable plugs (USB 1.x/2.0) ====
USB cables exist with various combinations of plugs on each end of the cable, as displayed below. Notes listed in the section above apply here as well.

{| class="wikitable"
|- |-
| Type-C current 3&nbsp;A{{Efn|optional for every USB-C host port. Mandatory for ports with even higher PD output.}}
! rowspan="2" | Plug<br/><small>(images not to scale)</small>
| {{right|3&nbsp;A}} || {{right|5&nbsp;V}} || {{right|15&nbsp;W}}
! colspan="6" | Plug <small>(images not to scale)</small>
|- |-
| ] {{abbr|SPR|Standard Power Range}}{{Efn|name="usbcOnly"}}
! ]
| {{right|5&nbsp;A}}{{efn|name="req_5A_cable"|>3&nbsp;A (>60&nbsp;W) operation requires an electronically marked cable rated at 5&nbsp;A.}} || {{right|up to 20&nbsp;V}} || {{right|100&nbsp;W}}
! ]
! ]
! ]
! ]
! ]
|- |-
| Power Delivery {{abbr|EPR|Extended Power Range}}{{Efn|name="usbcOnly"}}
! ]
| {{right|5&nbsp;A}}{{efn|name="req_5A_cable"}} || {{right|up to 48&nbsp;V}}{{efn|name="req_EPR_cable"|>20&nbsp;V (>100&nbsp;W) operation requires an electronically marked Extended Power Range (EPR) cable.}} || {{right|240&nbsp;W}}
| {{Depends|Non-standard}}
| {{Depends|Non-standard}}
| {{Depends|Non-standard}}
| {{Yes}}
| {{Yes}}
| {{Yes}}
|- |-
| colspan=4 | {{notelist}}
! ]
| {{Depends|Non-standard}}
| {{No}}
| {{No}}
| {{Partial|Deprecated}}
| {{Partial|Deprecated}}
| {{Depends|Non-standard}}
|-
! ]
| {{Depends|Non-standard}}
| {{No}}
| {{No}}
| {{Depends|Non-standard}}
| {{Depends|Non-standard}}
| {{Yes}}
|-
! ]
| {{Yes}}
| {{Partial|Deprecated}}
| {{Depends|Non-standard}}
| {{No}}
| {{No}}
| {{No}}
|-
! ]
| {{Yes}}
| {{Partial|Deprecated}}
| {{Depends|Non-standard}}
| {{No}}
| {{No}}
| {{No}}
|-
! ]
| {{Yes}}
| {{Depends|Non-standard}}
| {{Yes}}
| {{No}}
| {{No}}
| {{No}}
|} |}
;Non-standard:
Existing for specific ], and in most cases not inter-operable with USB-IF compliant equipment.
However, there do exist compliant A-to-A cables with a circuit in the middle that behaves as a pair of devices, such as the ].


To recognize Battery Charging mode, a dedicated charging port places a resistance not exceeding 200&nbsp;Ω across the D+ and D− terminals. Shorted or near-shorted data lanes with less than 200&nbsp;Ω of resistance across the D+ and D− terminals signify a dedicated charging port (DCP) with indefinite charging rates.<ref name="USBBC1.2">{{cite book| url = http://www.usb.org/developers/docs/devclass_docs/BCv1.2_070312.zip | title = Battery Charging Specification, Revision 1.2 | date = 7 December 2010 | access-date = 29 March 2016 | publisher = USB Implementers Forum | url-status=live | archive-url = https://web.archive.org/web/20160328102350/http://www.usb.org/developers/docs/devclass_docs/BCv1.2_070312.zip | archive-date = 28 March 2016 |chapter= Parameter Values |page=45}}</ref><ref>{{cite web |title=OVERVIEW OF USB BATTERY CHARGING REVISION 1.2 AND THE IMPORTANT ROLE OF ADAPTER EMULATORS |url=https://pdfserv.maximintegrated.com/en/an/TUT5801.pdf |publisher=maxim integrated |page=3 |date=2014 |access-date=12 August 2021 |archive-date=4 July 2021 |archive-url=https://web.archive.org/web/20210704221311/https://pdfserv.maximintegrated.com/en/an/TUT5801.pdf |url-status=live}}</ref>
In addition to the above cable assemblies comprising two plugs, an "adapter" cable with a Micro-A plug and a Standard-A receptacle is compliant with USB specifications.<ref name=microspec/> Other combinations of connectors are not compliant.


In addition to standard USB, there is a proprietary high-powered system known as ], developed in the 1990s, and mainly used in point-of-sale terminals such as cash registers.
;Deprecated:
Some older devices and cables with Mini-A connectors have been certified by USB-IF. The Mini-A connector is obsolete: no new Mini-A connectors and neither Mini-A nor Mini-AB receptacles will be certified.<ref name=depmini/>


== Signaling ==
==== Cable plugs (USB 3.0) ====
{{Main|USB (Communications)#Signaling (USB PHY)}}
]
{{See also|USB 3.0#CONNECTORS|l1=USB 3.0 Connectors}}


USB signals are transmitted using ] on ] data wires with {{nowrap|90 ] ± 15%}} ].<ref>{{cite web |title=USB in a NutShell&nbsp;— Chapter 2: Hardware |url=http://www.beyondlogic.org/usbnutshell/usb2.htm |publisher=Beyond Logic.org |access-date=25 August 2007 |url-status=live |archive-url=https://web.archive.org/web/20070820221226/http://www.beyondlogic.org/usbnutshell/usb2.htm |archive-date=20 August 2007 }}</ref> USB 2.0 and earlier specifications define a single pair in ] (HDx). USB 3.0 and later specifications define one dedicated pair for USB 2.0 compatibility and two or four pairs for data transfer: two data wire pairs realising full-duplex (FDx) for single lane (''×1'') variants require at least SuperSpeed (SS) connectors; four pairs realising full-duplex for two lane (''×2'') variants require USB-C connectors.
USB 3.0 introduced a new Micro-B cable plug, see photo on the right. It consists of a standard USB 1.x/2.0 Micro-B cable plug, with additional 5-pin plug "stacked" on side of it. That way, USB 3.0 Micro-A host connector preserved its backwards compatibility with the USB 1.x/2.0 Micro-B cable plugs.


USB4 Gen 4 requires the use of all four pairs but allow for asymmetrical pairs configuration.<ref>{{Cite web |title=USB4 Specification v2.0 {{!}} USB-IF |url=https://www.usb.org/document-library/usb4r-specification-v20 |access-date=2023-07-22 |website=www.usb.org}}</ref> In this case one data wire pair is used for the upstream data and the other three for the downstream data or vice-versa. USB4 Gen 4 use ] on 3 levels, providing a ] of information every ] transmitted, the transmission frequency of 12.8&nbsp;GHz translate to a transmission rate of 25.6&nbsp;GBd<ref>{{Cite web |title=USB4 Version 2.0 from Simulation to Tx, Rx, and Interconnect Test {{!}} Signal Integrity Journal |url=https://www.signalintegrityjournal.com/articles/3114-usb4-version-20-from-simulation-to-tx-rx-and-interconnect-test |access-date=2023-07-22 |website=www.signalintegrityjournal.com |language=en}}</ref> and the 11-bit–to–7-trit translation provides a theoretical maximum transmission speed just over 40.2&nbsp;Gbit/s.<ref>{{Cite web |title=Welcome to the 80Gpbs Ultra-High Speed Era of USB4 {{!}} GraniteRiverLabs |url=https://www.graniteriverlabs.com/en-us/technical-blog/usb4-80-cio80 |access-date=2023-07-22 |website=www.graniteriverlabs.com |language=en-us}}</ref>
=== Pinouts ===
{{mw-datatable}}
{{See also|USB 3.0#PINOUTS|l1=USB 3.0 pinouts}}
{| class="wikitable sortable mw-datatable" style="text-align:center;"

|+ USB Data operation modes|
USB is a serial bus, using four shielded wires for the USB&nbsp;2.0 variant: two for power (V<sub>BUS</sub> and GND), and two for ] (labelled as D+ and D− in ]s). ] (NRZI) encoding scheme is used for transferring data, with a sync field to synchronise the host and receiver clocks. D+ and D− signals are transmitted on a ], providing ] data transfers for USB&nbsp;2.0.
! colspan="2" |Operation mode name

! rowspan="2" |Introduced in
USB&nbsp;3.0 provides two additional twisted pairs (four wires, SSTx+, SSTx−, SSRx+ and SSRx−), providing ] data transfers at "super-speed", which makes it similar to ] or single-lane ].
! rowspan="2" |Lanes

! rowspan="2" |]
]
! rowspan="2" | # data wires
[[File:USB 3.0 Micro B plug.PNG|thumb|'''Micro-B USB 3.0 plug'''
! rowspan="2" data-sort-type=number | Nominal signaling rate
{{ordered list
! rowspan="2" |Original label
|1= Power (V<sub>BUS</sub>)
! colspan="2" |] current<ref></ref>
|2= USB&nbsp;2.0 Data− (D−)
|3= USB&nbsp;2.0 Data+ (D+)
|4= USB OTG ID line
|5= GND
|6= USB&nbsp;3.0 Transmit− (SSTx−)
|7= USB&nbsp;3.0 Transmit+ (SSTx+)
|8= GND
|9= USB&nbsp;3.0 Receive− (SSRx−)
|10= USB&nbsp;3.0 Receive+ (SSRx+)
}}]]

{| class="wikitable"
|+ USB 1.x/2.0 standard pinout
|- |-
! Pin ! current
! class=unsortable | old
! Name
! marketing name
! Wire color
! class=unsortable | logo
! Description
|- |-
|Low-Speed
| 1
| rowspan="3" {{CNone}}
| V<sub>BUS</sub>
| rowspan="2" |USB 1.0
|style=background:red;color:white| Red (or Orange)
| rowspan="3" |1 ]
| +5&nbsp;V
| rowspan="3" |]
| rowspan="3" | 2
| data-sort-value=0.001 | 1.5 Mbit/s<br />half-duplex
| Low-Speed USB (LS)
| rowspan="2" |Basic-Speed USB
| rowspan="2" |]
|- |-
|Full-Speed
| 2
| data-sort-value=0.012 | 12&nbsp;Mbit/s<br />half-duplex
| D−
|Full-Speed USB (FS)
|style=background:white| White (or Gold)
| Data−
|- |-
|High-Speed
| 3
| D+ |USB 2.0
| data-sort-value=0.480 | 480&nbsp;Mbit/s<br />half-duplex
|style=background:green;color:white| Green
|colspan=2|Hi-Speed USB (HS)
| Data+
| ]
|- |-
|USB 3.2 Gen&nbsp;1{{Abbr|×1|single-lane}}
| 4
|USB&nbsp;3.0,<br />USB&nbsp;3.1 Gen&nbsp;1
| ]
|]
|style=background:black;color:white| Black (or Blue)
| rowspan="2" |1 ] (+ 1 HDx){{efn|name="HDx"}}
| Ground
|]
|}
| rowspan="2" |6

|5 <abbr>Gbit/s</abbr><br />symmetric
{| class="wikitable"
|SuperSpeed USB (SS)
|+ USB 1.x/2.0 Mini/Micro pinout
|USB 5Gbps
|- style="background:#e0e0e0;"
|]
! Pin
! Name
! Wire color
! Description
|- |-
|USB&nbsp;3.2 Gen&nbsp;2{{Abbr|×1|single-lane}}
| 1
|USB&nbsp;3.1&nbsp;Gen&nbsp;2
| V<sub>BUS</sub>
|USB 3.1
|style=background:red;color:white| Red
|]
| +5&nbsp;V
|10 <abbr>Gbit/s</abbr><br />symmetric
|SuperSpeed+ (SS+)
|rowspan="2" |USB 10Gbps
|rowspan="2" |]
|- |-
|USB&nbsp;3.2 Gen&nbsp;1{{Abbr|×2|two-lane}}
| 2
|rowspan="9" {{CNone}}
| D−
|rowspan="2" |USB&nbsp;3.2
|style=background:white| White
| rowspan="2" | 2 FDx (+ 1 HDx){{efn|name="HDx"}}
| Data−
|8b/10b
|-
| rowspan="2" | 10
| 3
|10 <abbr>Gbit/s</abbr><br />symmetric
| D+
|style=background:green;color:white| Green
| Data+
|-
| 4
| ID
| {{n/a}} | {{n/a}}
| Permits detection of which end of a cable is plugged in:<br/>• "A" connector (host): connected to the signal ground<br>• "B" connector (device): not connected
|- |-
|USB&nbsp;3.2 Gen&nbsp;2{{Abbr|×2|two-lane}}
| 5
|128b/132b
| GND
|20 <abbr>Gbit/s</abbr><br />symmetric
|style=background:black;color:white| Black
|SuperSpeed USB 20Gbps
| Signal ground
|USB 20Gbps
|}{{clear}}
|]

==== Proprietary connectors and formats ====
Manufacturers of personal electronic devices might not include a USB standard connector on their product for technical or marketing reasons.<ref>{{cite web
| url = http://anythingbutipod.com/2008/04/proprietary-cable-vs-standard-usb/
| title = Proprietary Cables vs Standard USB
| date = 2008-04-30 | accessdate = 2013-10-29
| publisher = anythingbutipod.com
}}</ref> Some manufacturers provide proprietary cables that permit their devices to physically connect to a USB standard port. Full functionality of proprietary ports and cables with USB standard ports is not assured; for example, some devices only use the USB connection for battery charging and do not implement any data transfer functions.<ref>{{cite web
| url = http://www.macworld.com/article/2028888/review-logitechs-ultrathin-mini-keyboard-cover-makes-the-wrong-tradeoffs.html
| title = Review: Logitech's Ultrathin mini keyboard cover makes the wrong tradeoffs
| date = 2013-02-25 | accessdate = 2013-10-29
| author = Lex Friedman | publisher = macworld.com
}}</ref>

<gallery perrow=5 widths=180px heights=140px>
Image:Htc extmicrousb port and mhl-hdmi plug.png|] ExtMicro USB port and connector
Image:Popport.jpg|] ] connector
Image:Lightning to USB Cable.jpg|A ]-to-USB Cable
</gallery>

=== Colors ===
{| class="wikitable floatright" style="margin-left: 1.5em;"
|- |-
|USB4 Gen&nbsp;2{{Abbr|×1|single-lane}}
! Color
|rowspan="4" |]
! Description
|1 FDx (+ 1 HDx){{efn|name="HDx"}}
| rowspan="2" |64b/66b{{efn|name="rs-fec"}}
|6 (used of 10)
|10 <abbr>Gbit/s</abbr><br />symmetric
| colspan=2| USB 10Gbps
|]
|- |-
|USB4 Gen&nbsp;2{{Abbr|×2|two-lane}}
| Black or white
|2 FDx (+ 1 HDx){{efn|name="HDx"}}
| ] or ]
|10
|20 <abbr>Gbit/s</abbr><br />symmetric
|colspan="2" rowspan="2" |USB 20Gbps
|rowspan="2" |]
|- |-
|USB4 Gen&nbsp;3{{Abbr|×1|single-lane}}
| Blue
|1 FDx (+ 1 HDx){{efn|name="HDx"}}
| ]
| rowspan="2" |128b/132b{{efn|name="rs-fec"}}
|6 (used of 10)
|20 <abbr>Gbit/s</abbr><br />symmetric
|- |-
|USB4 Gen&nbsp;3{{Abbr|×2|two-lane}}
| Yellow or red<br/>(ports only)
|2 FDx (+ 1 HDx){{efn|name="HDx"}}
| High current and/or ]
|} |10
|40 <abbr>Gbit/s</abbr><br />symmetric

| colspan=2 | USB 40Gbps
USB ports and connectors are often color-coded to distinguish their different functions and USB versions. These colors are not part of the USB specification and can vary between manufacturers;{{citation needed|date=September 2013}} for example, USB&nbsp;3.0 specification mandates appropriate color-coding while it only recommends blue inserts for Standard-A USB&nbsp;3.0 connectors and plugs.<ref>{{cite web
|]
| url = http://www.usb.org/developers/docs/documents_archive/usb_30_spec_070113.zip
| title = Universal Serial Bus Revision 3.0 Specification
| section = Sections 3.1.1.1 and 5.3.1.3
| accessdate = 2014-05-19
}}</ref>
{{Clear}}

== Cabling ==
]. The wires are enclosed in a further layer of shielding.]]

The data cables for USB&nbsp;1.x and USB&nbsp;2.x use a ] to reduce ] and ]. USB&nbsp;3.0 cables contain twice as many wires as USB&nbsp;2.x to support SuperSpeed data transmission, and are thus larger in diameter.<ref>{{cite web| url= http://hantat.com/News-Read-459-1.html |title=What is the USB 3.0 Cable Difference |publisher=Hantat |date=2009-05-18 |accessdate=2011-12-12}}</ref>

The USB&nbsp;1.1 Standard specifies that a standard cable can have a maximum length of 5&nbsp;meters with devices operating at Full Speed (12&nbsp;Mbit/s), and a maximum length of 3&nbsp;meters with devices operating at Low Speed (1.5&nbsp;Mbit/s).<ref>{{cite web|url=http://www.cablesplususa.com/pdf/USB_Cable_Length_Limitations.pdf |title=USB Cable Length Limitations |publisher=cablesplususa.com |date=2010-11-03 |accessdate=2014-02-02 |format=PDF}}</ref><ref>{{cite web|url=http://www.usb.org/developers/usbfaq#cab1 |title=Cables and Long-Haul Solutions |work=USB FAQ |publisher=USB.org |date= |accessdate=2014-02-02}}</ref>

USB&nbsp;2.0 provides for a maximum cable length of 5 meters for devices running at Hi Speed (480&nbsp;Mbit/s). The primary reason for this limit is the maximum allowed round-trip delay of about 1.5&nbsp;μs. If USB host commands are unanswered by the USB device within the allowed time, the host considers the command lost. When adding USB device response time, delays from the maximum number of hubs added to the delays from connecting cables, the maximum acceptable delay per cable amounts to 26&nbsp;ns.<ref name="faq">{{Cite journal | url = http://www.usb.org/developers/usbfaq/#cab1 | title = USB Frequently Asked Questions | publisher = USB Implementers Forum | accessdate = 2010-12-10}}</ref> The USB&nbsp;2.0 specification requires that cable delay be less than 5.2&nbsp;ns per meter {{nowrap|(192 000 km/s}}, which is close to the maximum achievable transmission speed for standard copper wire).

The USB&nbsp;3.0 standard does not directly specify a maximum cable length, requiring only that all cables meet an electrical specification: for copper cabling with ] 26 wires the maximum practical length is {{convert|3|m|ft|1|sp=us}}.<ref>{{cite web|title=USB 3.0 Developers FAQ |url= http://www.lvr.com/usb3faq.htm#ca_maximum |accessdate=2011-10-29 | last=Axelson | first = Jan}}</ref>

== {{Anchor|POWER}}Power ==
{| class="wikitable" style="margin:0 0 1em 1em; float:right;"
|+ USB power standards
|-
! Specification !! Current !! Voltage !! Power
|-
| rowspan=2 | USB&nbsp;1.0<br>USB&nbsp;2.0 || 150&nbsp;mA || 5&nbsp;V || 0.75&nbsp;W
|-
| 500&nbsp;mA{{ref label|usb-power-2.0|a}} || 5&nbsp;V || 2.5&nbsp;W
|-
| USB&nbsp;3.0 || 900&nbsp;mA{{ref label|usb-power-3.0|b}} || 5&nbsp;V || 4.5&nbsp;W
|-
| rowspan=3 | USB&nbsp;3.1 || 2&nbsp;A || 5&nbsp;V || 10&nbsp;W
|-
| 5&nbsp;A || 12&nbsp;V || 60&nbsp;W
|-
| 5&nbsp;A || 20&nbsp;V || 100&nbsp;W
|-
| USB Battery Charging || 0.5&ndash;1.5&nbsp;A || 5&nbsp;V || 2.5&ndash;7.5&nbsp;W
|-
| rowspan=4 | USB Power Delivery || 2&nbsp;A || 5&nbsp;V || 10&nbsp;W
|-
| 3&nbsp;A || 12&nbsp;V || 36&nbsp;W
|- |-
| 3&nbsp;A || 20&nbsp;V || 60&nbsp;W | rowspan="3" |USB4 Gen&nbsp;4{{Abbr|×2|two-lane}}
| rowspan="3" |USB4 2.0
|2 FDx (+ 1 HDx){{efn|name="HDx"}}
| rowspan="3" |] 11b/7]
| rowspan="3" |10
|80 Gbit/s<br />symmetric
| colspan=2 | USB 80Gbps
|]
|- |-
| rowspan="2" | asymmetric (+ 1 HDx){{efn|name="HDx"}}
| 5&nbsp;A || 20&nbsp;V || 100&nbsp;W
|40&nbsp;Gbit/s&nbsp;up<br />120&nbsp;Gbit/s down
| colspan="3" rowspan="2" {{N/A}}
|- |-
|120&nbsp;Gbit/s&nbsp;up<br />40&nbsp;Gbit/s down
| colspan="7" style="max-width: 60em; font-size: 90%;" | <ol type="a">
<li>{{note label|usb-power-2.0}} Up to five unit loads; in USB&nbsp;2.0, unit load is 100&nbsp;mA.
<li>{{note label|usb-power-3.0}} Up to six unit loads; in USB&nbsp;3.0, unit load is 150&nbsp;mA.
</ol>
|} |}


{{notelist|refs=
]
{{efn|name="rs-fec"| USB4 can use optional ] ] (RS FEC). In this mode, 12 × 16&nbsp;B (128&nbsp;bit) symbols are assembled together with 2&nbsp;B (12&nbsp;bit + 4&nbsp;bit reserved) synchronization bits indicating the respective symbol types and 4&nbsp;B of RS FEC to allow to correct up to 1&nbsp;B of errors anywhere in the total 198&nbsp;B block.}}


{{efn|name="HDx"|USB 2.0 implementation}}
The USB&nbsp;1.x and 2.0 specifications provide a 5&nbsp;V supply on a single wire to power connected USB devices. The specification provides for no more than 5.25&nbsp;V and no less than 4.75&nbsp;V (5&nbsp;V&nbsp;±&nbsp;5%) between the positive and negative bus power lines (V<sub>BUS</sub> voltage). For USB&nbsp;3.0, the voltage supplied by low-powered hub ports is 4.45&ndash;5.25&nbsp;V.<ref>{{cite web |title=7.3.2 Bus Timing/Electrical Characteristics |work=Universal Serial Bus Specification |url=http://www.usb.org/developers/docs/ |publisher=USB.org}}</ref>
}}


* '''Low-speed (LS)''' and '''Full-speed (FS)''' modes use a single data wire pair, labeled D+ and D−, in ]. Transmitted signal levels are {{nowrap|0.0–0.3 V}} for logical low, and {{nowrap|2.8–3.6 V}} for logical high level. The signal lines are not ].
A unit load is defined as 100&nbsp;mA in USB&nbsp;2.0, and 150&nbsp;mA in USB&nbsp;3.0. A device may draw a maximum of five unit loads (500&nbsp;mA) from a port in USB&nbsp;2.0, or six unit loads (900&nbsp;mA) in USB&nbsp;3.0. There are two types of device: low-power and high-power. A low-power device (such as a ]) draws at most one-unit load, with minimum operating voltage of 4.4&nbsp;V in USB&nbsp;2.0, and 4&nbsp;V in USB&nbsp;3.0. A high-power device draws, at most, the maximum number of unit loads the standard permits. Every device functions initially as low-power (including high-power functions during their low-power enumeration phases), but may request high-power, and get it if available on the providing bus.<ref name="usb.org">{{cite web |url=http://www.usb.org/developers/docs/ |title=USB.org |publisher=USB.org |date= |accessdate=2010-06-22}}</ref><ref>{{cite web
* '''High-speed (HS)''' uses the same wire pair, but with different electrical conventions. Lower signal voltages of {{nowrap|−10 to 10 mV}} for low and {{nowrap|360 to 440 mV}} for logical high level, and termination of 45&nbsp;Ω to ground or 90&nbsp;Ω differential to match the data cable impedance.
| url = http://www.usb.org/developers/docs/usb_20_070113.zip
* '''SuperSpeed (SS)''' adds two additional pairs of shielded twisted data wires (and new, mostly compatible expanded connectors) besides another grounding wire. These are dedicated to full-duplex SuperSpeed operation. The SuperSpeed link operates independently from the USB&nbsp;2.0 channel and takes precedence on connection. Link configuration is performed using LFPS (Low Frequency Periodic Signaling, approximately at 20&nbsp;MHz frequency), and electrical features include voltage de-emphasis at the transmitter side, and adaptive linear equalization on the receiver side to combat electrical losses in transmission lines, and thus the link introduces the concept of ''link training''.
| title = 7.2.1.3 Low-power Bus-powered Functions
* '''SuperSpeed+ (SS+)''' uses a new coding scheme with an increased signaling rate (Gen 2×1 mode) and/or the additional lane of USB-C (Gen 1×2 and Gen 2×2 modes).
| work = Universal Serial Bus Specification
| date = 2000-04-27 | accessdate = 2014-01-11
| publisher = USB.org
}}</ref><ref>{{cite web
| url = http://www.usb.org/developers/docs/usb_20_070113.zip
| title = 7.2.1.4 High-power Bus-powered Functions
| work = Universal Serial Bus Specification
| date = 2000-04-27 | accessdate = 2014-01-11
| publisher = USB.org
}}</ref>


A USB connection is always between an ''A'' end, either a ''host'' or a downstream port of a hub, and a ''B'' end, either a ''peripheral device'' or the upstream port of a hub. Historically this was made clear by the fact that hosts had only Type-A and peripheral devices had only Type-B ports, and every compatible cable had one Type-A plug and one Type-B plug. USB-C (Type-C) is a single connector that replaces all legacy Type-A and Type-B connectors, so when both sides are equipment with USB Type-C ports they negotiate which is the ''host'' and which is the ''device''.
Some devices, such as high-speed external disk drives, require more than 500&nbsp;mA of current<ref>{{cite web |url=http://www.xbitlabs.com/articles/storage/display/25inch-500-640-750gb-hdd-roundup_17.html#sect0 |title=Roundup: 2.5-inch Hard Disk Drives with 500 GB, 640 GB and 750 GB Storage Capacities (page 17) |publisher=xbitlabs.com |date=2010-06-16 |accessdate=2010-07-09}}</ref> and therefore may have power issues if powered from just one USB&nbsp;2.0 port: erratic function, failure to function, or overloading/damaging the port. Such devices may come with an external power source or a Y-shaped cable that has two USB connectors (one for power and data, the other for power only) to plug into a computer. With such a cable, a device can draw power from two USB ports simultaneously.<ref>{{cite web|url=http://www.hitachigst.com/hdd/support/SignatureMiniFAQ.html#a2 |title= I have the drive plugged in but I cannot find the drive in "My Computer", why? |publisher=hitachigst.com |accessdate=2012-03-30}}</ref> However, USB compliance specification states that "use of a 'Y' cable (a cable with two A-plugs) is prohibited on any USB peripheral", meaning that "if a USB peripheral requires more power than allowed by the USB specification to which it is designed, then it must be self-powered."<ref>{{cite web|url=http://compliance.usb.org/index.asp?UpdateFile=Policies#72 |title=USB-IF Compliance Updates |publisher=Compliance.usb.org |date=2011-09-01 |accessdate=2014-01-22}}</ref>


== Protocol layer ==
A bus-powered hub initializes itself at one-unit load and transitions to maximum unit loads after it completes hub configuration. Any device connected to the hub draws one-unit load regardless of the current draw of devices connected to other ports of the hub (i.e., one device connected on a four-port hub draws only one-unit load despite the fact that more unit loads are being supplied to the hub).<ref name="usb.org"/>{{full|date=March 2014}}
{{Main|USB (Communications)#Protocol layer}}
During USB communication, data is transmitted as ]. Initially, all packets are sent from the host via the root hub, and possibly more hubs, to devices. Some of those packets direct a device to send some packets in reply.


== Transactions ==
A self-powered hub supplies maximum supported unit loads to any device connected to it. In addition, the ''V<sub>BUS</sub>'' presents one-unit load upstream for communication if parts of the Hub are powered down.{{Clarify|date=July 2012}}<ref name="usb.org"/>{{full|date=March 2014}}
{{Main|USB (Communications)#Transaction}}


The basic transactions of USB are:
=== {{Anchor|ACA|BCS|62680-3}}Charging ports ===
* OUT transaction
]
* IN transaction
* SETUP transaction
* Control transfer exchange


== Related standards <span class="anchor" id="HSIC"></span><span class="anchor" id="SSIC"></span>==
The ''USB Battery Charging Specification Revision 1.1'' (released in 2007) defines a new type of USB port, called the ''charging port''. Contrary to the ''standard downstream port'', for which current draw by a connected portable device can exceed 100&nbsp;mA only after digital negotiation with the host or hub, a charging port can supply currents between 500&nbsp;mA and 1.5&nbsp;A without the digital negotiation. A charging port supplies up to 500&nbsp;mA at 5&nbsp;V, up to the rated current at 3.6&nbsp;V or more, and drop its output voltage if the portable device attempts to draw more than the rated current. The charger port may shut down if the load is too high.<ref name="battchargespec">{{cite web|url=http://www.usb.org/developers/devclass_docs/batt_charging_1_1.zip|title= Battery Charging Specification, Revision 1.1|date= 15 April 2009|accessdate= 23 September 2009|publisher= USB Implementers Forum}}</ref>
]


=== Media Agnostic USB ===
Two types of charging port exist: the ''charging downstream port'' (CDP), supporting data transfers as well, and the ''dedicated charging port'' (DCP), without data support. A portable device can recognize the type of USB port; on a dedicated charging port, the D+ and D− pins are ] with a resistance not exceeding 200&nbsp;]s, while charging downstream ports provide additional detection logic so their presence can be determined by attached devices.<ref name="battchargespec" />
The USB Implementers Forum introduced the Media Agnostic USB (MA-USB) v.1.0 wireless communication standard based on the USB protocol on 29 July 2015. ] is a cable-replacement technology, and uses ] ] for data rates of up to 480&nbsp;Mbit/s.<ref>{{cite web |url=https://www.usb.org/document-library/media-agnostic-usb-v10a-spec-and-adopters-agreement |title=Media Agnostic USB v1.0a Spec and Adopters Agreement |website=usb.org |access-date=21 July 2021 |archive-date=31 July 2021 |archive-url=https://web.archive.org/web/20210731194632/https://www.usb.org/document-library/media-agnostic-usb-v10a-spec-and-adopters-agreement |url-status=live }}</ref>


The USB-IF used WiGig Serial Extension v1.2 specification as its initial foundation for the MA-USB specification and is compliant with SuperSpeed USB (3.0 and 3.1) and Hi-Speed USB (USB 2.0). Devices that use MA-USB will be branded as "Powered by MA-USB", provided the product qualifies its certification program.<ref>{{cite web |url=https://www.tweaktown.com/news/36420/usb-if-releases-final-specification-of-media-agnostic-usb/index.html |title=USB-IF releases final specification of Media Agnostic USB |work=tweaktown.com |last=Shaikh |first=Roshan Ashraf |date=3 November 2020 |access-date=21 July 2021 |archive-date=15 March 2021 |archive-url=https://web.archive.org/web/20210315204103/https://www.tweaktown.com/news/36420/usb-if-releases-final-specification-of-media-agnostic-usb/index.html |url-status=live }}</ref>
With charging downstream ports, current passing through the thin ground wire may interfere with high-speed data signals; therefore, current draw may not exceed 900&nbsp;mA during high-speed data transfer. A dedicated charge port may have a rated current between 500 and 1,500&nbsp;mA. For all charging ports, there is maximum current of 5&nbsp;A, as long as the connector can handle the current (standard USB&nbsp;2.0 A-connectors are rated at 1.5&nbsp;A).<ref name="battchargespec" />


=== InterChip USB ===
Before the battery charging specification was defined, there was no standardized way for the portable device to inquire how much current was available. For example, Apple's ] and ] chargers indicate the available current by voltages on the D− and D+ lines. When D+ = D− = 2.0&nbsp;V, the device may pull up to 500&nbsp;mA. When D+ = 2.0&nbsp;V and D− = 2.8&nbsp;V, the device may pull up to 1&nbsp;A of current.<ref>{{Citation | url = http://www.ladyada.net/make/mintyboost/icharge.html | title = Minty Boost | contribution = The mysteries of Apple device charging | publisher = Lady Ada | year = 2010, 2011}}</ref> When D+ = 2.8&nbsp;V and D− = 2.0&nbsp;V, the device may pull up to 2&nbsp;A of current.<ref>{{Citation | url = http://www.instructables.com/id/Modify-a-cheap-USB-charger-to-feed-an-iPod-iPhone/ | title = Instructables | year = 2013 | title = Modify a cheap USB charger to feed an iPod, iPhone}}</ref>
{{Main|InterChip USB}}
InterChip USB is a chip-to-chip variant that eliminates the conventional transceivers found in normal USB. The HSIC ] uses about 50% less power and 75% less ] area compared to USB&nbsp;2.0.<ref>{{cite web |title= Interchip Connectivity: HSIC, UniPro, HSI, C2C, LLI... oh my! |url= http://info.arteris.com/blog/bid/59433/Interchip-Connectivity-HSIC-UniPro-HSI-C2C-LLI-oh-my |first=Kurt |last=Shuler |date=31 March 2011 |access-date= 24 June 2011 |website= Arteris IP |url-status=live |archive-url= https://web.archive.org/web/20110619022557/http://info.arteris.com/blog/bid/59433/Interchip-Connectivity-HSIC-UniPro-HSI-C2C-LLI-oh-my |archive-date= 19 June 2011}}</ref> It is an alternative standard to ] and ].


=== USB-C ===
Dedicated charging ports can be found on USB power adapters that convert utility power or another power source (e.g., a car's electrical system) to run attached devices and battery packs. On a host (such as a laptop computer) with both standard and charging USB ports, the charging ports should be labeled as such.<ref name="battchargespec" />
{{Main|USB-C}}
USB-C (officially ''USB Type-C'') is a standard that defines a new connector, and several new connection features. Among them it supports ''Alternate Mode'', which allows transporting other protocols via the USB-C connector and cable. This is commonly used to support the ] or ] protocols, which allows connecting a display, such as a ] or ], via USB-C.


All other connectors are not capable of two-lane operations (Gen 1×2 and Gen 2×2) in USB 3.2, but can be used for one-lane operations (Gen&nbsp;1×1 and Gen&nbsp;2×1).<ref name="Black Box">{{cite web |title=USB 3.2 and Beyond |url=https://www.blackbox.co.uk/gb-gb/page/29254/Resources/Technical-Resources/Black-Box-Explains/USB(Universal-Serial-Bus)/USB-Connectivity-USB-32-and-Beyond |website=Black Box |access-date=4 March 2023}}</ref>
To support simultaneous charge and data communication, even if the communication port does not support charging a demanding device, so-called ''accessory charging adapters (ACA)'' are introduced. By using an accessory charging adapter, a device providing a single USB port can be attached to both a charger, and another USB device at the same time.<ref name="battchargespec" />


=== DisplayLink ===
The ''USB Battery Charging Specification Revision 1.2'' (released in 2010) makes clear that there are safety limits to the rated current at 5&nbsp;A coming from USB&nbsp;2.0. On the other hand, several changes are made and limits are increasing including allowing 1.5&nbsp;A on charging downstream ports for unconfigured devices, allowing high speed communication while having a current up to 1.5&nbsp;A, and allowing a maximum current of 5&nbsp;A. Also, revision 1.2 removes support for USB ports type detection via resistive detection mechanisms.<ref name="battchargespec1.2" />
{{Main|DisplayLink}}
DisplayLink is a technology which allows multiple displays to be connected to a computer via USB. It was introduced around 2006, and before the advent of Alternate Mode over USB-C it was the only way to connect displays via USB. It is a proprietary technology, not standardized by the USB Implementers Forum and typically requires a separate ] on the computer.


== Comparisons with other connection methods ==
=== Sleep-and-charge ports ===
=== FireWire (IEEE 1394) ===
]
At first, USB was considered a complement to FireWire (]) technology, which was designed as a high-bandwidth serial bus that efficiently interconnects peripherals such as disk drives, audio interfaces, and video equipment. In the initial design, USB operated at a far lower data rate and used less sophisticated hardware. It was suitable for small peripherals such as keyboards and pointing devices.


The most significant technical differences between FireWire and USB include:
Sleep-and-charge USB ports can be used to charge electronic devices even when the computer is switched off. Normally, when a computer is powered off, the USB ports are powered down. This prevents phones and other devices from being able to charge unless the computer is powered on. Sleep-and-charge USB ports remain powered even when the computer is off. On laptops, charging devices from the USB port when it is not being powered from AC drains the laptop battery faster; most laptops have a facility to stop charging if their own battery charge level gets too low.<ref>{{cite web|url= http://www.mytoshiba.com.au/file/download/resource/file/12585/NB200-UserGuide.pdf |title= Toshiba NB200 User Manual | location = UK |date=2009-03-01 |accessdate= 2014-01-26 }}</ref> Desktop machines need to remain plugged into AC power for Sleep-and-charge to work.<ref>{{cite web|url= http://www.usb-core.co.uk/03-04-2008-toshiba-announces-sleep-and-charge-usb-ports.html |title= Toshiba announces sleep‐and‐charge USB ports |publisher=USB core | location = UK |date=2008-04-03 |accessdate= 2010-06-22 | archiveurl = http://web.archive.org/web/20080614031146/http://www.usb-core.co.uk/03-04-2008-toshiba-announces-sleep-and-charge-usb-ports.html |archivedate = June 14, 2008}}</ref>


* USB networks use a ] topology, while IEEE&nbsp;1394 networks use a ] topology.
These ports are found colored differently (mostly red or yellow). On Dell laptops, the port is marked with the standard USB symbol with an added lightning bolt icon on the right side. Dell calls this feature "PowerShare."<ref>{{cite web
* USB&nbsp;1.0, 1.1, and 2.0 use a "speak-when-spoken-to" protocol, meaning that each peripheral communicates with the host when the host specifically requests communication. USB&nbsp;3.0 allows for device-initiated communications towards the host. A FireWire device can communicate with any other node at any time, subject to network conditions.
| url = http://www.dell.com/support/troubleshooting/an/en/andhs1/KCS/KcsArticles/ArticleView?c=an&l=en&s=dhs&docid=608993
* A USB network relies on a single host at the top of the tree to control the network. All communications are between the host and one peripheral. In a FireWire network, any capable node can control the network.
| title = USB PowerShare Feature
* USB runs with a 5&nbsp;] power line, while FireWire supplies 12&nbsp;V and theoretically can supply up to 30&nbsp;V.
| date = 2013-06-05 | accessdate = 2013-12-04
* Standard USB hub ports can provide from the typical 500&nbsp;mA/2.5&nbsp;W of current, only 100&nbsp;mA from non-hub ports. USB&nbsp;3.0 and USB On-The-Go supply 1.8&nbsp;A/9.0&nbsp;W (for dedicated battery charging, 1.5&nbsp;A/7.5&nbsp;W full bandwidth or 900&nbsp;mA/4.5&nbsp;W high bandwidth), while FireWire can in theory supply up to 60&nbsp;watts of power, although 10 to 20&nbsp;watts is more typical.
| publisher = dell.com
}}</ref>


These and other differences reflect the differing design goals of the two buses: USB was designed for simplicity and low cost, while FireWire was designed for high performance, particularly in time-sensitive applications such as audio and video. Although similar in theoretical maximum signaling rate, FireWire&nbsp;400 is faster than USB&nbsp;2.0 high-bandwidth in real-use,<ref>{{cite web|title=FireWire vs. USB 2.0|url=http://www.qimaging.com/support/pdfs/firewire_usb_technote.pdf|publisher=QImaging|access-date=20 July 2010|url-status=live|archive-url=https://web.archive.org/web/20101011050049/http://www.qimaging.com/support/pdfs/firewire_usb_technote.pdf|archive-date=11 October 2010}}</ref> especially in high-bandwidth use such as external hard drives.<ref>{{cite web |url= http://www.cwol.com/firewire/firewire-vs-usb.htm |title= FireWire vs. USB&nbsp;2.0 – Bandwidth Tests |access-date= 25 August 2007 |url-status=live |archive-url= https://web.archive.org/web/20070812045719/http://www.cwol.com/firewire/firewire-vs-usb.htm |archive-date= 12 August 2007}}</ref><ref>{{cite web |url=https://www.pricenfees.com/digit-life-archives/usb-2-0-vs-firewire |title=USB&nbsp;2.0 vs FireWire |publisher=Pricenfees |access-date=25 August 2007 |url-status=live |archive-url=https://web.archive.org/web/20161016063120/https://www.pricenfees.com/digit-life-archives/usb-2-0-vs-firewire |archive-date=16 October 2016}}</ref><ref>{{cite magazine |url=https://www.pcmag.com/article2/0,4149,847716,00.asp |title=The Great Interface-Off: FireWire Vs. USB&nbsp;2.0 |magazine=PC Magazine |access-date=25 August 2007 |last=Metz |first=Cade |date=25 February 2003 |url-status=live |archive-url=https://web.archive.org/web/20070930190355/http://www.pcmag.com/article2/0,4149,847716,00.asp |archive-date=30 September 2007 }}</ref><ref>{{cite web|url=http://www.g4tv.com/techtvvault/features/39129/USB_20_Versus_FireWire_pg3.html|title=USB&nbsp;2.0 Versus FireWire|access-date=25 August 2007|author=Heron, Robert|publisher=TechTV|url-status=live|archive-url=https://web.archive.org/web/20070929121843/http://www.g4tv.com/techtvvault/features/39129/USB_20_Versus_FireWire_pg3.html|archive-date=29 September 2007}}</ref> The newer FireWire&nbsp;800 standard is twice as fast as FireWire&nbsp;400 and faster than USB&nbsp;2.0 high-bandwidth both theoretically and practically.<ref>{{cite web | url = http://www.usb-ware.com/firewire-vs-usb.htm | title = FireWire vs. USB&nbsp;2.0 | publisher = USB Ware | access-date = 19 March 2007 | url-status=live | archive-url = https://web.archive.org/web/20070316072513/http://www.usb-ware.com/firewire-vs-usb.htm | archive-date = 16 March 2007}}</ref> However, FireWire's speed advantages rely on low-level techniques such as ] (DMA), which in turn have created opportunities for security exploits such as the ].
On ] and ] laptops, sleep-and-charge USB ports are marked with a non-standard symbol (the letters "USB" over a drawing of a battery); the feature is simply called "Power-off USB".<ref>{{cite web
| url = http://packard-bell-scandic.custhelp.com/app/answers/detail/a_id/29967/~/usb-charge-manager
| title = USB Charge Manager
| accessdate = 2014-04-25
| publisher = packardbell.com
}}</ref>


The chipset and drivers used to implement USB and FireWire have a crucial impact on how much of the bandwidth prescribed by the specification is achieved in the real world, along with compatibility with peripherals.<ref>{{cite web |url=http://www.anandtech.com/mb/showdoc.aspx?i=2602&p=15 |title=Firewire and USB Performance |access-date=1 February 2008 |last=Key |first=Gary |date=15 November 2005 |url-status=live |archive-url=https://web.archive.org/web/20080423214619/http://www.anandtech.com/mb/showdoc.aspx?i=2602&p=15 |archive-date=23 April 2008}}</ref>
=== Mobile device charger standards ===


====In China==== === Ethernet ===
The ''IEEE 802.3af'', ''802.3at'', and ''802.3bt'' ] (PoE) standards specify more elaborate power negotiation schemes than powered USB. They operate at 48&nbsp;V&nbsp;] and can supply more power (up to 12.95&nbsp;W for ''802.3af'', 25.5&nbsp;W for ''802.3at'', a.k.a. ''PoE+'', 71&nbsp;W for ''802.3bt'', a.k.a. ''4PPoE'') over a cable up to 100&nbsp;meters compared to USB&nbsp;2.0, which provides 2.5&nbsp;W with a maximum cable length of 5&nbsp;meters. This has made PoE popular for ] telephones, ]s, ]s, and other networked devices within buildings. However, USB is cheaper than PoE provided that the distance is short and power demand is low.
]s]]
{{As of |2007|6|14}}, all new ]s applying for a license in ] are required to use a USB port as a power port for battery charging.<ref>{{Cite news|url = http://www.eetimes.com/rss/showArticle.jhtml?articleID=199800238&cid=RSSfeed_eetimes_newsRSS | title = China to enforce universal cell phone charger | publisher = EE Times |author=Cai Yan |date=2007-05-31 |accessdate=2007-08-25}}</ref><ref>The Chinese FCC's technical standard: {{Cite journal | url = http://www.dianyuan.com/bbs/u/63/2015571206841181.pdf | title = YD/T 1591-2006, Technical Requirements and Test Method of Charger and Interface for Mobile Telecommunication Terminal Equipment | language = Chinese | publisher = Dian yuan | format = PDF}}</ref> This was the first standard to use the convention of shorting D+ and D-.<ref>{{cite web| first1 =Crystal Yan Yan | last1 = Lam | first2 = Harry Yao Hui | last2 = Liu |url=http://www.eetimes.com/document.asp?doc_id=1275077&print=yes | title= How to conform to China's new mobile phone interface standards | publisher= Wireless Net DesignLine |date = | accessdate = 2010-06-22}}</ref>


] standards require electrical isolation between the networked device (computer, phone, etc.) and the network cable up to 1500&nbsp;V&nbsp;AC or 2250&nbsp;V&nbsp;DC for 60&nbsp;seconds.<ref>{{cite web | url = http://standards.ieee.org/getieee802/download/802.3-2008_section1.pdf | title = 802.3, Section 14.3.1.1 | publisher = IEEE | url-status=dead | archive-url = https://web.archive.org/web/20101206030247/http://standards.ieee.org/getieee802/download/802.3-2008_section1.pdf | archive-date = 6 December 2010}}</ref> USB has no such requirement as it was designed for peripherals closely associated with a host computer, and in fact it connects the peripheral and host grounds. This gives Ethernet a significant safety advantage over USB with peripherals such as cable and DSL modems connected to external wiring that can assume hazardous voltages under certain fault conditions.<ref>{{cite web|date=8 March 2010|title=Powerbook Explodes After Comcast Plugs in Wrong Cable|url=http://consumerist.com/2006/12/powerbook-explodes-after-comcast-plugs-in-wrong-cable.html|url-status=dead|archive-url=https://web.archive.org/web/20100625052120/http://consumerist.com/2006/12/powerbook-explodes-after-comcast-plugs-in-wrong-cable.html|archive-date=25 June 2010|access-date=22 June 2010|publisher=Consumerist}}</ref><ref>{{Cite web|date=2021|title=Technical Note. Galvanic Isolation|url=https://www.isystem.com/files/content/downloads/documents/technical-notes/iSYSTEM_TN_Galvanic_Isolation.pdf#page=4|website=iSYSTEM|format=PDF|access-date=13 February 2022|archive-date=21 December 2021|archive-url=https://web.archive.org/web/20211221080208/https://www.isystem.com/files/content/downloads/documents/technical-notes/iSYSTEM_TN_Galvanic_Isolation.pdf#page=4|url-status=live}}</ref>
====OMTP/GSMA Universal Charging Solution====
In September 2007, the ] group (a forum of mobile network operators and manufacturers such as ], ], ], ] and ]) announced that its members had agreed on micro-USB as the future common connector for mobile devices.<ref>{{cite news | url=http://www.news.com/2100-1041_3-6209247.html |title=Pros seem to outdo cons in new phone charger standard |publisher= News |date=September 20, 2007 | accessdate = 2007-11-26}}</ref><ref>{{cite press release| url= http://www.omtp.org/News/Display.aspx?Id=4ec69ecb-0978-4df6-b045-34557aabbcbd | title = Broad Manufacturer Agreement Gives Universal Phone Cable Green Light |publisher=OTMP |date=September 17, 2007 |accessdate=2007-11-26}}</ref>


=== MIDI ===
The ] (GSMA) followed suit on 17 February 2009,<ref name="gsm20090217">{{Cite press release| url= http://www.gsmworld.com/newsroom/press-releases/2009/2548.htm | publisher = GSM World | title = Agreement on Mobile phone Standard Charger}}</ref><ref name="gsm20090217"/><ref>{{cite web| url= http://www.omtp.org/Publications/Display.aspx?Id=4dda105f-8472-4c12-ba04-75dd3c1d4ca6 | title=Common Charging and Local Data Connectivity | publisher=] | date= 11 February 2009 | accessdate= 2009-02-11}}{{Dead link|date=March 2011}}</ref><ref>{{cite web|url= http://www.gsmworld.com/our-work/mobile_planet/universal_charging_solution.htm |title=Universal Charging Solution ~ GSM World |publisher=GSM world |date= |accessdate= 2010-06-22}}</ref><ref>{{cite web|url= http://www.planetanalog.com/article/printableArticle.jhtml?articleID=218501515 |title= Meeting the challenge of the universal charge standard in mobile phones |publisher=Planet Analog |date= |accessdate=2010-06-22}}</ref> and on 22 April 2009, this was further endorsed by the ],<ref>{{cite press release| url = http://www.ctia.org/media/press/body.cfm/prid/1817 | title = The Wireless Association Announces One Universal Charger Solution to Celebrate Earth Day | publisher= CTIA | date= 2009-04-22 | accessdate = 2010-06-22}}</ref> with the ] (ITU) announcing on 22 October 2009 that it had also embraced the Universal Charging Solution as its "energy-efficient one-charger-fits-all new mobile phone solution", and added: "Based on the Micro-USB interface, UCS chargers will also include a 4-star or higher efficiency rating—up to three times more energy-efficient than an unrated charger."<ref>{{cite press release |url= http://www.itu.int/newsroom/press_releases/2009/49.html | title = ITU |date=2009-10-22 |accessdate=2010-06-22}}</ref>
The ''USB Device Class Definition for MIDI Devices'' transmits Music Instrument Digital Interface (]) music data over USB.<ref>{{cite web |url=https://www.usb.org/sites/default/files/midi10.pdf |title=Universal Serial Bus Device Class Definition for MIDI Devices |website=usb.org |date=1 November 1999 |access-date=21 July 2021 |archive-date=2 November 2021 |archive-url=https://web.archive.org/web/20211102080622/https://www.usb.org/sites/default/files/midi10.pdf |url-status=live}}</ref> The MIDI capability is extended to allow up to sixteen simultaneous ''virtual MIDI cables'', each of which can carry the usual MIDI sixteen channels and clocks.


USB is competitive for low-cost and physically adjacent devices. However, Power over Ethernet and the ] plug standard have an advantage in high-end devices that may have long cables. USB can cause ] problems between equipment, because it connects ground references on both transceivers. By contrast, the MIDI plug standard and ] have built-in isolation to {{gaps|500|V}} or more.
====EU Smartphone Power Supply Standard====
{{Main|Common External Power Supply}}


=== eSATA/eSATAp ===
In June 2009, many of the world's largest mobile phone manufacturers signed an ]-sponsored Memorandum of Understanding (MoU), agreeing to make most data-enabled mobile phones marketed in the ] compatible with a ] (EPS). The EU's common EPS specification (EN 62684:2010) references the USB Battery Charging standard and is similar to the GSMA/OMTP and Chinese charging solutions.<ref>{{cite web | url = http://ec.europa.eu/enterprise/rtte/chargers.htm |title= chargers | location = EU |publisher=EC |date= 2009-06-29 |accessdate= 2010-06-22}}</ref><ref>{{cite news | url= http://www.wired.com/gadgetlab/2009/06/europe-gets-universal-cellphone-charger-in-2010/ |title= Europe gets universal cellphone charger in 2010 |publisher=Wired |date=2009-06-13 |accessdate=2010-06-22}}</ref> In January 2011, the ] released its version of the (EU's) common EPS standard as IEC 62684:2011.<ref>{{cite web|url=http://www.iec.ch/newslog/2011/nr0311.htm |title=One size-fits-all mobile phone charger: IEC publishes first globally relevant standard |publisher=International Electrotechnical Commission |date=2011-02-01 |accessdate=2012-02-20}}</ref>
The ] connector is a more robust ] connector, intended for connection to external hard drives and SSDs. eSATA's transfer rate (up to 6&nbsp;Gbit/s) is similar to that of USB&nbsp;3.0 (up to 5&nbsp;Gbit/s) and USB&nbsp;3.1 (up to 10&nbsp;Gbit/s). A device connected by eSATA appears as an ordinary SATA device, giving both full performance and full compatibility associated with internal drives.


eSATA does not supply power to external devices. This is an increasing disadvantage compared to USB. Even though USB&nbsp;3.0's 4.5&nbsp;W is sometimes insufficient to power external hard drives, technology is advancing, and external drives gradually need less power, diminishing the eSATA advantage. ] (power over eSATA, a.k.a. ESATA/USB) is a connector introduced in 2009 that supplies power to attached devices using a new, backward compatible, connector. On a notebook eSATAp usually supplies only 5&nbsp;V to power a 2.5-inch HDD/SSD; on a desktop workstation it can additionally supply 12&nbsp;V to power larger devices including 3.5-inch HDD/SSD and 5.25-inch optical drives.
=== Non-standard devices ===
{{Unreferenced section|date=October 2011}}
]]]
] novelty device]]


eSATAp support can be added to a desktop machine in the form of a bracket connecting the motherboard SATA, power, and USB resources.
Some USB devices require more power than is permitted by the specifications for a single port. This is common for external hard and ]s, and generally for devices with ]s or ]. Such devices can use an ], which is allowed by the standard, or use a dual-input USB cable, one input of which is used for power and data transfer, the other solely for power, which makes the device a non-standard USB device. Some USB ports and external hubs can, in practice, supply more power to USB devices than required by the specification but a standard-compliant device may not depend on this.


eSATA, like USB, supports ], although this might be limited by OS drivers and device firmware.
In addition to limiting the total average power used by the device, the USB specification limits the ] (i.e., that used to charge decoupling and ]s) when the device is first connected. Otherwise, connecting a device could cause problems with the host's internal power. USB devices are also required to automatically enter ultra low-power suspend mode when the USB host is suspended. Nevertheless, many USB host interfaces do not cut off the power supply to USB devices when they are suspended.{{Citation needed|date=December 2011}}


=== Thunderbolt ===
Some non-standard USB devices use the 5&nbsp;V power supply without participating in a proper USB network, which negotiates power draw with the host interface. These are usually called '']s''.{{Citation needed|date=March 2013}} Examples include USB-powered keyboard lights, fans, mug coolers and heaters, battery chargers, miniature ]s, and even miniature ]s. In most cases, these items contain no digital circuitry, and thus are not standard compliant USB devices. This may cause problems with some computers, such as drawing too much current and damaging circuitry. Prior to the Battery Charging Specification, the USB specification required that devices connect in a low-power mode (100&nbsp;mA maximum) and communicate their current requirements to the host, which then permits the device to switch into high-power mode.
{{Main|Thunderbolt (interface)}}
Thunderbolt combines ] and ] into a new serial data interface. Original Thunderbolt implementations have two channels, each with a transfer speed of 10&nbsp;Gbit/s, resulting in an aggregate unidirectional bandwidth of 20&nbsp;Gbit/s.<ref>{{cite web |url=https://thunderbolttechnology.net/tech/how-it-works |title=How Thunderbolt Technology Works: Thunderbolt Technology Community |website=ThunderboltTechnology.net |access-date=22 January 2014 |url-status=live |archive-url=https://web.archive.org/web/20140210063142/https://thunderbolttechnology.net/tech/how-it-works |archive-date=10 February 2014 }}</ref>


] uses link aggregation to combine the two 10&nbsp;Gbit/s channels into one bidirectional 20&nbsp;Gbit/s channel.<ref>{{cite web |title=What you need to know about Thunderbolt 2 |url=https://www.macworld.com/article/222636/what-you-need-to-know-about-thunderbolt-2.html#:~:text=What%20is%20Thunderbolt%202%3F,20%20Gbps%20bi%2Ddirectional%20channel. |first=Jim |last=Galbraith |date=2 January 2014 |access-date=18 June 2021 |website=Macworld |publisher=IDG Communications, Inc. |archive-date=24 June 2021 |archive-url=https://web.archive.org/web/20210624202741/https://www.macworld.com/article/222636/what-you-need-to-know-about-thunderbolt-2.html#:~:text=What%20is%20Thunderbolt%202%3F,20%20Gbps%20bi%2Ddirectional%20channel. |url-status=live }}</ref>
Some devices, when plugged into charging ports, draw even more power (10 watts or 2.1 Amps) than the Battery Charging Specification allows. The ] and MiFi 2200 are two such devices.<ref>{{cite web|url=http://www.macobserver.com/tmo/article/watt_to_know_about_iphone_ipad_power_adapters/ |title=Watt to Know About iPhone & iPad Power Adapters &#124; Analysis |publisher=The Mac Observer |date= |accessdate=2011-12-12}}</ref>
Barnes & Noble NOOK Color devices also require a special charger that runs at 1.9 Amps.<ref>{{cite web|url=http://bookclubs.barnesandnoble.com/t5/NOOK-Color-General-Discussion/Nook-Color-charger-uses-special-micro-USB-connector/td-p/1093812 |title=Nook Color charger uses special micro-USB connector |publisher=barnesandnoble.com |date=2011-07-03}}</ref>


] and ] use ].<ref>{{cite web|url=https://www.cnet.com/news/thunderbolt-3-and-usb-type-c-join-forces-for-one-port-to-rule-them-all/|title=One port to rule them all: Thunderbolt&nbsp;3 and USB Type-C join forces|archive-url=https://web.archive.org/web/20150602195337/http://www.cnet.com/news/thunderbolt-3-and-usb-type-c-join-forces-for-one-port-to-rule-them-all/|archive-date=2 June 2015|url-status=live|access-date=2 June 2015}}</ref><ref>{{cite web |url=https://www.engadget.com/2015/06/02/thunderbolt-3-usb-c/ |title=Thunderbolt&nbsp;3 is twice as fast and uses reversible USB-C |date=2 June 2015 |access-date=2 June 2015 |url-status=live |archive-url=https://web.archive.org/web/20150603000428/http://www.engadget.com/2015/06/02/thunderbolt-3-usb-c/ |archive-date=3 June 2015 }}</ref><ref>{{cite web |url=https://arstechnica.com/gadgets/2015/06/thunderbolt-3-embraces-usb-type-c-connector-doubles-bandwidth-to-40gbps/ |title=Thunderbolt&nbsp;3 embraces USB Type-C connector, doubles bandwidth to 40&nbsp;Gbps |author=Sebastian Anthony |date=2 June 2015|website=Ars Technica |access-date=2 June 2015 |url-status=live |archive-url=https://web.archive.org/web/20150609183247/https://arstechnica.com/gadgets/2015/06/thunderbolt-3-embraces-usb-type-c-connector-doubles-bandwidth-to-40gbps/ |archive-date=9 June 2015 }}</ref> Thunderbolt&nbsp;3 has two physical 20&nbsp;Gbit/s bi-directional channels, aggregated to appear as a single logical 40&nbsp;Gbit/s bi-directional channel. Thunderbolt 3 controllers can incorporate a USB&nbsp;3.1 Gen&nbsp;2 controller to provide compatibility with USB devices. They are also capable of providing DisplayPort Alternate Mode as well as DisplayPort over USB4 Fabric, making the function of a Thunderbolt&nbsp;3 port a superset of that of a USB&nbsp;3.1 Gen&nbsp;2 port.
=== {{Anchor|PD|PD-R1.0|PD-R1.0V1.1|PD-R1.0V1.2}}USB Power Delivery ===
In July 2012, the USB Promoters Group announced the finalization of the USB Power Delivery ("PD") specification, an extension that specifies using certified "PD aware" USB cables with standard USB type A and B connectors to deliver up to 100&nbsp;W of power at 20&nbsp;V. For a PD-aware cable with a Micro-USB connector the maximum power supported is up to 60&nbsp;W at 20&nbsp;V, 36&nbsp;W at 12&nbsp;V and 10&nbsp;W at 5&nbsp;V. In all cases, both host-to-device and device-to-host configurations are supported.<ref>{{cite web|url=http://www.usb.org/press/USB_Power_Delivery_Spec_Completion_FINAL_072712.pdf |title=USB 3.0 Promoter Group Announces Availability of USB Power Delivery Specification |date=2012-07-18 |accessdate=2013-01-16}}</ref>


DisplayPort Alternate Mode 2.0: USB4 (requiring USB-C) requires that hubs support DisplayPort 2.0 over a USB-C Alternate Mode. DisplayPort 2.0 can support 8K resolution at 60&nbsp;Hz with HDR10 color.<ref name="displayport">{{cite web |title=New DisplayPort spec enables 16K video over USB-C |url=https://www.theverge.com/2020/4/30/21242445/vesa-displayport-alt-mode-2-0-usb-4-4k-144hz-hdr-8k-16k-displays |first=Jon |last=Porter |date=30 April 2020 |access-date=18 June 2021 |website=The Verge |publisher=Vox Media, LLC |archive-date=15 April 2021 |archive-url=https://web.archive.org/web/20210415051447/https://www.theverge.com/2020/4/30/21242445/vesa-displayport-alt-mode-2-0-usb-4-4k-144hz-hdr-8k-16k-displays |url-status=live }}</ref> DisplayPort 2.0 can use up to 80&nbsp;Gbit/s, which is double the amount available to USB data, because it sends all the data in one direction (to the monitor) and can thus use all eight data wires at once.<ref name="displayport"/>
The intent is to permit uniformly charging laptops, tablets, USB-powered disks and similarly higher power consumer electronics, as a natural extension of existing European and Chinese mobile telephone charging standards. This may also affect the way electric power used for small devices is transmitted and used in both residential and public buildings.<ref name="usb-power-delivery" /><ref>{{cite web|url=http://www.usb.org/developers/powerdelivery/PD_1.0_Introduction.pdf |title=USB Power Delivery — Introduction |date=2012-07-16 |accessdate=2013-01-06}}</ref>


After the specification was made royalty-free and custodianship of the Thunderbolt protocol was transferred from Intel to the USB Implementers Forum, Thunderbolt 3 has been effectively implemented in the USB4 specification – with compatibility with Thunderbolt 3 optional but encouraged for USB4 products.<ref>{{cite web|title=USB4 Thunderbolt3 Compatibility Requirements Specification|url=https://www.usb.org/sites/default/files/USB4%E2%84%A2%20Thunderbolt3%E2%84%A2%20Compatibility%20Requirements%20Specification%20Rev%201.0%20-%2020210129_0.pdf|date=January 2021|access-date=1 January 2021|website=USB|publisher=USB.org|archive-date=19 October 2021|archive-url=https://web.archive.org/web/20211019074211/https://usb.org/sites/default/files/USB4%E2%84%A2%20Thunderbolt3%E2%84%A2%20Compatibility%20Requirements%20Specification%20Rev%201.0%20-%2020210129_0.pdf|url-status=live}}</ref>
The USB Power Delivery revision 2.0 specification will be released along with the Type-C connector. Among other changes, the new specification will allow for an alternate physical layer which is not RF-based, thus it will reduce the possibilities for ]; see ] for more details.<ref>{{cite web|title=USB Future Specifications Industry Reviews| url = http://www.usb.org/developers/USB-Futures.pdf|accessdate=10 August 2014}}</ref>

=== PoweredUSB ===
{{Main|PoweredUSB}}

] is a proprietary extension that adds four additional pins supplying up to 6&nbsp;A at 5&nbsp;V, 12&nbsp;V, or 24&nbsp;V. It is commonly used in ] systems to power peripherals such as ]s, ]s, and printers.

==Signaling==
USB allows the following ] (the terms ''speed'' and ''bandwidth'' are used interchangeably, while "high-" is alternatively written as "hi-"):

* A ''low-speed'' (USB 1.0) rate of 1.5&nbsp;Mbit/s is defined by USB 1.0. It is very similar to full-bandwidth operation except each bit takes 8 times as long to transmit. It is intended primarily to save cost in low-bandwidth ]s (HID) such as keyboards, mice, and joysticks.
* The ''full-speed'' (USB 1.1) rate of 12&nbsp;] is the basic USB data rate defined by USB 1.0. All USB hubs can operate at this speed.
* A ''high-speed'' (USB 2.0) rate of 480&nbsp;Mbit/s was introduced in 2001. All hi-speed devices are capable of falling back to full-bandwidth operation if necessary; i.e., they are backward compatible with USB 1.1. Connectors are identical for USB 2.0 and USB 1.x.
* A ''SuperSpeed'' (USB 3.0) rate of 5.0&nbsp;Gbit/s. The written USB 3.0 specification was released by Intel and its partners in August 2008. The first USB 3.0 controller chips were sampled by ] in May 2009,<ref>{{cite web|url=http://www.reghardware.co.uk/2009/05/19/nec_usb_3_host/ | title=NEC ready to sample 'world's first' USB 3.0 controller chip | accessdate=2009-06-15}}</ref> and the first products using the USB 3.0 specification arrived in January 2010.<ref>{{cite web| url=http://www.everythingusb.com/superspeed-usb.html#6 | title=When will USB 3.0 products hit the market? | accessdate=2009-05-11}}</ref> USB 3.0 connectors are generally backwards compatible, but include new wiring and full duplex operation.

USB signals are transmitted on a ] data cable with 90] ±15% ],<ref>{{cite web|title=USB in a NutShell—Chapter 2—Hardware |url=http://www.beyondlogic.org/usbnutshell/usb2.htm |publisher=Beyond Logic.org |accessdate=2007-08-25}}</ref> labeled D+ and D−. Prior to USB 3.0, these collectively use ] ] to reduce the effects of electromagnetic ] on longer lines. Transmitted signal levels are 0.0 to 0.3 ]s for low and 2.8 to 3.6 ]s for high in full-bandwidth and low-bandwidth modes, and −10 to 10&nbsp;mV for low and 360 to 440&nbsp;mV for high in hi-bandwidth mode. In FS mode, the cable wires are not terminated, but the HS mode has ] of 45&nbsp;Ω to ground, or 90&nbsp;Ω differential to match the data cable impedance, reducing interference due to signal ]. USB 3.0 introduces two additional pairs of shielded twisted wire and new, mostly interoperable contacts in USB 3.0 cables, for them. They permit the higher data rate, and full duplex operation.

A USB connection is always between a host or hub at the "A" connector end, and a device or hub's "upstream" port at the other end. Originally, this was a "B" connector, preventing erroneous loop connections, but additional upstream connectors were specified, and some cable vendors designed and sold cables that permitted erroneous connections (and potential damage to circuitry). USB interconnections are not as fool-proof or as simple as originally intended.

The host includes 15&nbsp;kΩ pull-down resistors on each data line. When no device is connected, this pulls both data lines low into the so-called "single-ended zero" state (SE0 in the USB documentation), and indicates a reset or disconnected connection.

A USB device pulls one of the data lines high with a 1.5&nbsp;kΩ resistor. This overpowers one of the pull-down resistors in the host and leaves the data lines in an idle state called "J". For USB 1.x, the choice of data line indicates of what signal rates the device is capable; full-bandwidth devices pull D+ high, while low-bandwidth devices pull D− high. The "k" state is just the opposite polarity to the "j" state.

]

USB data is transmitted by toggling the data lines between the J state and the opposite K state. USB encodes data using the ] ]; a 0 bit is transmitted by toggling the data lines from J to K or vice-versa, while a 1 bit is transmitted by leaving the data lines as-is. To ensure a minimum density of signal transitions remains in the ], USB uses ]; an extra 0 bit is inserted into the data stream after any appearance of six consecutive 1 bits. Seven consecutive received 1 bits is always an error. USB 3.0 has introduced additional data transmission encodings.

A USB packet begins with an 8-bit synchronization sequence '00000001'. That is, after the initial idle state J, the data lines toggle KJKJKJKK. The final 1 bit (repeated K state) marks the end of the sync pattern and the beginning of the USB frame. For high bandwidth USB, the packet begins with a 32-bit synchronization sequence.

A USB packet's end, called EOP (end-of-packet), is indicated by the transmitter driving 2 bit times of SE0 (D+ and D− both below max) and 1 bit time of J state. After this, the transmitter ceases to drive the D+/D− lines and the aforementioned pull up resistors hold it in the J (idle) state. Sometimes skew due to hubs can add as much as one bit time before the SE0 of the end of packet. This extra bit can also result in a "bit stuff violation" if the six bits before it in the CRC are '1's. This bit should be ignored by receiver.

A USB bus is reset using a prolonged (10 to 20 milliseconds) SE0 signal.

USB 2.0 devices use a special protocol during reset, called "chirping", to negotiate the high bandwidth mode with the host/hub. A device that is HS capable first connects as an FS device (D+ pulled high), but upon receiving a USB RESET (both D+ and D− driven LOW by host for 10 to 20&nbsp;ms) it pulls the D− line high, known as chirp K. This indicates to the host that the device is high bandwidth. If the host/hub is also HS capable, it chirps (returns alternating J and K states on D− and D+ lines) letting the device know that the hub operates at high bandwidth. The device has to receive at least three sets of KJ chirps before it changes to high bandwidth terminations and begins high bandwidth signaling. Because USB 3.0 uses wiring separate and additional to that used by USB 2.0 and USB 1.x, such bandwidth negotiation is not required.

Clock tolerance is 480.00&nbsp;Mbit/s ±500 ], 12.000&nbsp;Mbit/s ±2500 ppm, 1.50&nbsp;Mbit/s ±15000 ppm.

Though high bandwidth devices are commonly referred to as "USB 2.0" and advertised as "up to 480&nbsp;Mbit/s", not all USB 2.0 devices are high bandwidth. The ] certifies devices and provides licenses to use special marketing logos for either "basic bandwidth" (low and full) or high bandwidth after passing a compliance test and paying a licensing fee. All devices are tested according to the latest specification, so recently compliant low bandwidth devices are also 2.0 devices.

USB 3 uses tinned copper stranded AWG-28 cables with {{val|90|7|u=Ω}} impedance for its high-speed differential pairs and ] and ]<!--6.2--> sent with a voltage of 1 V nominal<!--6.6.2--> with a 100 mV receiver threshold; the receiver uses equalization<!--6.8.2-->.<ref name="usb3_cablespec">{{cite web|title=Technical Specifications of the USB 3.0 SuperSpeed Cables|url=http://www.usb3.com/images/usb_superspeed_cable_spec.jpg}} 100717 usb3.com</ref> ]<!--6.5.3--> clock and {{nowrap|300 ppm}}<!--6.4.3--> precision is used. Packet headers are protected with CRC-16, while data payload is protected with CRC-32.<ref name="usb3_specfinal">{{cite web|title=Universal Serial Bus 3.0 Specification, Rev 1.0 November 12, 2008|url=http://www.usb3.com/whitepapers/USB%203%200%20(11132008)-final.pdf}} 100717 usb3.com</ref>
Power up to <!-- 0.900A*4V --> 3.6&nbsp;W may be used. One unit load in superspeed mode is equal to 150 mA.<ref name="usb3_specfinal"/>

== Transmission rates ==
The theoretical maximum data rate in USB 2.0 is 480&nbsp;Mbit/s (60&nbsp;MB/s) per controller and is shared amongst all attached devices. Some chipset manufacturers overcome this bottleneck by providing multiple USB 2.0 controllers within the ].

Typical hi-speed USB hard drives can be written to at rates around 25–30&nbsp;MB/s, and read from at rates of 30–42&nbsp;MB/s, according to routine testing done by ].<ref>{{cite web|url= http://reviews.cnet.com/external-hard-drives/seagate-freeagent-goflex-ultra/4505-3190_7-34183942-2.html |title=Seagate FreeAgent GoFlex Ultra-portable | type = review |publisher= CNet |accessdate=2011-05-22}}</ref> This is 70% of the total bandwidth available. Mask Tests, also known as ], are used to determine the quality of a signal in the time domain. They are defined in the referenced document as part of the electrical test description for the high-speed (HS) mode at 480&nbsp;Mbit/s.<ref>{{cite web | last = Schwarz | first = Rohde | url = http://www.rohde-schwarz.com/appnote/1MA188.pdf |title=USB 2.0 Mask Testing |date=2012-05-25 |accessdate= 2012-07-12}}</ref>

According to a USB-IF chairman, "at least 10 to 15 percent of the stated peak 60&nbsp;MB/s (480&nbsp;Mbit/s) of Hi-Speed USB goes to overhead—the communication protocol between the card and the peripheral. Overhead is a component of all connectivity standards".<ref>{{Citation | url = http://www.pcworld.com/article/82005/news_and_trends_usb_20s_real_deal.html | newspaper = PC world | title = News & Trends | contribution = USB 2.0's Real Deal | date = 2002-02-28}}</ref> Tables illustrating the transfer limits are shown in Chapter 5 of the USB spec.

For ] devices like audio streams, the bandwidth is constant, and reserved exclusively for a given device. The bus bandwidth therefore only has an effect on the number of channels that can be sent at a time, not the "speed" or ] of the transmission.

== Latency ==
For USB1 low-speed {{nowrap|(1.5&nbsp;Mbit/s)}} and full-speed {{nowrap|(12&nbsp;Mbit/s)}} devices the shortest time for a transaction in one direction is {{nowrap|1 ms.}}<ref>{{Citation | url = http://www.urbanterror.info/forums/topic/10515-mouse-stuff-you-ought-to-know-about/ | title = Urban terror | contribution = Mouse stuff you ought to know about | date = 2008-08-09}}</ref> USB2 high-speed {{nowrap |(480&nbsp;Mbit/s)}} uses transactions within each micro frame {{nowrap|(125 µs)}}<ref>{{Citation | url = http://wiki.osdev.org/Universal_Serial_Bus | title = OS dev | title = Universal Serial Bus | date = 2011-02-01}}</ref> where using 1-byte interrupt packet results in a minimal response time of {{nowrap|940 ns.}}<!--125 µs/133--> 4-byte interrupt packet results in {{nowrap|984 ns.}}<!--page 58/69--><ref>{{Citation | url = http://doc.utwente.nl/56345/1/Parchomov02real.pdf | publisher = U Twente | place = ] | title = Real Time Control on CAN |date=April 2002 | last = Parchomov | format = PDF}}</ref>

== Communication ==
During USB communication data is transmitted as ]. Initially, all packets are sent from the host, via the root hub and possibly more hubs, to devices. Some of those packets direct a device to send some packets in reply.

After the sync field, all packets are made of 8-bit bytes, transmitted ]. The first byte is a packet identifier (PID) byte. The PID is actually 4 bits; the byte consists of the 4-bit PID followed by its bitwise complement. This redundancy helps detect errors. (Note also that a PID byte contains at most four consecutive 1 bits, and thus never needs bit-stuffing, even when combined with the final 1 bit in the sync byte. However, trailing 1 bits in the PID may require bit-stuffing within the first few bits of the payload.)

{| class=wikitable
|+ USB PID bytes
! Type !! PID value<br />(]-first) !! Transmitted byte<br />(]-first) !! Name !! Description
|-
| ''Reserved'' ||align=center| 0000 ||align=center| 0000 1111 ||colspan=2|
|-
|rowspan=2| Token ||align=center| 1000 ||align=center| 0001 1110 || '''SPLIT''' || High-bandwidth (USB 2.0) split transaction
|-
|align=center| 0100 ||align=center| 0010 1101 || '''PING''' || Check if endpoint can accept data (USB 2.0)
|-
| Special || rowspan=2 style="text-align:center"| 1100 || rowspan=2 style="text-align:center"| 0011 1100 || '''PRE''' || Low-bandwidth USB preamble
|-
|rowspan=5| Handshake || '''ERR''' || Split transaction error (USB 2.0)
|-
|align=center| 0010 ||align=center| 0100 1011 || '''ACK''' || Data packet accepted
|-
|align=center| 1010 ||align=center| 0101 1010 || '''NAK''' || Data packet not accepted; please retransmit
|-
|align=center| 0110 ||align=center| 0110 1001 || '''NYET''' || Data not ready yet (USB 2.0)
|-
|align=center| 1110 ||align=center| 0111 1000 || '''STALL''' || Transfer impossible; do error recovery
|-
|rowspan=4| Token ||align=center| 0001 ||align=center| 1000 0111 || '''OUT''' || Address for host-to-device transfer
|-
|align=center| 1001 ||align=center| 1001 0110 || '''IN''' || Address for device-to-host transfer
|-
|align=center| 0101 ||align=center| 1010 0101 || '''SOF''' || Start of frame marker (sent each ms)
|-
|align=center| 1101 ||align=center| 1011 0100 || '''SETUP''' || Address for host-to-device control transfer
|-
|rowspan=4| Data ||align=center| 0011 ||align=center| 1100 0011 || '''DATA0''' || Even-numbered data packet
|-
|align=center| 1011 ||align=center| 1101 0010 || '''DATA1''' || Odd-numbered data packet
|-
|align=center| 0111 ||align=center| 1110 0001 || '''DATA2''' || Data packet for high-bandwidth isochronous transfer (USB 2.0)
|-
|align=center| 1111 ||align=center| 1111 0000 || '''MDATA''' || Data packet for high-bandwidth isochronous transfer (USB 2.0)
|}

Packets come in three basic types, each with a different format and CRC (]):

=== Handshake packets ===
Handshake packets consist of a PID byte, and are generally sent in response to data packets. The three basic types are ''ACK'', indicating that data was successfully received, ''NAK'', indicating that the data cannot be received and should be retried, and ''STALL'', indicating that the device has an error condition and cannot transfer data until some corrective action (such as device initialization) occurs.

USB 2.0 added two additional handshake packets: ''NYET'' and ''ERR''. NYET indicates that a split transaction is not yet complete. A NYET packet also tells the host that the receiver has accepted a data packet, but cannot accept any more due to full buffers. The host then sends PING packets and continues with data packets once the device ACK's the PING. The ''ERR'' handshake indicates that a split transaction failed.

The only handshake packet the USB host may generate is ACK. If it is not ready to receive data, it should not instruct a device to send.

=== Token packets ===
Token packets consist of a PID byte followed by two payload bytes: 11 bits of address and a 5-bit CRC. Tokens are only sent by the host, never a device.

''IN'' and ''OUT'' tokens contain a 7-bit device number and 4-bit function number (for multifunction devices) and command the device to transmit DATAx packets, or receive the following DATAx packets, respectively.

An IN token expects a response from a device. The response may be a NAK or STALL response, or a DATAx frame. In the latter case, the host issues an ACK handshake if appropriate.

An OUT token is followed immediately by a DATAx frame. The device responds with ACK, NAK, NYET, or STALL, as appropriate.

''SETUP'' operates much like an OUT token, but is used for initial device setup. It is followed by an 8-byte DATA0 frame with a standardized format.

Every millisecond (12000 full-bandwidth bit times), the USB host transmits a special ''SOF'' (start of frame) token, containing an 11-bit incrementing frame number in place of a device address. This is used to synchronize isochronous and interrupt data transfers. High-bandwidth USB 2.0 devices receive 7 additional SOF tokens per frame, each introducing a 125&nbsp;µs "microframe" (60000 high-bandwidth bit times each).

USB&nbsp;2.0 added a ''PING'' token, which asks a device if it is ready to receive an OUT/DATA packet pair. The device responds with ACK, NAK, or STALL, as appropriate. This avoids the need to send a large DATA packet to a device not willing to accept it.<ref>{{cite web
| url = http://www.usbmadesimple.co.uk/ums_7.htm#ping
| title = Part 7 - High Speed Transactions | section = Ping Protocol
| year = 2008 | accessdate = 2014-08-16
| website = usbmadesimple.co.uk
}}</ref>

USB&nbsp;2.0 also added a larger 3-byte ''SPLIT'' token with a 7-bit hub number, 12 bits of control flags, and a 5-bit CRC. This is used to perform split transactions. Rather than tie up the high-bandwidth USB bus sending data to a slower USB device, the nearest high-bandwidth capable hub receives a SPLIT token followed by one or two USB packets at high bandwidth, performs the data transfer at full or low bandwidth, and provides the response at high bandwidth when prompted by a second SPLIT token.

=== Data packets ===
A data packet consists of the PID followed by 0–1,023 bits of data payload (up to 1,024 in high bandwidth, at most 8 at low bandwidth), and a 16-bit CRC.

There are two basic forms of data packet, ''DATA0'' and ''DATA1''. A data packet must always be preceded by an address token, and is usually followed by a handshake token from the receiver back to the transmitter. The two packet types provide the 1-bit sequence number required by ]. If a USB host does not receive a response (such as an ACK) for data it has transmitted, it does not know if the data was received or not; the data might have been lost in transit, or it might have been received but the handshake response was lost.

To solve this problem, the device keeps track of the type of DATAx packet it last accepted. If it receives another DATAx packet of the same type, it is acknowledged but ignored as a duplicate. Only a DATAx packet of the opposite type is actually received.

If the data is corrupted while transmitted or received, the CRC check fails. When this happens, the receiver does not generate an ACK, which makes the sender resend the packet.<ref>{{cite web|url=http://www.totalphase.com/solutions/wp/debugging_usb/|title = Debugging Common USB Issues|accessdate=5 June 2013}}</ref>

When a device is reset with a SETUP packet, it expects an 8-byte DATA0 packet next.

USB 2.0 added ''DATA2'' and ''MDATA'' packet types as well. They are used only by high-bandwidth devices doing high-bandwidth isochronous transfers that must transfer more than 1024 bits per 125&nbsp;µs microframe (8,192&nbsp;kB/s).

=== PRE packet ===
Low-bandwidth devices are supported with a special PID value, ''PRE''. This marks the beginning of a low-bandwidth packet, and is used by hubs that normally do not send full-bandwidth packets to low-bandwidth devices. Since all PID bytes include four 0 bits, they leave the bus in the full-bandwidth K state, which is the same as the low-bandwidth J state. It is followed by a brief pause, during which hubs enable their low-bandwidth outputs, already idling in the J state. Then a low-bandwidth packet follows, beginning with a sync sequence and PID byte, and ending with a brief period of SE0. Full-bandwidth devices other than hubs can simply ignore the PRE packet and its low-bandwidth contents, until the final SE0 indicates that a new packet follows.

== Comparisons with other connection methods ==
]

=== FireWire ===
At first, USB was considered a complement to ] (FireWire) technology, which was designed as a high-bandwidth serial bus that efficiently interconnects peripherals such as disk drives, audio interfaces, and video equipment. In the initial design, USB operated at a far lower data rate and used less sophisticated hardware. It was suitable for small peripherals such as keyboards and pointing devices.

The most significant technical differences between FireWire and USB include:
* USB networks use a ]{{Clarify|tiered-star|date=October 2012}} topology, while IEEE 1394 networks use a ] topology.
* USB 1.0, 1.1 and 2.0 use a "speak-when-spoken-to" protocol; peripherals cannot communicate with the host unless the host specifically requests communication. USB 3.0 allows for device-initiated communications towards the host. A FireWire device can communicate with any other node at any time, subject to network conditions.
* A USB network relies on a single host at the top of the tree to control the network. In a FireWire network, any capable node can control the network.
* USB runs with a 5&nbsp;] power line, while FireWire in current implementations supplies 12&nbsp;V and theoretically can supply up to 30&nbsp;V.
* Standard USB hub ports can provide from the typical 500&nbsp;mA/2.5&nbsp;W of current, only 100&nbsp;mA from non-hub ports. USB 3.0 and USB On-The-Go supply 1.8&nbsp;A/9.0&nbsp;W (for dedicated battery charging, 1.5&nbsp;A/7.5&nbsp;W Full bandwidth or 900&nbsp;mA/4.5&nbsp;W High Bandwidth), while FireWire can in theory supply up to 60&nbsp;watts of power, although 10 to 20 watts is more typical.

These and other differences reflect the differing design goals of the two buses: USB was designed for simplicity and low cost, while FireWire was designed for high performance, particularly in time-sensitive applications such as audio and video. Although similar in theoretical maximum transfer rate, FireWire 400 is faster than USB 2.0 Hi-Bandwidth in real-use,<ref>{{cite web|title=FireWire vs. USB 2.0|url = http://www.qimaging.com/support/pdfs/firewire_usb_technote.pdf|publisher=QImaging|accessdate=20 July 2010 | format =PDF}}</ref> especially in high-bandwidth use such as external hard-drives.<ref>{{cite web|url= http://www.cwol.com/firewire/firewire-vs-usb.htm |title=FireWire vs. USB 2.0 – Bandwidth Tests |accessdate = 2007-08-25}}</ref><ref>{{cite web|url=http://www.digit-life.com/articles/usb20vsfirewire |title=USB 2.0 vs FireWire |publisher=Digit-Life |accessdate=2007-08-25}}</ref><ref>{{Cite news|url = http://www.pcmag.com/article2/0,4149,847716,00.asp |title=The Great Interface-Off: FireWire Vs. USB 2.0 |publisher=PC Magazine | accessdate =2007-08-25|last=Metz|first=Cade |date=2003-02-25}}</ref><ref>{{cite web|url= http://www.g4tv.com/techtvvault/features/39129/USB_20_Versus_FireWire_pg3.html |title=USB 2.0 Versus FireWire |accessdate = 2007-08-25|author=Heron, Robert|publisher=TechTV}}</ref> The newer FireWire 800 standard is twice as fast as FireWire 400 and faster than USB 2.0 Hi-Bandwidth both theoretically and practically.<ref>{{cite web| url = http://www.usb-ware.com/firewire-vs-usb.htm | title = FireWire vs. USB 2.0 | publisher = USB Ware | accessdate = 2007-03-19}}</ref> The chipset and drivers used to implement USB and FireWire have a crucial impact on how much of the bandwidth prescribed by the specification is achieved in the real world, along with compatibility with peripherals.<ref>{{cite web|url=http://www.anandtech.com/mb/showdoc.aspx?i=2602&p=15 |title=Firewire and USB Performance |accessdate=2008-02-01| last =Key | first = Gary|date=2005-11-15}}</ref>

=== Ethernet ===
The IEEE 802.3af ] (PoE) standard specifies a more elaborate power negotiation scheme than powered USB. It operates at 48&nbsp;V ] and can supply more power (up to 12.95&nbsp;W, PoE+ 25.5&nbsp;W) over a cable up to 100&nbsp;meters compared to USB 2.0, which provides 2.5&nbsp;W with a maximum cable length of 5&nbsp;meters. This has made PoE popular for ] telephones, ]s, ]s and other networked devices within buildings. However, USB is cheaper than PoE provided that the distance is short, and power demand is low.

Ethernet standards require electrical isolation between the networked device (computer, phone, etc.) and the network cable up to {{nowrap|1500 V AC}} or {{nowrap |2250 V DC}} for 60 seconds.<ref>{{cite web| url = http://standards.ieee.org/getieee802/download/802.3-2008_section1.pdf | title = 802.3 | format = PDF | publisher = IEEE | section =14.3.1.1}}</ref> USB has no such requirement as it was designed for peripherals closely associated with a host computer, and in fact it connects the peripheral and host grounds. This gives Ethernet a significant safety advantage over USB with peripherals such as cable and DSL modems connected to external wiring that can assume hazardous voltages under certain fault conditions.<ref>{{cite web |url= http://consumerist.com/2006/12/powerbook-explodes-after-comcast-plugs-in-wrong-cable.html |title=Powerbook Explodes After Comcast Plugs In Wrong Cable |publisher=Consumerist |date=2010-03-08 |accessdate=2010-06-22}}</ref>

=== MIDI ===
Digital musical instruments are another example where USB is competitive in low-cost devices. However ] and the ] plug standard have an advantage in high-end devices that may have long cables. USB can cause ] problems between equipment, because it connects ground references on both transceivers. By contrast, the MIDI plug standard and ] have built-in isolation to {{gaps|500|V}} or more.

=== eSATA/eSATAp ===
The ] connector is a more robust ] connector, intended for connection to external hard drives and SSDs. eSATA's transfer rate (up to 6&nbsp;Gbit/s) is similar to that of USB 3.0 (up to 5&nbsp;Gbit/s on current devices; 10&nbsp;Gbit/s speeds via USB 3.1, announced on July 31, 2013). A device connected by eSATA appears as an ordinary SATA device, giving both full performance and full compatibility associated with internal drives.

eSATA does not supply power to external devices. This is an increasing disadvantage compared to USB. Even though USB 3.0's 4.5&nbsp;W is sometimes insufficient to power external hard drives, technology is advancing and external drives gradually need less power, diminishing the eSATA advantage. ] (power over eSATA; aka ESATA/USB) is a connector introduced in 2009 that supplies power to attached devices using a new, backwards-compatible, connector. On a notebook eSATAp usually supplies only 5&nbsp;V to power a 2.5-inch HDD/SSD; on a desktop workstation it can additionally supply 12&nbsp;V to power larger devices including 3.5-inch HDD/SSD and 5.25-inch optical drives.

eSATAp support can be added to a desktop machine in the form of a bracket connecting to motherboard SATA, power, and USB resources.

eSATA, like USB, supports ], although this might be limited by OS drivers and device firmware.

=== Thunderbolt ===
] combines ] and ] into a new serial data interface. Current ] implementations have two channels, each with a transfer speed of 10&nbsp;Gbit/s, resulting in an aggregate unidirectional bandwidth of 20&nbsp;Gbit/s.<ref>{{cite web|url=https://thunderbolttechnology.net/tech/how-it-works |title=How Thunderbolt Technology Works: Thunderbolt Technology Community |publisher=Thunderbolttechnology.net |date= |accessdate=2014-01-22}}</ref>


== Interoperability == == Interoperability ==
{{main |USB adapter}} {{Main|USB-to-serial adapter}}
Various ]s that convert USB data signals to and from other communications standards. Various ]s are available that convert USB data signals to and from other communications standards.


== Related standards == == Security threats ==
Due to the prevalency of the USB standard, there are many exploits using the USB standard. One of the biggest instances of this today is known as the ], a device that damages USB devices by sending high voltage pulses across the data lines.
]
The USB Implementers Forum is working on a ] standard based on the USB protocol. ] is intended as a cable-replacement technology, and uses ] ] for data rates of up to 480&nbsp;Mbit/s.


In versions of ] before ], Windows would automatically run a script (if present) on certain devices via ], one of which are USB mass storage devices, which may contain malicious software.<ref>{{cite web |url=https://www.samlogic.net/articles/autorun-usb-flash-drive.htm |title=Using AutoRun with a USB Flash Drive (USB stick) |website=Positive Technologies |date=25 June 2022 |access-date=26 July 2022 |archive-date=26 April 2022 |archive-url=https://web.archive.org/web/20220426181327/https://www.samlogic.net/articles/autorun-usb-flash-drive.htm |url-status=live }}</ref>
USB 2.0 ] (HSIC) is a chip-to-chip variant of USB 2.0 that eliminates the conventional analog transceivers found in normal USB. It was adopted as a standard by the USB Implementers Forum in 2007. The HSIC ] uses about 50% less power and 75% less ] area compared to traditional USB 2.0. HSIC uses two signals at 1.2 V and has a throughput of 480&nbsp;Mbit/s using 240&nbsp;MHz DDR{{citation needed|date=February 2013}} signaling. Maximum ] trace length for HSIC is 10&nbsp;cm. It does not have low enough latency to support ] memory sharing between two chips.<ref>{{cite web|title= Interchip Connectivity: HSIC, UniPro, HSI, C2C, LLI... oh my!|url = http://info.arteris.com/blog/bid/59433/Interchip-Connectivity-HSIC-UniPro-HSI-C2C-LLI-oh-my |accessdate= 24 June 2011}}</ref><ref>{{cite web|title=USB High Speed Inter-Chip Interface|url= http://www.interfacebus.com/hsic-bus-high-speed-inter-chip-usb.html |accessdate=24 June 2011}}</ref>


== See also == == See also ==
{{Div col||30em}} {{Portal|Electronics}}

* ] – USB peer-to-peer transfer crossover bridge cable for Windows
=== USB ===
{{Div col|colwidth=24em}}
* ]
* ]
* ]
* ]
* ] (XHCI) * ] (XHCI)
* ] * {{section link|List of interface bit rates|Peripheral}}
* ]
* ] ] (incl. Fibre Channel, FCoE, iEEE 1394, InfiniBand, iSCSI, USB)
{{Div col end}}

=== Derived and related standards ===
{{Div col|colwidth=24em}}
* ]
* ]
* ] * ]
* ] * ]
* ] * ]
* ]
{{Div col end}} {{Div col end}}


== References == == References ==
{{Reflist|30em}} {{Reflist |refs=
<ref name="spec_3.0">{{cite book | url=http://www.usb.org/developers/docs/documents_archive/usb_30_spec_070113.zip | title=Universal Serial Bus 3.0 Specification | date=6 June 2011 | via=www.usb.org | publisher=] ] ] ] ] ] | format=] | url-status=live | archive-url=https://web.archive.org/web/20140519092924/http://www.usb.org/developers/docs/documents_archive/usb_30_spec_070113.zip | archive-date=19 May 2014}}<br />{{cite web |title= Universal Serial Bus 3.0 Specification |url= http://www.gaw.ru/pdf/interface/usb/USB%203%200_english.pdf |date= 12 November 2008 |access-date= 29 December 2012 |via= www.gaw.ru |archive-date= 6 October 2012 |archive-url= https://web.archive.org/web/20121006160059/http://www.gaw.ru/pdf/interface/usb/USB%203%200_english.pdf |url-status= live }}</ref>
<ref name="usb.org 3.1">{{cite web |title=USB 3.1 Specification&nbsp;— Language Usage Guidelines from USB-IF |url=http://www.usb.org/developers/ssusb/USB_3_1_Language_Product_and_Packaging_Guidelines_FINAL.pdf |via=www.usb.org |url-status=live |archive-url=https://web.archive.org/web/20160312135950/http://www.usb.org/developers/ssusb/USB_3_1_Language_Product_and_Packaging_Guidelines_FINAL.pdf |archive-date=12 March 2016 }}</ref>
<ref name="xmos2015">{{Cite web
|url=http://www.epsglobal.com/downloads/XMOS/Why-do-you-need-USB-Audio-Class-2.pdf
|title=Why do you need USB Audio Class 2? |last=Strong |first=Laurence |date=2015 |publisher=XMOS |url-status=dead |archive-url=https://web.archive.org/web/20171124080752/http://www.epsglobal.com/downloads/XMOS/Why-do-you-need-USB-Audio-Class-2.pdf |archive-date=24 November 2017 |access-date=11 December 2020 |quote=In applications where streaming latency is important, UAC2 offers up to an 8x reduction over UAC1. ... Each clocking method has pros and cons and best-fit applications.}}</ref>
}}


== Further reading == == Further reading ==
*{{Cite book| first = Jan | last = Axelson | date = September 1, 2006 | title = USB Mass Storage: Designing and Programming Devices and Embedded Hosts | publisher = ] | edition = 1st | isbn = 978-1-931-44804-8 | url = http://www.lvr.com/usbms.htm}} 287 pp. * {{Cite book | first = Jan | last = Axelson | date = 1 September 2006 | title = USB Mass Storage: Designing and Programming Devices and Embedded Hosts | publisher = ] | edition = 1st | isbn = 978-1-931-44804-8 | url = https://archive.org/details/isbn_9781931448048 | url-access = registration }}
*{{Cite book| first = Jan | last = Axelson | author-mask = 3 | date = December 1, 2007 | title = Serial Port Complete: COM Ports, USB Virtual COM Ports, and Ports for Embedded Systems | publisher = ] | edition = 2nd | isbn = 978-1-931-44806-2 | url = http://www.lvr.com/spc.htm}} 380 pp. * {{Cite book| first = Jan | last = Axelson | author-mask = 3 | date = 1 December 2007 | title = Serial Port Complete: COM Ports, USB Virtual COM Ports, and Ports for Embedded Systems | publisher = Lakeview Research | edition = 2nd | isbn = 978-1-931-44806-2 | url = http://janaxelson.com/spc.htm}}
*{{Cite book| first = Jan | last = Axelson | author-mask = 3 | year = 2009 | title = USB Complete: The Developer's Guide | publisher = ] | edition = 4th | isbn = 978-1-931-44808-6 | url = http://www.lvr.com/usbc.htm}} 506 pp. * {{Cite book| first = Jan | last = Axelson | author-mask = 3 | year = 2015 | title = USB Complete: The Developer's Guide | publisher = Lakeview Research | edition = 5th | isbn = 978-1-931448-28-4 | url = http://janaxelson.com/usbc.htm}}
*{{Cite book| first = John | last = Hyde | date = February 2001 | title = USB Design by Example: A Practical Guide to Building I/O Devices | publisher = ] | edition = 2nd | isbn = 978-0-970-28465-5 | url = http://www.intel.com/intelpress/usb/}} 510 pp. * {{Cite book| first = John | last = Hyde | date = February 2001 | title = USB Design by Example: A Practical Guide to Building I/O Devices | publisher = ] | edition = 2nd | isbn = 978-0-970-28465-5 | url = http://www.intel.com/intelpress/usb/}}
* {{Cite journal | title = Debugging USB 2.0 for Compliance: It's Not Just a Digital World | publisher = Agilent | series = Technologies Application Note | issue = 1382–3 | format = PDF | url = http://cp.literature.agilent.com/litweb/pdf/5988-4794EN.pdf}}. * {{Cite journal|title=Debugging USB 2.0 for Compliance: It's Not Just a Digital World|publisher=Keysight|series=Technologies Application Note|issue=1382–3|journal=Keysight Technologies|url=http://literature.cdn.keysight.com/litweb/pdf/5988-4794EN.pdf}}


== External links == == External links ==
{{Commons category|USB}} {{Commons|Universal Serial Bus}}
{{wikibooks |Serial Programming:USB Technical Manual|USB connectors}} {{Wikibooks|Serial Programming:USB Technical Manual|USB connectors}}
* {{cite web | url = http://www.usb.org/ | title = USB Implementers Forum}}
* {{Cite journal | url = http://stuff.mit.edu/afs/sipb/contrib/doc/specs/protocol/usb/UHCI11D.PDF | publisher = Intel | title = Universal Host Controller Interface (UHCI) | format = PDF}}
* {{Cite journal | url = http://pinoutsguide.com/Slots/usb_3_0_connector_pinout.shtml | title = USB 3.0 Standard-A, Standard-B, Powered-B connectors | publisher = Pinouts guide}}
* {{Cite journal | url = http://www.agilent.com/find/USB | publisher = Agilent | title = Characterization and compliance test}}
* Muller, Henk. ''Electronic Design'', July 2012.


=== General overview ===
* {{cite web |url = https://www.fastcompany.com/3060705/an-oral-history-of-the-usb |title = The unlikely origins of USB, the port that changed everything |author = Joel Johnson |publisher = ] |date = 29 May 2019 }}
* {{cite AV media |url = https://www.youtube.com/watch?v=36CKsP9YQ1E |title = Why Does USB Keep Changing? |first=Peter |last=Leigh |date = 24 May 2020 |medium = video }}
* {{cite news |last=Parikh |first=Bijal |title=USB (Universal Serial Bus): An Overview |url=https://www.engineersgarage.com/usb-universal-serial-bus-an-overview/ |access-date=7 May 2022 |work=Engineers Garage |publisher=WTWH Media}}
* {{cite AV media |url = https://www.youtube.com/watch?v=PctX3kcTj5U |title = Explaining USB: From 1.0 to USB4 V2.0 (ExplainingComputers)|first=Christopher |last=Barnatt |date = 25 September 2022 |medium = video }}

=== Technical documents ===
* {{cite web |url = https://www.usb.org/ |title = USB Implementers Forum (USB-IF) |website = USB.org }}
* {{cite web |url = https://www.usb.org/documents |title = USB Document Library (USB&nbsp;3.2, USB&nbsp;2.0, Wireless USB, USB-C, USB Power Delivery) |website = USB.org }}
* {{cite web |url = http://stuff.mit.edu/afs/sipb/contrib/doc/specs/protocol/usb/UHCI11D.PDF |title = Universal Host Controller Interface (UHCI) |publisher = ] |via=mit.edu}}
* {{cite web |url = http://pinoutsguide.com/Slots/usb_3_0_connector_pinout.shtml |title = USB 3.0 Standard-A, Standard-B, Powered-B connectors |website=Pinouts guide |archive-url=https://web.archive.org/web/20160514121804/http://pinoutsguide.com/Slots/usb_3_0_connector_pinout.shtml |archive-date=14 May 2016}}
* {{cite web |url = https://www.electronicdesign.com/boards/how-create-and-program-usb-devices |title = How To Create And Program USB Devices |first=Henk |last=Muller |publisher = ] |date = July 2012 }}
* {{cite web |url = https://usb.org/sites/default/files/bwpaper2.pdf |title = An Analysis of Throughput Characteristics of Universal Serial Bus |first=John |last=Garney |date = June 1996 }}
* {{cite web |url=https://engineering.biu.ac.il/files/engineering/shared/PE_project_book_0.pdf |title=USB&nbsp;2.0 Protocol Engine |first1=Razi |last1=Hershenhoren |first2=Omer |last2=Reznik |date=October 2010 |access-date=30 January 2019 |archive-date=4 August 2020 |archive-url=https://web.archive.org/web/20200804030543/https://engineering.biu.ac.il/files/engineering/shared/PE_project_book_0.pdf |url-status=dead }}
* ] 62680 (Universal Serial Bus interfaces for data and power):
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{{List of IEC standards}}
{{Basic computer components}} {{Basic computer components}}
{{Computer-bus}} {{Computer bus}}
{{DC power delivery standards}} {{DC power delivery standards}}
{{USB}}
{{Solid-state drive}}


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Latest revision as of 01:32, 12 January 2025

Standard for computer data connections This article is about the computer bus standard. For other uses, see USB (disambiguation).

USB
Universal Serial Bus
The current connector for USB, Thunderbolt, and other protocols, USB-C (plug and receptacle shown)
Type Bus
Production history
Designer
Designed January 1996; 29 years ago (1996-01)
Produced Since May 1996
Superseded Serial port, parallel port, game port, Apple Desktop Bus, PS/2 port, and FireWire (IEEE 1394)

Universal Serial Bus (USB) is an industry standard, developed by USB Implementers Forum (USB-IF), that allows data exchange and delivery of power between many types of electronics. It specifies its architecture, in particular its physical interface, and communication protocols for data transfer and power delivery to and from hosts, such as personal computers, to and from peripheral devices, e.g. displays, keyboards, and mass storage devices, and to and from intermediate hubs, which multiply the number of a host's ports.

Introduced in 1996, USB was originally designed to standardize the connection of peripherals to computers, replacing various interfaces such as serial ports, parallel ports, game ports, and ADB ports. Early versions of USB became commonplace on a wide range of devices, such as keyboards, mice, cameras, printers, scanners, flash drives, smartphones, game consoles, and power banks. USB has since evolved into a standard to replace virtually all common ports on computers, mobile devices, peripherals, power supplies, and manifold other small electronics.

In the current standard, the USB-C connector replaces the many various connectors for power (up to 240 W), displays (e.g. DisplayPort, HDMI), and many other uses, as well as all previous USB connectors.

As of 2024, USB consists of four generations of specifications: USB 1.x, USB 2.0, USB 3.x, and USB4. USB4 enhances the data transfer and power delivery functionality with

... a connection-oriented, tunneling architecture designed to combine multiple protocols onto a single physical interface so that the total speed and performance of the USB4 Fabric can be dynamically shared.

USB4 particularly supports the tunneling of the Thunderbolt 3 protocols, namely PCI Express (PCIe, load/store interface) and DisplayPort (display interface). USB4 also adds host-to-host interfaces.

Each specification sub-version supports different signaling rates from 1.5 and 12 Mbit/s half-duplex in USB 1.0/1.1 to 80 Gbit/s full-duplex in USB4 2.0. USB also provides power to peripheral devices; the latest versions of the standard extend the power delivery limits for battery charging and devices requiring up to 240 watts as defined in USB Power Delivery (USB-PD) Rev. V3.1. Over the years, USB(-PD) has been adopted as the standard power supply and charging format for many mobile devices, such as mobile phones, reducing the need for proprietary chargers.

Overview

USB was designed to standardize the connection of peripherals to personal computers, both to exchange data and to supply electric power. It has largely replaced interfaces such as serial ports and parallel ports and has become commonplace on various devices. Peripherals connected via USB include computer keyboards and mice, video cameras, printers, portable media players, mobile (portable) digital telephones, disk drives, and network adapters.

USB connectors have been increasingly replacing other types of charging cables for portable devices.

USB connector interfaces are classified into three types: the many various legacy Type-A (upstream) and Type-B (downstream) connectors found on hosts, hubs, and peripheral devices, and the modern Type-C (USB-C) connector, which replaces the many legacy connectors as the only applicable connector for USB4.

The Type-A and Type-B connectors came in Standard, Mini, and Micro sizes. The standard format was the largest and was mainly used for desktop and larger peripheral equipment. The Mini-USB connectors (Mini-A, Mini-B, Mini-AB) were introduced for mobile devices. Still, they were quickly replaced by the thinner Micro-USB connectors (Micro-A, Micro-B, Micro-AB). The Type-C connector, also known as USB-C, is not exclusive to USB, is the only current standard for USB, is required for USB4, and is required by other standards, including modern DisplayPort and Thunderbolt. It is reversible and can support various functionalities and protocols, including USB; some are mandatory, and many are optional, depending on the type of hardware: host, peripheral device, or hub.

USB specifications provide backward compatibility, usually resulting in decreased signaling rates, maximal power offered, and other capabilities. The USB 1.1 specification replaces USB 1.0. The USB 2.0 specification is backward-compatible with USB 1.0/1.1. The USB 3.2 specification replaces USB 3.1 (and USB 3.0) while including the USB 2.0 specification. USB4 "functionally replaces" USB 3.2 while retaining the USB 2.0 bus operating in parallel.

The USB 3.0 specification defined a new architecture and protocol named SuperSpeed (aka SuperSpeed USB, marketed as SS), which included a new lane for a new signal coding scheme (8b/10b symbols, 5 Gbit/s; later also known as Gen 1) providing full-duplex data transfers that physically required five additional wires and pins, while preserving the USB 2.0 architecture and protocols and therefore keeping the original four pins/wires for the USB 2.0 backward-compatibility resulting in 9 wires (with 9 or 10 pins at connector interfaces; ID-pin is not wired) in total.

The USB 3.1 specification introduced an Enhanced SuperSpeed System – while preserving the SuperSpeed architecture and protocol (SuperSpeed USB) – with an additional SuperSpeedPlus architecture and protocol (aka SuperSpeedPlus USB) adding a new coding schema (128b/132b symbols, 10 Gbit/s; also known as Gen 2); for some time marketed as SuperSpeed+ (SS+).

The USB 3.2 specification added a second lane to the Enhanced SuperSpeed System besides other enhancements so that the SuperSpeedPlus USB system part implements the Gen 1×2, Gen 2×1, and Gen 2×2 operation modes. However, the SuperSpeed USB part of the system still implements the one-lane Gen 1×1 operation mode. Therefore, two-lane operations, namely USB 3.2 Gen 1×2 (10 Gbit/s) and Gen 2×2 (20 Gbit/s), are only possible with Full-Featured USB-C. As of 2023, they are somewhat rarely implemented; Intel, however, started to include them in its 11th-generation SoC processor models, but Apple never provided them. On the other hand, USB 3.2 Gen 1(×1) (5 Gbit/s) and Gen 2(×1) (10 Gbit/s) have been quite common for some years.

Connector type quick reference

Main article: USB hardware § Connectors

Each USB connection is made using two connectors: a receptacle and a plug. Pictures show only receptacles:

Available connectors by USB standard
Standard USB 1.0
1996 		USB 1.1
1998 		USB 2.0
2000 		USB 2.0
Revised 		USB 3.0
2008 		USB 3.1
2013 		USB 3.2
2017 		USB4
2019 		USB4 2.0
2022
Max Speed Current marketing name Basic-Speed High-Speed USB 5Gbps USB 10Gbps USB 20Gbps USB 40Gbps USB 80Gbps
Original label Low-Speed & Full-Speed SuperSpeed, or SS SuperSpeed+, or SS+ SuperSpeed USB 20Gbps
Operation mode USB 3.2 Gen 1×1 USB 3.2 Gen 2×1 USB 3.2 Gen 2×2 USB4 Gen 3×2 USB4 Gen 4×2
Signaling rate 1.5 Mbit/s & 12 Mbit/s 480 Mbit/s 5 Gbit/s 10 Gbit/s 20 Gbit/s 40 Gbit/s 80 Gbit/s
Connector Standard-A —
Standard-B
Mini-A —
Mini-AB
Mini-B
Micro-A   —
Micro-AB
Micro-B
Type-C (USB-C)
(Enlarged to show detail)
Remarks:
  1. ^ Limited to max speed at 10 Gbit/s, since only one-lane (×1) operation mode is possible.
  2. ^ Backward compatibility given.
  3. ^ Only as receptacle.
  4. Accepts both Mini-A and Mini-B plugs.
  5. Only as plug.
  6. ^ Backward compatibility given by USB 2.0 implementation.
  7. Accepts both Micro-A and Micro-B plugs.

Objectives

The Universal Serial Bus was developed to simplify and improve the interface between personal computers and peripheral devices, such as cell phones, computer accessories, and monitors, when compared with previously existing standard or ad hoc proprietary interfaces.

From the computer user's perspective, the USB interface improves ease of use in several ways:

  • The USB interface is self-configuring, eliminating the need for the user to adjust the device's settings for speed or data format, or configure interrupts, input/output addresses, or direct memory access channels.
  • USB connectors are standardized at the host, so any peripheral can use most available receptacles.
  • USB takes full advantage of the additional processing power that can be economically put into peripheral devices so that they can manage themselves. As such, USB devices often do not have user-adjustable interface settings.
  • The USB interface is hot-swappable (devices can be exchanged without shutting the host computer down).
  • Small devices can be powered directly from the USB interface, eliminating the need for additional power supply cables.
  • Because use of the USB logo is only permitted after compliance testing, the user can have confidence that a USB device will work as expected without extensive interaction with settings and configuration.
  • The USB interface defines protocols for recovery from common errors, improving reliability over previous interfaces.
  • Installing a device that relies on the USB standard requires minimal operator action. When a user plugs a device into a port on a running computer, it either entirely automatically configures using existing device drivers, or the system prompts the user to locate a driver, which it then installs and configures automatically.

The USB standard also provides multiple benefits for hardware manufacturers and software developers, specifically in the relative ease of implementation:

  • The USB standard eliminates the requirement to develop proprietary interfaces to new peripherals.
  • The wide range of transfer speeds available from a USB interface suits devices ranging from keyboards and mice up to streaming video interfaces.
  • A USB interface can be designed to provide the best available latency for time-critical functions or can be set up to do background transfers of bulk data with little impact on system resources.
  • The USB interface is generalized with no signal lines dedicated to only one function of one device.

Limitations

As with all standards, USB possesses multiple limitations to its design:

  • USB cables are limited in length, as the standard was intended for peripherals on the same tabletop, not between rooms or buildings. However, a USB port can be connected to a gateway that accesses distant devices.
  • USB data transfer rates are slower than those of other interconnects such as 100 Gigabit Ethernet.
  • USB has a strict tree network topology and master/slave protocol for addressing peripheral devices; slave devices cannot interact with one another except via the host, and two hosts cannot communicate over their USB ports directly. Some extension to this limitation is possible through USB On-The-Go, Dual-Role-Devices and protocol bridge.
  • A host cannot broadcast signals to all peripherals at once; each must be addressed individually.
  • While converters exist between certain legacy interfaces and USB, they might not provide a full implementation of the legacy hardware. For example, a USB-to-parallel-port converter might work well with a printer, but not with a scanner that requires bidirectional use of the data pins.

For a product developer, using USB requires the implementation of a complex protocol and implies an "intelligent" controller in the peripheral device. Developers of USB devices intended for public sale generally must obtain a USB ID, which requires that they pay a fee to the USB Implementers Forum (USB-IF). Developers of products that use the USB specification must sign an agreement with the USB-IF. Use of the USB logos on the product requires annual fees and membership in the organization.

History

Large circle is left end of horizontal line. The line forks into three branches ending in circle, triangle and square symbols.
The basic USB trident logo

A group of seven companies began the development of USB in 1995: Compaq, DEC, IBM, Intel, Microsoft, NEC, and Nortel. The goal was to make it fundamentally easier to connect external devices to PCs by replacing the multitude of connectors at the back of PCs, addressing the usability issues of existing interfaces, and simplifying software configuration of all devices connected to USB, as well as permitting greater data transfer rates for external devices and plug and play features. Ajay Bhatt and his team worked on the standard at Intel; the first integrated circuits supporting USB were produced by Intel in 1995.

USB 1.x

The Basic-Speed USB logo

Released in January 1996, USB 1.0 specified signaling rates of 1.5 Mbit/s (Low Bandwidth or Low Speed) and 12 Mbit/s (Full Speed). It did not allow for extension cables, due to timing and power limitations. Few USB devices made it to the market until USB 1.1 was released in August 1998. USB 1.1 was the earliest revision that was widely adopted and led to what Microsoft designated the "Legacy-free PC".

Neither USB 1.0 nor 1.1 specified a design for any connector smaller than the standard type A or type B. Though many designs for a miniaturized type B connector appeared on many peripherals, conformity to the USB 1.x standard was hampered by treating peripherals that had miniature connectors as though they had a tethered connection (that is: no plug or receptacle at the peripheral end). There was no known miniature type A connector until USB 2.0 (revision 1.01) introduced one.

USB 2.0

The Hi-Speed USB logo

USB 2.0 was released in April 2000, adding a higher maximum signaling rate of 480 Mbit/s (maximum theoretical data throughput 53 MByte/s) named High Speed or High Bandwidth, in addition to the USB 1.x Full Speed signaling rate of 12 Mbit/s (maximum theoretical data throughput 1.2 MByte/s).

Modifications to the USB specification have been made via engineering change notices (ECNs). The most important of these ECNs are included into the USB 2.0 specification package available from USB.org:

  • Mini-A and Mini-B Connector
  • Micro-USB Cables and Connectors Specification 1.01
  • InterChip USB Supplement
  • On-The-Go Supplement 1.3 USB On-The-Go makes it possible for two USB devices to communicate with each other without requiring a separate USB host
  • Battery Charging Specification 1.1 Added support for dedicated chargers, host chargers behavior for devices with dead batteries
  • Battery Charging Specification 1.2: with increased current of 1.5 A on charging ports for unconfigured devices, allowing high-speed communication while having a current up to 1.5 A
  • Link Power Management Addendum ECN, which adds a sleep power state

USB 3.x

Main article: USB 3.0
Deprecated SuperSpeed USB logo

The USB 3.0 specification was released on 12 November 2008, with its management transferring from USB 3.0 Promoter Group to the USB Implementers Forum (USB-IF) and announced on 17 November 2008 at the SuperSpeed USB Developers Conference.

USB 3.0 adds a new architecture and protocol named SuperSpeed, with associated backward-compatible plugs, receptacles, and cables. SuperSpeed plugs and receptacles are identified with a distinct logo and blue inserts in standard format receptacles.

The SuperSpeed architecture provides for an operation mode at a rate of 5.0 Gbit/s, in addition to the three existing operation modes. Its efficiency is dependent on a number of factors including physical symbol encoding and link-level overhead. At a 5 Gbit/s signaling rate with 8b/10b encoding, each byte needs 10 bits to transmit, so the raw throughput is 500 MB/s. When flow control, packet framing and protocol overhead are considered, it is realistic for about two thirds of the raw throughput, or 330 MB/s to transmit to an application. SuperSpeed's architecture is full-duplex; all earlier implementations, USB 1.0-2.0, are all half-duplex, arbitrated by the host.

Low-power and high-power devices remain operational with this standard, but devices implementing SuperSpeed can provide increased current of between 150 mA and 900 mA, by discrete steps of 150 mA.

USB 3.0 also introduced the USB Attached SCSI protocol (UASP), which provides generally faster transfer speeds than the BOT (Bulk-Only-Transfer) protocol.

USB 3.1, released in July 2013 has two variants. The first one preserves USB 3.0's SuperSpeed architecture and protocol and its operation mode is newly named USB 3.1 Gen 1, and the second version introduces a distinctively new SuperSpeedPlus architecture and protocol with a second operation mode named as USB 3.1 Gen 2 (marketed as SuperSpeed+ USB). SuperSpeed+ doubles the maximum signaling rate to 10 Gbit/s (later marketed as SuperSpeed USB 10 Gbps by the USB 3.2 specification), while reducing line encoding overhead to just 3% by changing the encoding scheme to 128b/132b.

USB 3.2, released in September 2017, preserves existing USB 3.1 SuperSpeed and SuperSpeedPlus architectures and protocols and their respective operation modes, but introduces two additional SuperSpeedPlus operation modes (USB 3.2 Gen 1×2 and USB 3.2 Gen 2×2) with the new USB-C Fabric with signaling rates of 10 and 20 Gbit/s (raw data rates of 1212 and 2424 MB/s). The increase in bandwidth is a result of two-lane operation over existing wires that were originally intended for flip-flop capabilities of the USB-C connector.

Naming scheme

Starting with the USB 3.2 specification, USB-IF introduced a new naming scheme. To help companies with the branding of the different operation modes, USB-IF recommended branding the 5, 10, and 20 Gbit/s capabilities as SuperSpeed USB 5Gbps, SuperSpeed USB 10 Gbps, and SuperSpeed USB 20 Gbps, respectively.

In 2023, they were replaced again, removing "SuperSpeed", with USB 5Gbps, USB 10Gbps, and USB 20Gbps. With new Packaging and Port logos.

USB4

This section needs to be updated. The reason given is: Incomplete, erroneous and not up-to-date; e.g. lacks differences between USB4 first version and 2.0. Applies also to main article.. Please help update this article to reflect recent events or newly available information. (August 2024)
Main article: USB4
Deprecated Certified USB4 logo

The USB4 specification was released on 29 August 2019 by the USB Implementers Forum.

The USB4 2.0 specification was released on 1 September 2022 by the USB Implementers Forum.

USB4 is based on the Thunderbolt 3 protocol. It supports 40 Gbit/s throughput, is compatible with Thunderbolt 3, and backward compatible with USB 3.2 and USB 2.0. The architecture defines a method to share a single high-speed link with multiple end device types dynamically that best serves the transfer of data by type and application.

During CES 2020, USB-IF and Intel stated their intention to allow USB4 products that support all the optional functionality as Thunderbolt 4 products.

USB4 2.0 with 80 Gbit/s speeds was to be revealed in November 2022. Further technical details were to be released at two USB developer days scheduled for November 2022.

The USB4 specification states that the following technologies shall be supported by USB4:

Connection Mandatory for Remarks
host hub device
USB 2.0 (480 Mbit/s) Yes Yes Yes Contrary to other functions – which use the multiplexing of high-speed links – USB 2.0 over USB-C utilizes its own differential pair of wires.
Tunneled USB 3.2 Gen 2×1 (10 Gbit/s) Yes Yes No
Tunneled USB 3.2 Gen 2×2 (20 Gbit/s) No No No
Tunneled USB 3 Gen T (5–80 Gbit/s) No No No A type of USB 3 Tunneling architecture where the Enhanced SuperSpeed System is extended to allow operation at the maximum bandwidth available on the USB4 Link.
USB4 Gen 2 (10 or 20 Gbit/s) Yes Yes Yes Either one or two lanes
USB4 Gen 3 (20 or 40 Gbit/s) No Yes No
Tunneled DisplayPort 1.4a Yes Yes No The specification requires that hosts and hubs support the DisplayPort Alternate Mode.
Tunneled PCI Express 3.0 No Yes No The PCI Express function of USB4 replicates the functionality of previous versions of the Thunderbolt specification.
Host-to-Host communications Yes Yes — A LAN-like connection between two peers.
Thunderbolt 3 Alternate Mode No Yes No Thunderbolt 3 uses USB-C cables; the USB4 specification allows hosts and devices and requires hubs to support interoperability with the standard using the Thunderbolt 3 Alternate Mode (namely DisplayPort and PCIe).
Other Alternate Modes No No No USB4 products may optionally offer interoperability with the HDMI, MHL, and VirtualLink Alternate Modes.

September 2022 naming scheme

An overview of USB naming scheme that was put in place in September 2022
(A mix of USB specifications and their marketing names are being displayed
because specifications are sometimes wrongly used as marketing names.)

Because of the previous confusing naming schemes, USB-IF decided to change it once again. As of 2 September 2022, marketing names follow the syntax "USB xGbps", where x is the speed of transfer in Gbit/s. Overview of the updated names and logos can be seen in the adjacent table.

The operation modes USB 3.2 Gen 2×2 and USB4 Gen 2×2 – or: USB 3.2 Gen 2×1 and USB4 Gen 2×1 – are not interchangeable or compatible; all participating controllers must operate with the same mode.

Version history

Release versions

Name Release date Maximum signaling rate Note
USB 0.7 November 1994 ? Pre-release.
USB 0.8 December 1994 ?
USB 0.9 April 1995 12 Mbit/s: Full Speed (FS)
USB 0.99 August 1995 ?
USB 1.0-RC November 1995 ? Release Candidate.
USB 1.0 January 1996 1.5 Mbit/s: Low Speed (LS)
12 Mbit/s: Full Speed (FS) 		Renamed to Basic-Speed.
USB 1.1 September 1998
USB 2.0 April 2000 480 Mbit/s: High Speed (HS)
USB 3.0 November 2008 5 Gbit/s: SuperSpeed (SS) Renamed to USB 3.1 Gen 1, and later to USB 3.2 Gen 1×1.
USB 3.1 July 2013 10 Gbit/s: SuperSpeed+ (SS+) Renamed to USB 3.1 Gen 2, and later to USB 3.2 Gen 2×1.
USB 3.2 August 2017 20 Gbit/s: SuperSpeed+ two-lane Includes new USB 3.2 Gen 1×2 and Gen 2×2 two-lane modes. Requires Full-Featured USB-C.
USB4 August 2019 40 Gbit/s: two-lane Includes new USB4 Gen 2×2 (64b/66b encoding) and Gen 3×2 (128b/132b encoding) modes and introduces USB4 routing for tunneling of USB 3.2, DisplayPort 1.4a and PCI Express traffic and host-to-host transfers, based on the Thunderbolt 3 protocol; requires USB4 Fabric.
USB4 2.0 September 2022 120 ⇄ 40 Gbit/s: asymmetric Includes new USB4 Gen 4×2 (PAM-3 encoding) mode to get 80 and 120 Gbit/s over Type-C connector. Requires USB4 Fabric.

Power-related standards

Release name Release date Max. power Note
USB Battery Charging Rev. 1.0 2007-03-08 7.5 W (5 V, 1.5 A)
USB Battery Charging Rev. 1.1 2009-04-15 7.5 W (5 V, 1.5 A) Page 28, Table 5–2, but with limitation on paragraph 3.5. In ordinary USB 2.0's standard-A port, 1.5 A only.
USB Battery Charging Rev. 1.2 2010-12-07 7.5 W (5 V, 1.5 A)
USB Power Delivery Rev. 1.0 (V. 1.0) 2012-07-05 100 W (20 V, 5 A) Using FSK protocol over bus power (VBUS)
USB Power Delivery Rev. 1.0 (V. 1.3) 2014-03-11 100 W (20 V, 5 A)
USB Type-C Rev. 1.0 2014-08-11 15 W (5 V, 3 A) New connector and cable specification
USB Power Delivery Rev. 2.0 (V. 1.0) 2014-08-11 100 W (20 V, 5 A) Using BMC protocol over communication channel (CC) on USB-C cables.
USB Type-C Rev. 1.1 2015-04-03 15 W (5 V, 3 A)
USB Power Delivery Rev. 2.0 (V. 1.1) 2015-05-07 100 W (20 V, 5 A)
USB Type-C Rev. 1.2 2016-03-25 15 W (5 V, 3 A)
USB Power Delivery Rev. 2.0 (V. 1.2) 2016-03-25 100 W (20 V, 5 A)
USB Power Delivery Rev. 2.0 (V. 1.3) 2017-01-12 100 W (20 V, 5 A)
USB Power Delivery Rev. 3.0 (V. 1.1) 2017-01-12 100 W (20 V, 5 A)
USB Type-C Rev. 1.3 2017-07-14 15 W (5 V, 3 A)
USB Power Delivery Rev. 3.0 (V. 1.2) 2018-06-21 100 W (20 V, 5 A)
USB Type-C Rev. 1.4 2019-03-29 15 W (5 V, 3 A)
USB Type-C Rev. 2.0 2019-08-29 15 W (5 V, 3 A) Enabling USB4 over USB Type-C connectors and cables.
USB Power Delivery Rev. 3.0 (V. 2.0) 2019-08-29 100 W (20 V, 5 A)
USB Power Delivery Rev. 3.1 (V. 1.0) 2021-05-24 240 W (48 V, 5 A)
USB Type-C Rev. 2.1 2021-05-25 15 W (5 V, 3 A)
USB Power Delivery Rev. 3.1 (V. 1.1) 2021-07-06 240 W (48 V, 5 A)
USB Power Delivery Rev. 3.1 (V. 1.2) 2021-10-26 240 W (48 V, 5 A) Including errata through October 2021

This version incorporates the following ECNs:

  • Clarify use of Retries
  • Battery Capabilities
  • FRS timing problem
  • PPS power rule clarifications
  • Peak current support for EPR AVS APDO

System design

A USB system consists of a host with one or more downstream facing ports (DFP), and multiple peripherals, forming a tiered-star topology. Additional USB hubs may be included, allowing up to five tiers. A USB host may have multiple controllers, each with one or more ports. Up to 127 devices may be connected to a single host controller. USB devices are linked in series through hubs. The hub built into the host controller is called the root hub.

A USB device may consist of several logical sub-devices that are referred to as device functions. A composite device may provide several functions, for example, a webcam (video device function) with a built-in microphone (audio device function). An alternative to this is a compound device, in which the host assigns each logical device a distinct address and all logical devices connect to a built-in hub that connects to the physical USB cable.

Diagram: inside a device are several endpoints, each of which connects by a logical pipe to a host controller. Data in each pipe flows in one direction, though there are a mixture going to and from the host controller.
USB endpoints reside on the connected device: the channels to the host are referred to as pipes.

USB device communication is based on pipes (logical channels). A pipe connects the host controller to a logical entity within a device, called an endpoint. Because pipes correspond to endpoints, the terms are sometimes used interchangeably. Each USB device can have up to 32 endpoints (16 in and 16 out), though it is rare to have so many. Endpoints are defined and numbered by the device during initialization (the period after physical connection called "enumeration") and so are relatively permanent, whereas pipes may be opened and closed.

There are two types of pipe: stream and message.

  • A message pipe is bi-directional and is used for control transfers. Message pipes are typically used for short, simple commands to the device, and for status responses from the device, used, for example, by the bus control pipe number 0.
  • A stream pipe is a uni-directional pipe connected to a uni-directional endpoint that transfers data using an isochronous, interrupt, or bulk transfer:
    Isochronous transfers
    At some guaranteed data rate (for fixed-bandwidth streaming data) but with possible data loss (e.g., realtime audio or video)
    Interrupt transfers
    Devices that need guaranteed quick responses (bounded latency) such as pointing devices, mice, and keyboards
    Bulk transfers
    Large sporadic transfers using all remaining available bandwidth, but with no guarantees on bandwidth or latency (e.g., file transfers)

When a host starts a data transfer, it sends a TOKEN packet containing an endpoint specified with a tuple of (device_address, endpoint_number). If the transfer is from the host to the endpoint, the host sends an OUT packet (a specialization of a TOKEN packet) with the desired device address and endpoint number. If the data transfer is from the device to the host, the host sends an IN packet instead. If the destination endpoint is a uni-directional endpoint whose manufacturer's designated direction does not match the TOKEN packet (e.g. the manufacturer's designated direction is IN while the TOKEN packet is an OUT packet), the TOKEN packet is ignored. Otherwise, it is accepted and the data transaction can start. A bi-directional endpoint, on the other hand, accepts both IN and OUT packets.

Rectangular opening where the width is twice the height. The opening has a metal rim, and within the opening a flat rectangular bar runs parallel to the top side.
Two USB 3.0 Standard-A receptacles (left) and two USB 2.0 Standard-A receptacles (right) on a computer's front panel

Endpoints are grouped into interfaces and each interface is associated with a single device function. An exception to this is endpoint zero, which is used for device configuration and is not associated with any interface. A single device function composed of independently controlled interfaces is called a composite device. A composite device only has a single device address because the host only assigns a device address to a function.

When a USB device is first connected to a USB host, the USB device enumeration process is started. The enumeration starts by sending a reset signal to the USB device. The signaling rate of the USB device is determined during the reset signaling. After reset, the USB device's information is read by the host and the device is assigned a unique 7-bit address. If the device is supported by the host, the device drivers needed for communicating with the device are loaded and the device is set to a configured state. If the USB host is restarted, the enumeration process is repeated for all connected devices.

The host controller directs traffic flow to devices, so no USB device can transfer any data on the bus without an explicit request from the host controller. In USB 2.0, the host controller polls the bus for traffic, usually in a round-robin fashion. The throughput of each USB port is determined by the slower speed of either the USB port or the USB device connected to the port.

High-speed USB 2.0 hubs contain devices called transaction translators that convert between high-speed USB 2.0 buses and full and low speed buses. There may be one translator per hub or per port.

Because there are two separate controllers in each USB 3.0 host, USB 3.0 devices transmit and receive at USB 3.0 signaling rates regardless of USB 2.0 or earlier devices connected to that host. Operating signaling rates for earlier devices are set in the legacy manner.

Device classes

The functionality of a USB device is defined by a class code sent to a USB host. This allows the host to load software modules for the device and to support new devices from different manufacturers.

Device classes include:

Class
(hexadecimal) 		Usage Description Examples, or exception
00 Device Unspecified Device class is unspecified, interface descriptors are used to determine needed drivers
01 Interface Audio Speaker, microphone, sound card, MIDI
02 Both Communications and CDC control UART and RS-232 serial adapter, modem, Wi-Fi adapter, Ethernet adapter. Used together with class 0Ah (CDC-Data) below
03 Interface Human interface device (HID) Keyboard, mouse, joystick
05 Interface Physical interface device (PID) Force feedback joystick
06 Interface Media (PTP/MTP) Scanner, Camera
07 Interface Printer Laser printer, inkjet printer, CNC machine
08 Interface USB mass storage, USB Attached SCSI USB flash drive, memory card reader, digital audio player, digital camera, external drive
09 Device USB hub High speed USB hub
0A Interface CDC-Data Used together with class 02h (Communications and CDC Control) above
0B Interface Smart card USB smart card reader
0D Interface Content security Fingerprint reader
0E Interface Video Webcam
0F Interface Personal healthcare device class (PHDC) Pulse monitor (watch)
10 Interface Audio/Video (AV) Webcam, TV
11 Device Billboard Describes USB-C alternate modes supported by device
DC Both Diagnostic device USB compliance testing device
E0 Interface Wireless Controller Bluetooth adapter
EF Both Miscellaneous ActiveSync device
FE Interface Application-specific IrDA Bridge, RNDIS, Test & Measurement Class (USBTMC), USB DFU (Device Firmware Upgrade)
FFh Both Vendor-specific Indicates that a device needs vendor-specific drivers

USB mass storage / USB drive

See also: USB mass storage device class, Disk enclosure, and External hard disk drive
A flash drive, a typical USB mass-storage device
An M.2 (2242) solid-state-drive (SSD) connected into USB 3.0 adapter and connected to computer

The USB mass storage device class (MSC or UMS) standardizes connections to storage devices. At first intended for magnetic and optical drives, it has been extended to support flash drives and SD card readers. The ability to boot a write-locked SD card with a USB adapter is particularly advantageous for maintaining the integrity and non-corruptible, pristine state of the booting medium.

Though most personal computers since early 2005 can boot from USB mass storage devices, USB is not intended as a primary bus for a computer's internal storage. However, USB has the advantage of allowing hot-swapping, making it useful for mobile peripherals, including drives of various kinds.

Several manufacturers offer external portable USB hard disk drives, or empty enclosures for disk drives. These offer performance comparable to internal drives, limited by the number and types of attached USB devices, and by the upper limit of the USB interface. Other competing standards for external drive connectivity include eSATA, ExpressCard, FireWire (IEEE 1394), and most recently Thunderbolt.

Another use for USB mass storage devices is the portable execution of software applications (such as web browsers and VoIP clients) with no need to install them on the host computer.

Media Transfer Protocol

See also: Picture Transfer Protocol

Media Transfer Protocol (MTP) was designed by Microsoft to give higher-level access to a device's filesystem than USB mass storage, at the level of files rather than disk blocks. It also has optional DRM features. MTP was designed for use with portable media players, but it has since been adopted as the primary storage access protocol of the Android operating system from the version 4.1 Jelly Bean as well as Windows Phone 8 (Windows Phone 7 devices had used the Zune protocol—an evolution of MTP). The primary reason for this is that MTP does not require exclusive access to the storage device the way UMS does, alleviating potential problems should an Android program request the storage while it is attached to a computer. The main drawback is that MTP is not as well supported outside of Windows operating systems.

Human interface devices

Main article: USB human interface device class

A USB mouse or keyboard can usually be used with older computers that have PS/2 ports with the aid of a small USB-to-PS/2 adapter. For mice and keyboards with dual-protocol support, a passive adapter that contains no logic circuitry may be used: the USB hardware in the keyboard or mouse is designed to detect whether it is connected to a USB or PS/2 port, and communicate using the appropriate protocol. Active converters that connect USB keyboards and mice (usually one of each) to PS/2 ports also exist.

Device Firmware Upgrade mechanism

Device Firmware Upgrade (DFU) is a generic mechanism for upgrading the firmware of USB devices with improved versions provided by their manufacturers, offering (for example) a way to deploy firmware bug fixes. During the firmware upgrade operation, USB devices change their operating mode effectively becoming a PROM programmer. Any class of USB device can implement this capability by following the official DFU specifications. Doing so allows use of DFU-compatible host tools to update the device.

DFU is sometimes used as a flash memory programming protocol in microcontrollers with built-in USB bootloader functionality.

Audio streaming

The USB Device Working Group has laid out specifications for audio streaming, and specific standards have been developed and implemented for audio class uses, such as microphones, speakers, headsets, telephones, musical instruments, etc. The working group has published three versions of audio device specifications: USB Audio 1.0, 2.0, and 3.0, referred to as "UAC" or "ADC".

UAC 3.0 primarily introduces improvements for portable devices, such as reduced power usage by bursting the data and staying in low power mode more often, and power domains for different components of the device, allowing them to be shut down when not in use.

UAC 2.0 introduced support for High Speed USB (in addition to Full Speed), allowing greater bandwidth for multi-channel interfaces, higher sample rates, lower inherent latency, and 8× improvement in timing resolution in synchronous and adaptive modes. UAC2 also introduced the concept of clock domains, which provides information to the host about which input and output terminals derive their clocks from the same source, as well as improved support for audio encodings like DSD, audio effects, channel clustering, user controls, and device descriptions.

UAC 1.0 devices are still common, however, due to their cross-platform driverless compatibility, and also partly due to Microsoft's failure to implement UAC 2.0 for over a decade after its publication, having finally added support to Windows 10 through the Creators Update on 20 March 2017. UAC 2.0 is also supported by macOS, iOS, and Linux, however Android only implements a subset of the UAC 1.0 specification.

USB provides three isochronous (fixed-bandwidth) synchronization types, all of which are used by audio devices:

  • Asynchronous — The ADC or DAC are not synced to the host computer's clock at all, operating off a free-running clock local to the device.
  • Synchronous — The device's clock is synced to the USB start-of-frame (SOF) or Bus Interval signals. For instance, this can require syncing an 11.2896 MHz clock to a 1 kHz SOF signal, a large frequency multiplication.
  • Adaptive — The device's clock is synced to the amount of data sent per frame by the host

While the USB spec originally described asynchronous mode being used in "low cost speakers" and adaptive mode in "high-end digital speakers", the opposite perception exists in the hi-fi world, where asynchronous mode is advertised as a feature, and adaptive/synchronous modes have a bad reputation. In reality, all types can be high-quality or low-quality, depending on the quality of their engineering and the application. Asynchronous has the benefit of being untied from the computer's clock, but the disadvantage of requiring sample rate conversion when combining multiple sources.

Connectors

Main article: USB hardware § Connectors

The connectors the USB committee specifies support a number of USB's underlying goals, and reflect lessons learned from the many connectors the computer industry has used. The female connector mounted on the host or device is called the receptacle, and the male connector attached to the cable is called the plug. The official USB specification documents also periodically define the term male to represent the plug, and female to represent the receptacle.

USB Type-A plug
The legacy USB Type-A plug. This is one of many legacy types of USB connector.

The design is intended to make it difficult to insert a USB plug into its receptacle incorrectly. The USB specification requires that the cable plug and receptacle be marked so the user can recognize the proper orientation. The USB-C plug however is reversible. USB cables and small USB devices are held in place by the gripping force from the receptacle, with no screws, clips, or thumb-turns as some connectors use.

The different A and B plugs prevent accidentally connecting two power sources. However, some of this directed topology is lost with the advent of multi-purpose USB connections (such as USB On-The-Go in smartphones, and USB-powered Wi-Fi routers), which require A-to-A, B-to-B, and sometimes Y/splitter cables.

USB connector types multiplied as the specification progressed. The original USB specification detailed standard-A and standard-B plugs and receptacles. The connectors were different so that users could not connect one computer receptacle to another. The data pins in the standard plugs are recessed compared to the power pins, so that the device can power up before establishing a data connection. Some devices operate in different modes depending on whether the data connection is made. Charging docks supply power, and do not include a host device or data pins, allowing any capable USB device to charge or operate from a standard USB cable. Charging cables provide power connections but not data. In a charge-only cable, the data wires are shorted at the device end; otherwise, the device may reject the charger as unsuitable.

Cabling

Main article: USB hardware § Cabling
A variety of USB cables for sale in Hong Kong

The USB 1.1 standard specifies that a standard cable can have a maximum length of 5 meters (16 ft 5 in) with devices operating at full speed (12 Mbit/s), and a maximum length of 3 meters (9 ft 10 in) with devices operating at low speed (1.5 Mbit/s).

USB 2.0 provides for a maximum cable length of 5 meters (16 ft 5 in) for devices running at high speed (480 Mbit/s).

The USB 3.0 standard does not directly specify a maximum cable length, requiring only that all cables meet an electrical specification: for copper cabling with AWG 26 wires the maximum practical length is 3 meters (9 ft 10 in).

USB bridge cables

USB bridge cables, or data transfer cables can be found within the market, offering direct PC to PC connections. A bridge cable is a special cable with a chip and active electronics in the middle of the cable. The chip in the middle of the cable acts as a peripheral to both computers and allows for peer-to-peer communication between the computers. The USB bridge cables are used to transfer files between two computers via their USB ports.

Popularized by Microsoft as Windows Easy Transfer, the Microsoft utility used a special USB bridge cable to transfer personal files and settings from a computer running an earlier version of Windows to a computer running a newer version. In the context of the use of Windows Easy Transfer software, the bridge cable can sometimes be referenced as Easy Transfer cable.

Many USB bridge / data transfer cables are still USB 2.0, but there are also a number of USB 3.0 transfer cables. Despite USB 3.0 being 10 times faster than USB 2.0, USB 3.0 transfer cables are only 2 to 3 times faster given their design.

The USB 3.0 specification introduced an A-to-A cross-over cable without power for connecting two PCs. These are not meant for data transfer but are aimed at diagnostic uses.

Dual-role USB connections

USB bridge cables have become less important with USB dual-role-device capabilities introduced with the USB 3.1 specification. Under the most recent specifications, USB supports most scenarios connecting systems directly with a Type-C cable. For the capability to work, however, connected systems must support role-switching. Dual-role capabilities requires there be two controllers within the system, as well as a role controller. While this can be expected in a mobile platform such as a tablet or a phone, desktop PCs and laptops often will not support dual roles.

Power

Main article: USB hardware § Power

Upstream USB connectors supply power at a nominal 5 V DC via the V_BUS pin to downstream USB devices.

Low-power and high-power devices

This section describes the power distribution model of USB that existed before Power-Delivery (USB-PD). On devices that do not use PD, USB provides up to 4.5 W through Type-A and Type-B connectors, and up to 15 W through USB-C. All pre-PD USB power is provided at 5 V.

For a host providing power to devices, USB has a concept of the unit load. Any device may draw power of one unit, and devices may request more power in these discrete steps. It is not required that the host provide requested power, and a device may not draw more power than negotiated.

Devices that draw no more than one unit are said to be low-power devices. All devices must act as low-power devices when starting out as unconfigured. For USB devices up to USB 2.0 a unit load is 100 mA (or 500 mW), while USB 3.0 defines a unit load as 150 mA (750 mW). Full-featured USB-C can support low-power devices with a unit load of 250 mA (or 1250 mW).

Devices that draw more than one unit are high-power devices (such as typical 2.5-inch hard disk drives). USB up to 2.0 allows a host or hub to provide up to 2.5 W to each device, in five discrete steps of 100 mA, and SuperSpeed devices (USB 3.x) allows a host or a hub to provide up to 4.5 W in six steps of 150 mA. USB-C allows for dual-lane operation of USB 3.x with larger unit load (250 mA; up to 7.5 W). USB-C also allows for Type-C Current as a replacement for USB BC, signaling power availability in a simple way, without needing any data connection.

USB power standards
Specification max current Voltage max power
Low-power device 100 mA 5 V 0.50 W
Low-power SuperSpeed / USB 3.x device 150 mA 5 V 0.75 W
High-power device 500 mA 5 V 2.5 W
High-power SuperSpeed / USB 3.x single-lane device 900 mA 5 V 4.5 W
High-power SuperSpeed / USB 3.x dual-lane device 1.5 A 5 V 7.5 W
Battery Charging (BC) 1.5 A 5 V 7.5 W
USB4 1.5 A 5 V 7.5 W
Type-C current 1.5 A 1.5 A 5 V 7.5 W
Type-C current 3 A 3 A 5 V 15 W
Power Delivery SPR 5 A up to 20 V 100 W
Power Delivery EPR 5 A up to 48 V 240 W
  1. ^ The VBUS supply from a low-powered hub port may drop to 4.40 V.
  2. Up to five unit loads; with non-SuperSpeed devices, one unit load is 100 mA.
  3. Up to six unit loads; with SuperSpeed devices, one unit load is 150 mA.
  4. ^ for USB-C only
  5. Up to six unit loads; with multi-lane devices, one unit load is 250 mA.
  6. Not Type-C current, only available after starting USB4 connection. Can be combined with Type-C current.
  7. optional for every USB-C host port. Mandatory for USB-C ports with USB-BC or for even higher PD output.
  8. optional for every USB-C host port. Mandatory for ports with even higher PD output.
  9. ^ >3 A (>60 W) operation requires an electronically marked cable rated at 5 A.
  10. >20 V (>100 W) operation requires an electronically marked Extended Power Range (EPR) cable.

To recognize Battery Charging mode, a dedicated charging port places a resistance not exceeding 200 Ω across the D+ and D− terminals. Shorted or near-shorted data lanes with less than 200 Ω of resistance across the D+ and D− terminals signify a dedicated charging port (DCP) with indefinite charging rates.

In addition to standard USB, there is a proprietary high-powered system known as PoweredUSB, developed in the 1990s, and mainly used in point-of-sale terminals such as cash registers.

Signaling

Main article: USB (Communications) § Signaling (USB PHY)

USB signals are transmitted using differential signaling on twisted-pair data wires with 90 Ω ± 15% characteristic impedance. USB 2.0 and earlier specifications define a single pair in half-duplex (HDx). USB 3.0 and later specifications define one dedicated pair for USB 2.0 compatibility and two or four pairs for data transfer: two data wire pairs realising full-duplex (FDx) for single lane (×1) variants require at least SuperSpeed (SS) connectors; four pairs realising full-duplex for two lane (×2) variants require USB-C connectors.

USB4 Gen 4 requires the use of all four pairs but allow for asymmetrical pairs configuration. In this case one data wire pair is used for the upstream data and the other three for the downstream data or vice-versa. USB4 Gen 4 use pulse amplitude modulation on 3 levels, providing a trit of information every baud transmitted, the transmission frequency of 12.8 GHz translate to a transmission rate of 25.6 GBd and the 11-bit–to–7-trit translation provides a theoretical maximum transmission speed just over 40.2 Gbit/s.

Operation mode name Introduced in Lanes Encoding # data wires Nominal signaling rate Original label USB-IF current
current old marketing name logo
Low-Speed Does not appear USB 1.0 1 HDx NRZI 2 1.5 Mbit/s
half-duplex 		Low-Speed USB (LS) Basic-Speed USB
Full-Speed 12 Mbit/s
half-duplex 		Full-Speed USB (FS)
High-Speed USB 2.0 480 Mbit/s
half-duplex 		Hi-Speed USB (HS)
USB 3.2 Gen 1×1 USB 3.0,
USB 3.1 Gen 1 		USB 3.0 1 FDx (+ 1 HDx) 8b/10b 6 5 Gbit/s
symmetric 		SuperSpeed USB (SS) USB 5Gbps
USB 3.2 Gen 2×1 USB 3.1 Gen 2 USB 3.1 128b/132b 10 Gbit/s
symmetric 		SuperSpeed+ (SS+) USB 10Gbps
USB 3.2 Gen 1×2 Does not appear USB 3.2 2 FDx (+ 1 HDx) 8b/10b 10 10 Gbit/s
symmetric 		—
USB 3.2 Gen 2×2 128b/132b 20 Gbit/s
symmetric 		SuperSpeed USB 20Gbps USB 20Gbps
USB4 Gen 2×1 USB4 1 FDx (+ 1 HDx) 64b/66b 6 (used of 10) 10 Gbit/s
symmetric 		USB 10Gbps
USB4 Gen 2×2 2 FDx (+ 1 HDx) 10 20 Gbit/s
symmetric 		USB 20Gbps
USB4 Gen 3×1 1 FDx (+ 1 HDx) 128b/132b 6 (used of 10) 20 Gbit/s
symmetric
USB4 Gen 3×2 2 FDx (+ 1 HDx) 10 40 Gbit/s
symmetric 		USB 40Gbps
USB4 Gen 4×2 USB4 2.0 2 FDx (+ 1 HDx) PAM-3 11b/7t 10 80 Gbit/s
symmetric 		USB 80Gbps
asymmetric (+ 1 HDx) 40 Gbit/s up
120 Gbit/s down 		—
120 Gbit/s up
40 Gbit/s down
  1. ^ USB 2.0 implementation
  2. ^ USB4 can use optional Reed–Solomon forward error correction (RS FEC). In this mode, 12 × 16 B (128 bit) symbols are assembled together with 2 B (12 bit + 4 bit reserved) synchronization bits indicating the respective symbol types and 4 B of RS FEC to allow to correct up to 1 B of errors anywhere in the total 198 B block.
  • Low-speed (LS) and Full-speed (FS) modes use a single data wire pair, labeled D+ and D−, in half-duplex. Transmitted signal levels are 0.0–0.3 V for logical low, and 2.8–3.6 V for logical high level. The signal lines are not terminated.
  • High-speed (HS) uses the same wire pair, but with different electrical conventions. Lower signal voltages of −10 to 10 mV for low and 360 to 440 mV for logical high level, and termination of 45 Ω to ground or 90 Ω differential to match the data cable impedance.
  • SuperSpeed (SS) adds two additional pairs of shielded twisted data wires (and new, mostly compatible expanded connectors) besides another grounding wire. These are dedicated to full-duplex SuperSpeed operation. The SuperSpeed link operates independently from the USB 2.0 channel and takes precedence on connection. Link configuration is performed using LFPS (Low Frequency Periodic Signaling, approximately at 20 MHz frequency), and electrical features include voltage de-emphasis at the transmitter side, and adaptive linear equalization on the receiver side to combat electrical losses in transmission lines, and thus the link introduces the concept of link training.
  • SuperSpeed+ (SS+) uses a new coding scheme with an increased signaling rate (Gen 2×1 mode) and/or the additional lane of USB-C (Gen 1×2 and Gen 2×2 modes).

A USB connection is always between an A end, either a host or a downstream port of a hub, and a B end, either a peripheral device or the upstream port of a hub. Historically this was made clear by the fact that hosts had only Type-A and peripheral devices had only Type-B ports, and every compatible cable had one Type-A plug and one Type-B plug. USB-C (Type-C) is a single connector that replaces all legacy Type-A and Type-B connectors, so when both sides are equipment with USB Type-C ports they negotiate which is the host and which is the device.

Protocol layer

Main article: USB (Communications) § Protocol layer

During USB communication, data is transmitted as packets. Initially, all packets are sent from the host via the root hub, and possibly more hubs, to devices. Some of those packets direct a device to send some packets in reply.

Transactions

Main article: USB (Communications) § Transaction

The basic transactions of USB are:

  • OUT transaction
  • IN transaction
  • SETUP transaction
  • Control transfer exchange

Related standards

The Wireless USB logo

Media Agnostic USB

The USB Implementers Forum introduced the Media Agnostic USB (MA-USB) v.1.0 wireless communication standard based on the USB protocol on 29 July 2015. Wireless USB is a cable-replacement technology, and uses ultra-wideband wireless technology for data rates of up to 480 Mbit/s.

The USB-IF used WiGig Serial Extension v1.2 specification as its initial foundation for the MA-USB specification and is compliant with SuperSpeed USB (3.0 and 3.1) and Hi-Speed USB (USB 2.0). Devices that use MA-USB will be branded as "Powered by MA-USB", provided the product qualifies its certification program.

InterChip USB

Main article: InterChip USB

InterChip USB is a chip-to-chip variant that eliminates the conventional transceivers found in normal USB. The HSIC physical layer uses about 50% less power and 75% less board area compared to USB 2.0. It is an alternative standard to SPI and I2C.

USB-C

Main article: USB-C

USB-C (officially USB Type-C) is a standard that defines a new connector, and several new connection features. Among them it supports Alternate Mode, which allows transporting other protocols via the USB-C connector and cable. This is commonly used to support the DisplayPort or HDMI protocols, which allows connecting a display, such as a computer monitor or television set, via USB-C.

All other connectors are not capable of two-lane operations (Gen 1×2 and Gen 2×2) in USB 3.2, but can be used for one-lane operations (Gen 1×1 and Gen 2×1).

DisplayLink

Main article: DisplayLink

DisplayLink is a technology which allows multiple displays to be connected to a computer via USB. It was introduced around 2006, and before the advent of Alternate Mode over USB-C it was the only way to connect displays via USB. It is a proprietary technology, not standardized by the USB Implementers Forum and typically requires a separate device driver on the computer.

Comparisons with other connection methods

FireWire (IEEE 1394)

At first, USB was considered a complement to FireWire (IEEE 1394) technology, which was designed as a high-bandwidth serial bus that efficiently interconnects peripherals such as disk drives, audio interfaces, and video equipment. In the initial design, USB operated at a far lower data rate and used less sophisticated hardware. It was suitable for small peripherals such as keyboards and pointing devices.

The most significant technical differences between FireWire and USB include:

  • USB networks use a tiered-star topology, while IEEE 1394 networks use a tree topology.
  • USB 1.0, 1.1, and 2.0 use a "speak-when-spoken-to" protocol, meaning that each peripheral communicates with the host when the host specifically requests communication. USB 3.0 allows for device-initiated communications towards the host. A FireWire device can communicate with any other node at any time, subject to network conditions.
  • A USB network relies on a single host at the top of the tree to control the network. All communications are between the host and one peripheral. In a FireWire network, any capable node can control the network.
  • USB runs with a 5 V power line, while FireWire supplies 12 V and theoretically can supply up to 30 V.
  • Standard USB hub ports can provide from the typical 500 mA/2.5 W of current, only 100 mA from non-hub ports. USB 3.0 and USB On-The-Go supply 1.8 A/9.0 W (for dedicated battery charging, 1.5 A/7.5 W full bandwidth or 900 mA/4.5 W high bandwidth), while FireWire can in theory supply up to 60 watts of power, although 10 to 20 watts is more typical.

These and other differences reflect the differing design goals of the two buses: USB was designed for simplicity and low cost, while FireWire was designed for high performance, particularly in time-sensitive applications such as audio and video. Although similar in theoretical maximum signaling rate, FireWire 400 is faster than USB 2.0 high-bandwidth in real-use, especially in high-bandwidth use such as external hard drives. The newer FireWire 800 standard is twice as fast as FireWire 400 and faster than USB 2.0 high-bandwidth both theoretically and practically. However, FireWire's speed advantages rely on low-level techniques such as direct memory access (DMA), which in turn have created opportunities for security exploits such as the DMA attack.

The chipset and drivers used to implement USB and FireWire have a crucial impact on how much of the bandwidth prescribed by the specification is achieved in the real world, along with compatibility with peripherals.

Ethernet

The IEEE 802.3af, 802.3at, and 802.3bt Power over Ethernet (PoE) standards specify more elaborate power negotiation schemes than powered USB. They operate at 48 V DC and can supply more power (up to 12.95 W for 802.3af, 25.5 W for 802.3at, a.k.a. PoE+, 71 W for 802.3bt, a.k.a. 4PPoE) over a cable up to 100 meters compared to USB 2.0, which provides 2.5 W with a maximum cable length of 5 meters. This has made PoE popular for Voice over IP telephones, security cameras, wireless access points, and other networked devices within buildings. However, USB is cheaper than PoE provided that the distance is short and power demand is low.

Ethernet standards require electrical isolation between the networked device (computer, phone, etc.) and the network cable up to 1500 V AC or 2250 V DC for 60 seconds. USB has no such requirement as it was designed for peripherals closely associated with a host computer, and in fact it connects the peripheral and host grounds. This gives Ethernet a significant safety advantage over USB with peripherals such as cable and DSL modems connected to external wiring that can assume hazardous voltages under certain fault conditions.

MIDI

The USB Device Class Definition for MIDI Devices transmits Music Instrument Digital Interface (MIDI) music data over USB. The MIDI capability is extended to allow up to sixteen simultaneous virtual MIDI cables, each of which can carry the usual MIDI sixteen channels and clocks.

USB is competitive for low-cost and physically adjacent devices. However, Power over Ethernet and the MIDI plug standard have an advantage in high-end devices that may have long cables. USB can cause ground loop problems between equipment, because it connects ground references on both transceivers. By contrast, the MIDI plug standard and Ethernet have built-in isolation to 500V or more.

eSATA/eSATAp

The eSATA connector is a more robust SATA connector, intended for connection to external hard drives and SSDs. eSATA's transfer rate (up to 6 Gbit/s) is similar to that of USB 3.0 (up to 5 Gbit/s) and USB 3.1 (up to 10 Gbit/s). A device connected by eSATA appears as an ordinary SATA device, giving both full performance and full compatibility associated with internal drives.

eSATA does not supply power to external devices. This is an increasing disadvantage compared to USB. Even though USB 3.0's 4.5 W is sometimes insufficient to power external hard drives, technology is advancing, and external drives gradually need less power, diminishing the eSATA advantage. eSATAp (power over eSATA, a.k.a. ESATA/USB) is a connector introduced in 2009 that supplies power to attached devices using a new, backward compatible, connector. On a notebook eSATAp usually supplies only 5 V to power a 2.5-inch HDD/SSD; on a desktop workstation it can additionally supply 12 V to power larger devices including 3.5-inch HDD/SSD and 5.25-inch optical drives.

eSATAp support can be added to a desktop machine in the form of a bracket connecting the motherboard SATA, power, and USB resources.

eSATA, like USB, supports hot plugging, although this might be limited by OS drivers and device firmware.

Thunderbolt

Main article: Thunderbolt (interface)

Thunderbolt combines PCI Express and DisplayPort into a new serial data interface. Original Thunderbolt implementations have two channels, each with a transfer speed of 10 Gbit/s, resulting in an aggregate unidirectional bandwidth of 20 Gbit/s.

Thunderbolt 2 uses link aggregation to combine the two 10 Gbit/s channels into one bidirectional 20 Gbit/s channel.

Thunderbolt 3 and Thunderbolt 4 use USB-C. Thunderbolt 3 has two physical 20 Gbit/s bi-directional channels, aggregated to appear as a single logical 40 Gbit/s bi-directional channel. Thunderbolt 3 controllers can incorporate a USB 3.1 Gen 2 controller to provide compatibility with USB devices. They are also capable of providing DisplayPort Alternate Mode as well as DisplayPort over USB4 Fabric, making the function of a Thunderbolt 3 port a superset of that of a USB 3.1 Gen 2 port.

DisplayPort Alternate Mode 2.0: USB4 (requiring USB-C) requires that hubs support DisplayPort 2.0 over a USB-C Alternate Mode. DisplayPort 2.0 can support 8K resolution at 60 Hz with HDR10 color. DisplayPort 2.0 can use up to 80 Gbit/s, which is double the amount available to USB data, because it sends all the data in one direction (to the monitor) and can thus use all eight data wires at once.

After the specification was made royalty-free and custodianship of the Thunderbolt protocol was transferred from Intel to the USB Implementers Forum, Thunderbolt 3 has been effectively implemented in the USB4 specification – with compatibility with Thunderbolt 3 optional but encouraged for USB4 products.

Interoperability

Main article: USB-to-serial adapter

Various protocol converters are available that convert USB data signals to and from other communications standards.

Security threats

Due to the prevalency of the USB standard, there are many exploits using the USB standard. One of the biggest instances of this today is known as the USB killer, a device that damages USB devices by sending high voltage pulses across the data lines.

In versions of Microsoft Windows before Windows XP, Windows would automatically run a script (if present) on certain devices via AutoRun, one of which are USB mass storage devices, which may contain malicious software.

See also

USB

Derived and related standards

References

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