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Wikipedia

USB

November 13, 2023

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

Universal Serial Bus (USB) is an industry standard that allows data exchange and delivery of power between many various 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.[2] 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,[3] and prior 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.[4][5] It has 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.

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; 27 years ago (1996-01)
Produced Since May 1996[1]
Superseded Serial port, parallel port, game port, Apple Desktop Bus, PS/2 port, and FireWire (IEEE 1394)
Various legacy USB and related connectors (left to right): USB Micro-B plug, proprietary UC-E6 plug, USB Mini-B plug, Standard-A-like receptacle (non-compliant), Standard-A plug, Standard-B plug. Shown with a centimeter ruler.

As of 2023, 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.[2]

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.[2]

Each specification sub-version supports different signaling rates from 1.5 and 12 Mbit/s total in USB 1.0 to 80 Gbit/s (in each direction) in USB4.[4][6][7][2] 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 (USB Power Delivery (USB-PD)).[8] 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.[9]

Overview edit

USB was designed to standardize the connection of peripherals to personal computers, both to exchange data with and to supply electric power. It has largely replaced interfaces such as serial ports and parallel ports and has become commonplace on a wide range of devices. Examples of peripherals that are 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 as charging cables of 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, respectively, and the modern Type-C (USB-C) connector (which as of 2014 starts to replace all of the many legacy connectors and is 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, but 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 as well, including modern DisplayPort and Thunderbolt. It is reversible and can support various functionalities and protocols including USB; some are mandatory, many optional, depending on the type of the hardware: host, peripheral device, or hub.[10][11]

USB specifications provide backward compatibility, but usually results in a decrease in signalling rates, maximal power offered, and other provided capabilities. The USB 1.1 specification replaces USB 1.0. 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 also including the USB 2.0 specification. USB4 "functionally replaces" USB 3.2 while retaining USB 2.0 bus operating in parallel.[4][6][7][2]

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 Gbps; 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 4 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 Gbps; also known as Gen 2); for some time period marketed as SupeerSpeed+ (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. The SuperSpeed USB part of the system however 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 Fabrics.[7] As of 2023, they are somewhat rarely implemented; Intel, however, starts to include them in its 11th generation SoC processor models, but Apple never provided them. On the other hand, USB 3.2 Gen 1(x1) (5 Gbit/s) and Gen 2(x1) (10 Gbit/s) has been quite common for some years.

Connector type quick reference edit

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
2001 		USB 2.0
Revised 		USB 3.0
2008 		USB 3.1
2013 		USB 3.2
2017 		USB4
2019 		USB4 v2.0
2022
Max Speed Marketing name (operation mode) Low-Speed & Full-Speed High-Speed SuperSpeed USB 5Gbps, original: SuperSpeed (Gen 1) SuperSpeed USB 10Gbps, original: SuperSpeed+ (Gen 2) SuperSpeed USB 20Gbps (USB 3.2 Gen 2×2) USB4 40Gbps (USB4 Gen 3×2) USB4 80Gbps (USB4 Gen 4)
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        [rem 1] —
Standard-B      [rem 1] —
Mini-A [rem 2]   —
Mini-AB[rem 3][rem 4]   —
Mini-B   —
Micro-A[rem 5]  [rem 2] [rem 6]      [rem 1] —
Micro-AB[rem 3][rem 7]      [rem 1] —
Micro-B      [rem 1] —
Type-C (USB-C) [rem 6]  
(Enlarged to show detail)
Remarks:
  1. ^ a b c d e Limited to max speed at 10 Gbit/s, since only one-lane (Gen ×1) operation modes possible.
  2. ^ a b Backward compatibility given.
  3. ^ a b Only as receptacle.
  4. ^ Accepts both Mini-A and Mini-B plugs.
  5. ^ Only as plug.
  6. ^ a b Backward compatibility given by USB 2.0 implementation.
  7. ^ Accepts both Micro-A and Micro-B plugs.

Objectives edit

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.[12]

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.[13]
  • 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.[12]
  • 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.[12]

Limitations edit

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 table-top, 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; those 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 in, Dual-Role-Devices[14] 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.[12]

History edit

 
The basic USB trident logo[15]
 
USB logo on the head of a standard Type-A (Standard-A) plug

A group of seven companies began the development of USB in 1995:[16] 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.[17] Ajay Bhatt and his team worked on the standard at Intel;[18][19] the first integrated circuits supporting USB were produced by Intel in 1995.[20]

USB 1.x edit

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).[21] 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".[22][23][24]

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 edit

 
The Hi-Speed USB logo
 
A USB 2.0 PCI expansion card

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[25]) 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[26]).

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:[27]

  • 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:[28] 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 edit

Main article: USB 3.0
 
The 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.[29]

USB 3.0 adds a SuperSpeed operation mode, 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 bus 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 400 MB/s (3.2 Gbit/s) or more to transmit to an application.[30]: 4–19  Communication is full-duplex in SuperSpeed operation mode; earlier modes are half-duplex, arbitrated by the host.[31]

 
USB 3.2 Gen 1(x1), Standard-A (formerly known as USB 3.0; later renamed USB 3.1 Gen 1 – marketed as SuperSpeed USB (USB 3.0) and later (USB 3.2) as SuperSpeed USB 5Gbps)

Low-power and high-power devices remain operational with this standard, but devices using SuperSpeed can take advantage of increased available current of between 150 mA and 900 mA, respectively.[30]: 9–9 

USB 3.1, released in July 2013 has two variants. The first one preserves USB 3.0's SuperSpeed operation mode and is newly labeled USB 3.1 Gen 1,[32][33] and the second version introduces a new SuperSpeedPlus operation mode under the label of USB 3.1 Gen 2 (marketed as SuperSpeed+ USB). SuperSpeed+ doubles the maximum data signaling rate to 10 Gbit/s (later (USB 3.2) marketed as SuperSpeed USB 10Gbps), while reducing line encoding overhead to just 3% by changing the encoding scheme to 128b/132b.[32][34]

USB 3.2, released in September 2017,[35] preserves existing USB 3.1 SuperSpeed and SuperSpeedPlus operation modes but introduces two new SuperSpeedPlus operation modes with the new USB-C Fabric with signaling rates of 10 and 20 Gbit/s (nominal data rates of 1212 and 2424 MB/s). The increase in bandwidth is a result of multi-lane operation over existing wires that were intended for flip-flop capabilities of the USB-C connector.[36]

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

Naming scheme edit

Starting with the USB 3.2 specification, USB-IF introduced a new naming scheme.[37] To help companies with branding of the different operation modes, USB-IF recommended branding the 5, 10, and 20 Gbit/s operation modes as SuperSpeed USB 5Gbps, SuperSpeed USB 10Gbps, and SuperSpeed USB 20Gbps, respectively.[38].

USB4 edit

Main article: USB4
 
 
The Certified USB4 40Gbps logo and trident logo

The USB4 specification was released on 29 August 2019 by the USB Implementers Forum.[39]

The USB4 2.0 specification was released on 1 September 2022 by the USB Implementers Forum.[40]

USB4 is based on the Thunderbolt 3 protocol.[41] It supports 40 Gbit/s throughput, is compatible with Thunderbolt 3, and backward compatible with USB 3.2 and USB 2.0.[42][43] 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.

The USB4 specification states that the following technologies shall be supported by USB4:[39]

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 USB3 Gen T (5–80 Gbit/s) No No No A type of USB3 Tunneling architecture where the Enhanced SuperSpeed protocol 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
DisplayPort Yes Yes No The specification requires that hosts and hubs support the DisplayPort Alternate Mode.
Host-to-Host communications Yes Yes — A LAN-like connection between two peers.
PCI Express No Yes No The PCI Express function of USB4 replicates the functionality of previous versions of the Thunderbolt specification.
Thunderbolt 3 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.
Other Alternate Modes No No No USB4 products may optionally offer interoperability with the HDMI, MHL, and VirtualLink Alternate Modes.

During CES 2020, USB-IF and Intel stated their intention to allow USB4 products that support all the optional functionality as Thunderbolt 4 products. The first products compatible with USB4 are expected to be Intel's Tiger Lake series and AMD's Zen 3 series of CPU, released in 2020.

September 2022 naming scheme edit

 
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.)[disputed (for: USB4 20Gbps does not exist; USB4 2×2 is not interchangeable with USB 3.2 2×2 as indicated by the logo; logos for USB 3.x and USB4 are different.)  – discuss]

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 Gb/s.[44] 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 edit

Release versions edit

Name Release date Maximum signaling rate Note
USB 0.7 November 1994 ? Pre-release
USB 0.8 December 1994 ? Pre-release
USB 0.9 April 1995 12 Mbit/s: Full Speed (FS) Pre-release
USB 0.99 August 1995 ? Pre-release
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)
USB 1.1 August 1998
USB 2.0 April 2000 480 Mbit/s: High Speed (HS)
USB 3.0 November 2008 5 Gbit/s: SuperSpeed (SS) Also referred to as USB 3.1 Gen 1[32] and USB 3.2 Gen 1×1.
USB 3.1 July 2013 10 Gbit/s: SuperSpeed+ (SS+) Includes new USB 3.1 Gen 2,[32] also named USB 3.2 Gen 2×1 in later specification.
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.[45] Requires Full-Featured USB-C Fabrics.
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 Gbit/s Includes new 80 and 120 Gbit/s modes over Type-C connector.[46] Requires USB4 Fabric.

Power-related standards edit

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.[47]
USB Battery Charging Rev. 1.2 2010-12-07 7.5 W (5 V, 1.5 A) [48]
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) [49]
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) [50]
USB Power Delivery Rev. 3.1 (V. 1.1) 2021-07-06 240 W (48 V, 5 A) [51]
USB Power Delivery Rev. 3.1 (V. 1.2) 2021-10-26 240 W (48 V, 5 A) Including errata through October 2021[51]

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 edit

A USB system consists of a host with one or more downstream facing ports (DFP),[52] 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.[53][30]: 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 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.

 
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 is a connection from 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,[54] 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.

 
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 edit

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:[55]

Class
(hexadecimal) 		Usage Description Examples, or exception
00 Device Unspecified[56] 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, Microsoft RNDIS
EF Both Miscellaneous ActiveSync device
FE Interface Application-specific IrDA Bridge, Test & Measurement Class (USBTMC),[57] USB DFU (Device Firmware Upgrade)[58]
FFh Both Vendor-specific Indicates that a device needs vendor-specific drivers

USB mass storage / USB drive edit

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.[59][60]

Media Transfer Protocol edit

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 edit

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.[citation needed] Active converters that connect USB keyboards and mice (usually one of each) to PS/2 ports also exist.[61]

Device Firmware Upgrade mechanism edit

Device Firmware Upgrade (DFU) is a vendor- and device-independent 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.[58][62][63]

DFU can also give the user the freedom to flash USB devices with alternative firmware. One consequence of this is that USB devices after being re-flashed may act as various unexpected device types. For example, a USB device that the seller intends to be just a flash drive can "spoof" an input device like a keyboard. See BadUSB.[64]

Audio streaming edit

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:[65][66] USB Audio 1.0, 2.0, and 3.0, referred to as "UAC"[67] or "ADC".[68]

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.[69]

UAC 2.0 introduced support for High Speed USB (in addition to Full Speed), allowing greater bandwidth for multi-channel interfaces, higher sample rates,[70] lower inherent latency,[71][67] and 8× improvement in timing resolution in synchronous and adaptive modes.[67] 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.[67][72]

UAC 1.0 devices are still common, however, due to their cross-platform driverless compatibility,[70] 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.[73][74][72] UAC 2.0 is also supported by macOS, iOS, and Linux,[67] however Android only implements a subset of the UAC 1.0 specification.[75]

USB provides three isochronous (fixed-bandwidth) synchronization types,[76] all of which are used by audio devices:[77]

  • 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.[78][79]
  • Adaptive – The device's clock is synced to the amount of data sent per frame by the host[80]

While the USB spec originally described asynchronous mode being used in "low cost speakers" and adaptive mode in "high-end digital speakers",[81] 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.[82][83][75] In reality, all types can be high-quality or low-quality, depending on the quality of their engineering and the application.[79][67][84] 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 edit

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.[30]: 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.[85]

 
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.[30] 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 edit

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).[86][87][88]

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).[88]

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).[89]

USB bridge cables edit

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 - 3 times faster given their design.[clarification needed]

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 edit

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.[90]

Power edit

Main article: USB hardware § Power

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

Low-power and high-power devices edit

Note that this describes the power distribution model of USB that existed before Power-Delivery (USB-PD). Before PD, USB provided up to 7.5 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 Fabrics 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 3.0 and up) allows a host or a hub to provide up to 4.5 W in six steps of 150mA. USB-C supports high-power devices with up to 7.5 W, in six steps of 250 mA. Full-Featured USB-C Fabrics can support up to 15 W.

USB power standards
Specification Current Voltage Power (max.)
Low-power device 100 mA 5 V[a] 0.50 W
Low-power SuperSpeed (USB 3.0) device 150 mA 5 V[a] 0.75 W
High-power device 500 mA[b] 5 V 2.5 W
High-power SuperSpeed (USB 3.0) device 900 mA[c] 5 V 4.5 W
Multi-lane SuperSpeed (USB 3.2 Gen x2) device 1.5 A[d] 5 V 7.5 W
Battery Charging (BC) 1.1 1.5 A 5 V 7.5 W
Battery Charging (BC) 1.2 1.5 A 5 V 7.5 W
USB-C (multi-lane) 1.5 A[d] 5 V 7.5 W
3 A 5 V 15 W
Power Delivery 1.0/2.0/3.0 Type-C 5 A[e] 20 V 100 W
Power Delivery 3.1 Type-C 5 A[e] 48 V[f] 240 W
  1. ^ a b 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. ^ a b Up to six unit loads; with multi-lane devices, one unit load is 250 mA.
  5. ^ a b >3 A (>60 W) operation requires an electronically marked cable rated at 5 A.
  6. ^ >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.[91][92]

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 edit

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.[93] 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 pairs in full-duplex (FDx) for single lane variants require SuperSpeed connectors; four pairs in 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,[94] in this case one lane 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[95] and the 11-bit–to–7-trit translation provides a theoretical maximum transmission speed just over 40.2 Gbit/s.[96]

Operation mode Old name Introduced in Encoding Data wire pairs Nominal rate USB-IF marketing

name[97][98][99]

 Logo
Low-Speed Does not appear USB 1.0 NRZI 1 HDx 1.5 Mbit/s
half-duplex 		Basic-Speed USB  
Full-Speed 12 Mbit/s
half-duplex
High-Speed USB 2.0 480 Mbit/s
half-duplex 		Hi-Speed USB
USB 3.2 Gen 1×1 USB 3.0,
USB 3.1 Gen 1 		USB 3.0 8b/10b 2 FDx (+ 1 HDx)[a] 5 Gbit/s
symmetric 		SuperSpeed USB 5Gbps
(formerly SuperSpeed USB) 		 
USB 3.2 Gen 2×1 USB 3.1 Gen 2 USB 3.1 128b/132b 2 FDx (+ 1 HDx)[a] 10 Gbit/s
symmetric 		SuperSpeed USB 10Gbps  
USB 3.2 Gen 1×2 Does not appear USB 3.2 8b/10b 4 FDx (+ 1 HDx)[a] 10 Gbit/s
symmetric 		—
USB 3.2 Gen 2×2 128b/132b 4 FDx (+ 1 HDx)[a] 20 Gbit/s
symmetric 		SuperSpeed USB 20Gbps  
USB4 Gen 2×1 USB4 64b/66b[b] 2 FDx (+ 1 HDx)[a] 10 Gbit/s
symmetric 		—
USB4 Gen 2×2 64b/66b[b] 4 FDx (+ 1 HDx)[a] 20 Gbit/s
symmetric 		USB4 20Gbps  
USB4 Gen 3×1 128b/132b[b] 2 FDx (+ 1 HDx)[a] 20 Gbit/s
symmetric 		—
USB4 Gen 3×2 128b/132b[b] 4 FDx (+ 1 HDx)[a] 40 Gbit/s
symmetric 		USB4 40Gbps  
USB4 Gen 4 USB4 version 2.0 PAM-3 11b/7t 4 FDx (+ 1 HDx)[a] 80 Gbit/s
symmetric 		USB4 80Gbps
40 Gbit/s up
120 Gbit/s down
120 Gbit/s up
40 Gbit/s down
  1. ^ a b c d e f g h i USB 2.0 implementation
  2. ^ a b c d 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 USB 2.0 channel and takes a 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 transmitter side, and adaptive linear equalization on 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 ports and peripheral devices had only Type-B, and every (valid) cable had one Type-A plug and one Type-B plug. USB-C (Type-C) is a single connector that replaces all Type-A and Type-B connectors (legacy connectors), so when both sides are modern equipment with USB-C ports they negotiate which is the host (A) and which is the device (B).

Protocol layer edit

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 edit

Main article: USB (Communications) § Transaction

The basic transactions of USB are:

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

Related standards edit

 
The Wireless USB logo

Media Agnostic USB edit

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.[100]

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.[101]

InterChip USB edit

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.[102] It is an alternative standard to SPI and I2C.

USB-C edit

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 (Gen1×1 and Gen2×1).[103]

DisplayLink edit

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 edit

FireWire (IEEE 1394) edit

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,[104] especially in high-bandwidth use such as external hard drives.[105][106][107][108] The newer FireWire 800 standard is twice as fast as FireWire 400 and faster than USB 2.0 high-bandwidth both theoretically and practically.[109] 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.[110]

Ethernet edit

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 aka PoE+, 71 W for 802.3bt aka 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 VoIP 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.[111] 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.[112][113]

MIDI edit

The USB Device Class Definition for MIDI Devices transmits Music Instrument Digital Interface (MIDI) music data over USB.[114] 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 edit

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; aka 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 edit

Thunderbolt combines PCI Express and Mini 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.[115]

Thunderbolt 2 uses link aggregation to combine the two 10 Gbit/s channels into one bidirectional 20 Gbit/s channel.[116]

Thunderbolt 3 and Thunderbolt 4 use USB-C.[117][118][119] 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.[120] 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.[120]

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.[121]

Interoperability edit

Main article: USB adapter

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

Security threats edit

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 which damages 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.[122]

See also edit

USB edit

Derived and related standards edit

References edit

  1. ^ (PDF). Intel. May 1996. Archived from the original (PDF) on 13 March 2016. Retrieved 12 March 2016.
  2. ^ a b c d e "USB4® Specification v2.0" (ZIP) (Version 2.0 ed.). USB. 30 June 2023. Retrieved 23 October 2023.
  3. ^ "About USB-IF". USB Implementers Forum. Retrieved 27 April 2023.
  4. ^ a b c "Universal Serial Bus 3.1 Specification" (ZIP). USB Implementers Forum. Retrieved 27 April 2023.
  5. ^ "USB deserves more support". Business. Boston Globe Online. Simson. 31 December 1995. from the original on 6 April 2012. Retrieved 12 December 2011.
  6. ^ a b "Universal Serial Bus 2.0 Specification" (ZIP) (Revision 2.0 ed.). USB Implementers Forum. 27 April 2000. Retrieved 27 April 2023.
  7. ^ a b c "Universal Serial Bus 3.2 Specification" (ZIP) (Revision 1.1 ed.). USB Implementers Forum. June 2022. Retrieved 27 April 2023.
  8. ^ "Universal Serial Bus Power Delivery Specification Revision 3.0 Version 2.0a (Released)" (ZIP). USB Implementers Forum. Retrieved 27 April 2023.
  9. ^ "Universal Charging Solution". GSMA. 17 February 2009. from the original on 30 November 2011. Retrieved 12 December 2011.
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Further reading edit

  • Axelson, Jan (1 September 2006). USB Mass Storage: Designing and Programming Devices and Embedded Hosts (1st ed.). Lakeview Research. ISBN 978-1-931-44804-8.
  • ——— (1 December 2007). Serial Port Complete: COM Ports, USB Virtual COM Ports, and Ports for Embedded Systems (2nd ed.). Lakeview Research. ISBN 978-1-931-44806-2.
  • ——— (2015). USB Complete: The Developer's Guide (5th ed.). Lakeview Research. ISBN 978-1-931448-28-4.
  • Hyde, John (February 2001). USB Design by Example: A Practical Guide to Building I/O Devices (2nd ed.). Intel Press. ISBN 978-0-970-28465-5.
  • "Debugging USB 2.0 for Compliance: It's Not Just a Digital World" (PDF). Keysight Technologies. Technologies Application Note. Keysight (1382–3).

External links edit

 
Wikimedia Commons has media related to Universal Serial Bus.
 
The Wikibook Serial Programming:USB Technical Manual has a page on the topic of: USB connectors

General overview edit

  • Joel Johnson (29 May 2019). "The unlikely origins of USB, the port that changed everything". Fast Company.
  • Leigh, Peter (24 May 2020). Why Does USB Keep Changing? (video).
  • Parikh, Bijal. "USB (Universal Serial Bus): An Overview". Engineers Garage. WTWH Media. Retrieved 7 May 2022.
  • Barnatt, Christopher (25 September 2022). Explaining USB: From 1.0 to USB4 V2.0 (ExplainingComputers) (video).

Technical documents edit

  • "USB Implementers Forum (USB-IF)". USB.org.
  • "USB Document Library (USB 3.2, USB 2.0, Wireless USB, USB-C, USB Power Delivery)". USB.org.
  • "Universal Host Controller Interface (UHCI)" (PDF). Intel – via mit.edu.
  • . Pinouts guide. Archived from the original on 14 May 2016.
  • Muller, Henk (July 2012). "How To Create And Program USB Devices". Electronic Design.
  • Garney, John (June 1996). "An Analysis of Throughput Characteristics of Universal Serial Bus" (PDF).
  • Hershenhoren, Razi; Reznik, Omer (October 2010). (PDF). Archived from the original (PDF) on 4 August 2020. Retrieved 30 January 2019.
  • IEC 62680 (Universal Serial Bus interfaces for data and power):
    • IEC 62680-1.1:2015 - Part 1-1: Common components - USB Battery Charging Specification, Revision 1.2
    • IEC 62680-1-2:2018 - Part 1-2: Common components - USB Power Delivery specification
    • IEC 62680-1-3:2018 - Part 1-3: Common components - USB Type-C Cable and Connector Specification
    • IEC 62680-1-4:2018 - Part 1-4: Common components - USB Type-C Authentication Specification
    • IEC 62680-2-1:2015 - Part 2-1: Universal Serial Bus Specification, Revision 2.0
    • IEC 62680-2-2:2015 - Part 2-2: Micro-USB Cables and Connectors Specification, Revision 1.01
    • IEC 62680-2-3:2015 - Part 2-3: Universal Serial Bus Cables and Connectors Class Document Revision 2.0
    • IEC 62680-3-1:2017 - Part 3-1: Universal Serial Bus 3.1 Specification

November 13, 2023
this, article, about, computer, standard, other, uses, disambiguation, universal, serial, industry, standard, that, allows, data, exchange, delivery, power, between, many, various, types, electronics, specifies, architecture, particular, physical, interface, c. This article is about the computer bus standard For other uses see USB disambiguation Universal Serial Bus USB is an industry standard that allows data exchange and delivery of power between many various 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 2 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 3 and prior 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 4 5 It has 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 USBUniversal Serial BusThe current connector for USB Thunderbolt and other protocols USB C plug and receptacle shown TypeBusProduction historyDesignerCompaq DEC IBM Intel Microsoft NEC NortelDesignedJanuary 1996 27 years ago 1996 01 ProducedSince May 1996 1 SupersededSerial port parallel port game port Apple Desktop Bus PS 2 port and FireWire IEEE 1394 Various legacy USB and related connectors left to right USB Micro B plug proprietary UC E6 plug USB Mini B plug Standard A like receptacle non compliant Standard A plug Standard B plug Shown with a centimeter ruler As of 2023 update 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 2 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 2 Each specification sub version supports different signaling rates from 1 5 and 12 Mbit s total in USB 1 0 to 80 Gbit s in each direction in USB4 4 6 7 2 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 USB Power Delivery USB PD 8 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 9 Contents 1 Overview 1 1 Connector type quick reference 1 2 Objectives 1 3 Limitations 2 History 2 1 USB 1 x 2 2 USB 2 0 2 3 USB 3 x 2 3 1 Naming scheme 2 4 USB4 2 4 1 September 2022 naming scheme 2 5 Version history 2 5 1 Release versions 2 5 2 Power related standards 3 System design 4 Device classes 4 1 USB mass storage USB drive 4 2 Media Transfer Protocol 4 3 Human interface devices 4 4 Device Firmware Upgrade mechanism 4 5 Audio streaming 5 Connectors 6 Cabling 6 1 USB bridge cables 6 1 1 Dual role USB connections 7 Power 7 1 Low power and high power devices 8 Signaling 9 Protocol layer 10 Transactions 11 Related standards 11 1 Media Agnostic USB 11 2 InterChip USB 11 3 USB C 11 4 DisplayLink 12 Comparisons with other connection methods 12 1 FireWire IEEE 1394 12 2 Ethernet 12 3 MIDI 12 4 eSATA eSATAp 12 5 Thunderbolt 13 Interoperability 14 Security threats 15 See also 15 1 USB 15 2 Derived and related standards 16 References 17 Further reading 18 External links 18 1 General overview 18 2 Technical documentsOverview editUSB was designed to standardize the connection of peripherals to personal computers both to exchange data with and to supply electric power It has largely replaced interfaces such as serial ports and parallel ports and has become commonplace on a wide range of devices Examples of peripherals that are 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 as charging cables of 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 respectively and the modern Type C USB C connector which as of 2014 starts to replace all of the many legacy connectors and is 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 but 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 as well including modern DisplayPort and Thunderbolt It is reversible and can support various functionalities and protocols including USB some are mandatory many optional depending on the type of the hardware host peripheral device or hub 10 11 USB specifications provide backward compatibility but usually results in a decrease in signalling rates maximal power offered and other provided capabilities The USB 1 1 specification replaces USB 1 0 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 also including the USB 2 0 specification USB4 functionally replaces USB 3 2 while retaining USB 2 0 bus operating in parallel 4 6 7 2 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 Gbps 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 4 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 Gbps also known as Gen 2 for some time period marketed as SupeerSpeed 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 The SuperSpeed USB part of the system however 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 Fabrics 7 As of 2023 update they are somewhat rarely implemented Intel however starts to include them in its 11th generation SoC processor models but Apple never provided them On the other hand USB 3 2 Gen 1 x1 5 Gbit s and Gen 2 x1 10 Gbit s has been quite common for some years Connector type quick reference edit 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 01996 USB 1 11998 USB 2 02001 USB 2 0Revised USB 3 02008 USB 3 12013 USB 3 22017 USB42019 USB4 v2 02022Max Speed Marketing name operation mode Low Speed amp Full Speed High Speed SuperSpeed USB 5Gbps original SuperSpeed Gen 1 SuperSpeed USB 10Gbps original SuperSpeed Gen 2 SuperSpeed USB 20Gbps USB 3 2 Gen 2 2 USB4 40Gbps USB4 Gen 3 2 USB4 80Gbps USB4 Gen 4 Signaling rate 1 5 Mbit s amp 12 Mbit s 480 Mbit s 5 Gbit s 10 Gbit s 20 Gbit s 40 Gbit s 80 Gbit sConnector Standard A nbsp nbsp nbsp nbsp rem 1 Standard B nbsp nbsp nbsp rem 1 Mini A rem 2 nbsp Mini AB rem 3 rem 4 nbsp Mini B nbsp Micro A rem 5 rem 2 rem 6 nbsp nbsp nbsp rem 1 Micro AB rem 3 rem 7 nbsp nbsp nbsp rem 1 Micro B nbsp nbsp nbsp rem 1 Type C USB C rem 6 nbsp Enlarged to show detail Remarks a b c d e Limited to max speed at 10 Gbit s since only one lane Gen 1 operation modes possible a b Backward compatibility given a b Only as receptacle Accepts both Mini A and Mini B plugs Only as plug a b Backward compatibility given by USB 2 0 implementation Accepts both Micro A and Micro B plugs Objectives edit 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 12 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 13 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 12 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 12 Limitations edit 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 table top 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 those 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 in Dual Role Devices 14 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 12 History edit nbsp The basic USB trident logo 15 nbsp USB logo on the head of a standard Type A Standard A plugA group of seven companies began the development of USB in 1995 16 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 17 Ajay Bhatt and his team worked on the standard at Intel 18 19 the first integrated circuits supporting USB were produced by Intel in 1995 20 USB 1 x edit 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 21 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 22 23 24 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 edit nbsp The Hi Speed USB logo nbsp A USB 2 0 PCI expansion cardUSB 2 0 was released in April 2000 adding a higher maximum signaling rate of 480 Mbit s maximum theoretical data throughput 53 MByte s 25 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 26 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 27 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 28 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 stateUSB 3 x edit Main article USB 3 0 nbsp The SuperSpeed USB logoThe 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 29 USB 3 0 adds a SuperSpeed operation mode 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 bus 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 400 MB s 3 2 Gbit s or more to transmit to an application 30 4 19 Communication is full duplex in SuperSpeed operation mode earlier modes are half duplex arbitrated by the host 31 nbsp USB 3 2 Gen 1 x1 Standard A formerly known as USB 3 0 later renamed USB 3 1 Gen 1 marketed as SuperSpeed USB USB 3 0 and later USB 3 2 as SuperSpeed USB 5Gbps Low power and high power devices remain operational with this standard but devices using SuperSpeed can take advantage of increased available current of between 150 mA and 900 mA respectively 30 9 9 USB 3 1 released in July 2013 has two variants The first one preserves USB 3 0 s SuperSpeed operation mode and is newly labeled USB 3 1 Gen 1 32 33 and the second version introduces a new SuperSpeedPlus operation mode under the label of USB 3 1 Gen 2 marketed as SuperSpeed USB SuperSpeed doubles the maximum data signaling rate to 10 Gbit s later USB 3 2 marketed as SuperSpeed USB 10Gbps while reducing line encoding overhead to just 3 by changing the encoding scheme to 128b 132b 32 34 USB 3 2 released in September 2017 35 preserves existing USB 3 1 SuperSpeed and SuperSpeedPlus operation modes but introduces two new SuperSpeedPlus operation modes with the new USB C Fabric with signaling rates of 10 and 20 Gbit s nominal data rates of 1212 and 2424 MB s The increase in bandwidth is a result of multi lane operation over existing wires that were intended for flip flop capabilities of the USB C connector 36 USB 3 0 also introduced the USB Attached SCSI protocol UASP which provides generally faster transfer speeds than the BOT Bulk Only Transfer protocol Naming scheme edit Starting with the USB 3 2 specification USB IF introduced a new naming scheme 37 To help companies with branding of the different operation modes USB IF recommended branding the 5 10 and 20 Gbit s operation modes as SuperSpeed USB 5Gbps SuperSpeed USB 10Gbps and SuperSpeed USB 20Gbps respectively 38 USB4 edit Main article USB4 nbsp nbsp The Certified USB4 40Gbps logo and trident logo The USB4 specification was released on 29 August 2019 by the USB Implementers Forum 39 The USB4 2 0 specification was released on 1 September 2022 by the USB Implementers Forum 40 USB4 is based on the Thunderbolt 3 protocol 41 It supports 40 Gbit s throughput is compatible with Thunderbolt 3 and backward compatible with USB 3 2 and USB 2 0 42 43 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 The USB4 specification states that the following technologies shall be supported by USB4 39 Connection Mandatory for Remarkshost hub deviceUSB 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 NoTunneled USB 3 2 Gen 2 2 20 Gbit s No No NoTunneled USB3 Gen T 5 80 Gbit s No No No A type of USB3 Tunneling architecture where the Enhanced SuperSpeed protocol 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 lanesUSB4 Gen 3 20 or 40 Gbit s No Yes NoDisplayPort Yes Yes No The specification requires that hosts and hubs support the DisplayPort Alternate Mode Host to Host communications Yes Yes A LAN like connection between two peers PCI Express No Yes No The PCI Express function of USB4 replicates the functionality of previous versions of the Thunderbolt specification Thunderbolt 3 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 Other Alternate Modes No No No USB4 products may optionally offer interoperability with the HDMI MHL and VirtualLink Alternate Modes During CES 2020 USB IF and Intel stated their intention to allow USB4 products that support all the optional functionality as Thunderbolt 4 products The first products compatible with USB4 are expected to be Intel s Tiger Lake series and AMD s Zen 3 series of CPU released in 2020 September 2022 naming scheme edit nbsp 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 disputed for USB4 20Gbps does not exist USB4 2 2 is not interchangeable with USB 3 2 2 2 as indicated by the logo logos for USB 3 x and USB4 are different discuss 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 Gb s 44 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 edit Release versions edit Name Release date Maximum signaling rate NoteUSB 0 7 November 1994 Pre releaseUSB 0 8 December 1994 Pre releaseUSB 0 9 April 1995 12 Mbit s Full Speed FS Pre releaseUSB 0 99 August 1995 Pre releaseUSB 1 0 RC November 1995 Release CandidateUSB 1 0 January 1996 1 5 Mbit s Low Speed LS 12 Mbit s Full Speed FS USB 1 1 August 1998USB 2 0 April 2000 480 Mbit s High Speed HS USB 3 0 November 2008 5 Gbit s SuperSpeed SS Also referred to as USB 3 1 Gen 1 32 and USB 3 2 Gen 1 1 USB 3 1 July 2013 10 Gbit s SuperSpeed SS Includes new USB 3 1 Gen 2 32 also named USB 3 2 Gen 2 1 in later specification 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 45 Requires Full Featured USB C Fabrics 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 Gbit s Includes new 80 and 120 Gbit s modes over Type C connector 46 Requires USB4 Fabric Power related standards edit Release name Release date Max power NoteUSB 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 47 USB Battery Charging Rev 1 2 2010 12 07 7 5 W 5 V 1 5 A 48 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 specificationUSB 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 49 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 50 USB Power Delivery Rev 3 1 V 1 1 2021 07 06 240 W 48 V 5 A 51 USB Power Delivery Rev 3 1 V 1 2 2021 10 26 240 W 48 V 5 A Including errata through October 2021 51 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 APDOSystem design editA USB system consists of a host with one or more downstream facing ports DFP 52 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 53 30 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 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 nbsp 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 is a connection from 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 54 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 nbsp Two USB 3 0 Standard A receptacles left and two USB 2 0 Standard A receptacles right on a computer s front panelEndpoints 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 editThe 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 55 Class hexadecimal Usage Description Examples or exception00 Device Unspecified 56 Device class is unspecified interface descriptors are used to determine needed drivers01 Interface Audio Speaker microphone sound card MIDI02 Both Communications and CDC control UART and RS 232 serial adapter modem Wi Fi adapter Ethernet adapter Used together with class 0Ah CDC Data below03 Interface Human interface device HID Keyboard mouse joystick05 Interface Physical interface device PID Force feedback joystick06 Interface Media PTP MTP Scanner Camera07 Interface Printer Laser printer inkjet printer CNC machine08 Interface USB mass storage USB Attached SCSI USB flash drive memory card reader digital audio player digital camera external drive09 Device USB hub High speed USB hub0A Interface CDC Data Used together with class 02h Communications and CDC Control above0B Interface Smart card USB smart card reader0D Interface Content security Fingerprint reader0E Interface Video Webcam0F Interface Personal healthcare device class PHDC Pulse monitor watch 10 Interface Audio Video AV Webcam TV11 Device Billboard Describes USB C alternate modes supported by deviceDC Both Diagnostic device USB compliance testing deviceE0 Interface Wireless Controller Bluetooth adapter Microsoft RNDISEF Both Miscellaneous ActiveSync deviceFE Interface Application specific IrDA Bridge Test amp Measurement Class USBTMC 57 USB DFU Device Firmware Upgrade 58 FFh Both Vendor specific Indicates that a device needs vendor specific driversUSB mass storage USB drive edit See also USB mass storage device class Disk enclosure and External hard disk drive nbsp A flash drive a typical USB mass storage device nbsp An M 2 2242 solid state drive SSD connected into USB 3 0 adapter and connected to computerThe 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 59 60 Media Transfer Protocol edit 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 edit 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 citation needed Active converters that connect USB keyboards and mice usually one of each to PS 2 ports also exist 61 Device Firmware Upgrade mechanism edit Device Firmware Upgrade DFU is a vendor and device independent 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 58 62 63 DFU can also give the user the freedom to flash USB devices with alternative firmware One consequence of this is that USB devices after being re flashed may act as various unexpected device types For example a USB device that the seller intends to be just a flash drive can spoof an input device like a keyboard See BadUSB 64 Audio streaming edit 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 65 66 USB Audio 1 0 2 0 and 3 0 referred to as UAC 67 or ADC 68 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 69 UAC 2 0 introduced support for High Speed USB in addition to Full Speed allowing greater bandwidth for multi channel interfaces higher sample rates 70 lower inherent latency 71 67 and 8 improvement in timing resolution in synchronous and adaptive modes 67 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 67 72 UAC 1 0 devices are still common however due to their cross platform driverless compatibility 70 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 73 74 72 UAC 2 0 is also supported by macOS iOS and Linux 67 however Android only implements a subset of the UAC 1 0 specification 75 USB provides three isochronous fixed bandwidth synchronization types 76 all of which are used by audio devices 77 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 78 79 Adaptive The device s clock is synced to the amount of data sent per frame by the host 80 While the USB spec originally described asynchronous mode being used in low cost speakers and adaptive mode in high end digital speakers 81 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 82 83 75 In reality all types can be high quality or low quality depending on the quality of their engineering and the application 79 67 84 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 editMain 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 30 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 85 nbsp 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 30 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 editMain article USB hardware Cabling nbsp A variety of USB cables for sale in Hong KongThe 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 86 87 88 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 88 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 89 USB bridge cables edit 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 3 times faster given their design clarification needed 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 edit 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 90 Power editMain article USB hardware Power Upstream USB connectors supply power at a nominal 5V DC via the V BUS pin to downstream USB devices Low power and high power devices edit Note that this describes the power distribution model of USB that existed before Power Delivery USB PD Before PD USB provided up to 7 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 Fabrics 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 0 and up allows a host or a hub to provide up to 4 5 W in six steps of 150mA USB C supports high power devices with up to 7 5 W in six steps of 250 mA Full Featured USB C Fabrics can support up to 15 W USB power standards Specification Current Voltage Power max Low power device 100 mA 5 V a 0 50 WLow power SuperSpeed USB 3 0 device 150 mA 5 V a 0 75 WHigh power device 500 mA b 5 V 2 5 WHigh power SuperSpeed USB 3 0 device 900 mA c 5 V 4 5 WMulti lane SuperSpeed USB 3 2 Gen x2 device 1 5 A d 5 V 7 5 WBattery Charging BC 1 1 1 5 A 5 V 7 5 WBattery Charging BC 1 2 1 5 A 5 V 7 5 WUSB C multi lane 1 5 A d 5 V 7 5 W3 A 5 V 15 WPower Delivery 1 0 2 0 3 0 Type C 5 A e 20 V 100 WPower Delivery 3 1 Type C 5 A e 48 V f 240 W a b The VBUS supply from a low powered hub port may drop to 4 40 V Up to five unit loads with non SuperSpeed devices one unit load is 100 mA Up to six unit loads with SuperSpeed devices one unit load is 150 mA a b Up to six unit loads with multi lane devices one unit load is 250 mA a b gt 3 A gt 60 W operation requires an electronically marked cable rated at 5 A gt 20 V gt 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 W across the D and D terminals Shorted or near shorted data lanes with less than 200 W of resistance across the D and D terminals signify a dedicated charging port DCP with indefinite charging rates 91 92 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 editMain article USB Communications Signaling USB PHY USB signals are transmitted using differential signaling on twisted pair data wires with 90 W 15 characteristic impedance 93 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 pairs in full duplex FDx for single lane variants require SuperSpeed connectors four pairs in 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 94 in this case one lane 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 95 and the 11 bit to 7 trit translation provides a theoretical maximum transmission speed just over 40 2 Gbit s 96 Operation mode Old name Introduced in Encoding Data wire pairs Nominal rate USB IF marketing name 97 98 99 LogoLow Speed Does not appear USB 1 0 NRZI 1 HDx 1 5 Mbit shalf duplex Basic Speed USB nbsp Full Speed 12 Mbit shalf duplexHigh Speed USB 2 0 480 Mbit shalf duplex Hi Speed USBUSB 3 2 Gen 1 1 USB 3 0 USB 3 1 Gen 1 USB 3 0 8b 10b 2 FDx 1 HDx a 5 Gbit ssymmetric SuperSpeed USB 5Gbps formerly SuperSpeed USB nbsp USB 3 2 Gen 2 1 USB 3 1 Gen 2 USB 3 1 128b 132b 2 FDx 1 HDx a 10 Gbit ssymmetric SuperSpeed USB 10Gbps nbsp USB 3 2 Gen 1 2 Does not appear USB 3 2 8b 10b 4 FDx 1 HDx a 10 Gbit ssymmetric USB 3 2 Gen 2 2 128b 132b 4 FDx 1 HDx a 20 Gbit ssymmetric SuperSpeed USB 20Gbps nbsp USB4 Gen 2 1 USB4 64b 66b b 2 FDx 1 HDx a 10 Gbit ssymmetric USB4 Gen 2 2 64b 66b b 4 FDx 1 HDx a 20 Gbit ssymmetric USB4 20Gbps nbsp USB4 Gen 3 1 128b 132b b 2 FDx 1 HDx a 20 Gbit ssymmetric USB4 Gen 3 2 128b 132b b 4 FDx 1 HDx a 40 Gbit ssymmetric USB4 40Gbps nbsp USB4 Gen 4 USB4 version 2 0 PAM 3 11b 7t 4 FDx 1 HDx a 80 Gbit ssymmetric USB4 80Gbps40 Gbit s up120 Gbit s down120 Gbit s up40 Gbit s down a b c d e f g h i USB 2 0 implementation a b c d 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 W to ground or 90 W 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 USB 2 0 channel and takes a 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 transmitter side and adaptive linear equalization on 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 ports and peripheral devices had only Type B and every valid cable had one Type A plug and one Type B plug USB C Type C is a single connector that replaces all Type A and Type B connectors legacy connectors so when both sides are modern equipment with USB C ports they negotiate which is the host A and which is the device B Protocol layer editMain 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 editMain article USB Communications Transaction The basic transactions of USB are OUT transaction IN transaction SETUP transaction Control transfer exchangeRelated standards edit nbsp The Wireless USB logoMedia Agnostic USB edit 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 100 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 101 InterChip USB edit 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 0 102 It is an alternative standard to SPI and I2C USB C edit 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 Gen1 1 and Gen2 1 103 DisplayLink edit 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 editFireWire IEEE 1394 edit 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 104 especially in high bandwidth use such as external hard drives 105 106 107 108 The newer FireWire 800 standard is twice as fast as FireWire 400 and faster than USB 2 0 high bandwidth both theoretically and practically 109 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 110 Ethernet edit 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 aka PoE 71 W for 802 3bt aka 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 VoIP 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 111 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 112 113 MIDI edit The USB Device Class Definition for MIDI Devices transmits Music Instrument Digital Interface MIDI music data over USB 114 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 edit 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 aka 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 edit Thunderbolt combines PCI Express and Mini 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 115 Thunderbolt 2 uses link aggregation to combine the two 10 Gbit s channels into one bidirectional 20 Gbit s channel 116 Thunderbolt 3 and Thunderbolt 4 use USB C 117 118 119 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 120 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 120 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 121 Interoperability editMain article USB adapter Various protocol converters are available that convert USB data signals to and from other communications standards Security threats editDue 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 which damages 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 122 See also edit nbsp Electronics portalUSB edit USB hardware USB protocol USB C USB hub Extensible Host Controller Interface XHCI List of device bit rates Peripheral WebUSB Derived and related standards edit DockPort LIO Target Media Transfer Protocol Mobile High Definition Link Thunderbolt interface Windows Easy 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the original on 23 September 2016 Retrieved 7 May 2018 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 Plummer Gregg 20 September 2017 Ampliozone USB Audio Class 2 0 Support in Windows 10 FINALLY Ampliozone Archived from the original on 7 May 2018 Retrieved 7 May 2018 a b USB Digital Audio Android Open Source Project Retrieved 16 February 2023 Synchronous sub mode is not commonly used with audio because both host and peripheral are at the mercy of the USB clock 32 bit Atmel Microcontroller Application Note PDF Atmel Corporation 2011 Archived PDF from the original on 6 May 2016 Retrieved 13 April 2016 PCM2906C datasheet PDF Texas Instruments November 2011 Archived PDF from the original on 4 May 2018 Retrieved 4 May 2018 The PCM2906C employs SpAct architecture TI s unique system that recovers the audio clock from USB packet data Castor Perry Kendall October 2010 Designing Modern 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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 USB 2 0 Specification Engineering Change Notice ECN 1 Mini B connector PDF 20 October 2000 Archived PDF from the original on 12 April 2015 Retrieved 29 December 2014 via www usb org USB Cable Length Limitations PDF CablesPlusUSA com 3 November 2010 Archived from the original PDF on 11 October 2014 Retrieved 2 February 2014 What Is the Maximum Length of a USB Cable Techwalla com Archived from the original on 1 December 2017 Retrieved 18 November 2017 a b Cables and Long Haul Solutions USB 2 0 Frequently Asked Questions USB Implementers Forum Archived from the original on 18 January 2011 Retrieved 28 April 2019 Axelson Jan USB 3 0 Developers FAQ Archived from the original on 20 December 2016 Retrieved 20 October 2016 USB 3 1 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a Digital World PDF Keysight Technologies Technologies Application Note Keysight 1382 3 External links edit nbsp Wikimedia Commons has media related to Universal Serial Bus nbsp The Wikibook Serial Programming USB Technical Manual has a page on the topic of USB connectors General overview edit Joel Johnson 29 May 2019 The unlikely origins of USB the port that changed everything Fast Company Leigh Peter 24 May 2020 Why Does USB Keep Changing video Parikh Bijal USB Universal Serial Bus An Overview Engineers Garage WTWH Media Retrieved 7 May 2022 Barnatt Christopher 25 September 2022 Explaining USB From 1 0 to USB4 V2 0 ExplainingComputers video Technical documents edit USB Implementers Forum USB IF USB org USB Document Library USB 3 2 USB 2 0 Wireless USB USB C USB Power Delivery USB org Universal Host Controller Interface UHCI PDF Intel via mit edu USB 3 0 Standard A Standard B Powered B connectors Pinouts guide Archived from the original on 14 May 2016 Muller Henk July 2012 How To Create And Program USB Devices Electronic Design Garney John June 1996 An Analysis of Throughput Characteristics of Universal Serial Bus PDF Hershenhoren Razi Reznik Omer October 2010 USB 2 0 Protocol Engine PDF Archived from the original PDF on 4 August 2020 Retrieved 30 January 2019 IEC 62680 Universal Serial Bus interfaces for data and power IEC 62680 1 1 2015 Part 1 1 Common components USB Battery Charging Specification Revision 1 2 IEC 62680 1 2 2018 Part 1 2 Common components USB Power Delivery specification IEC 62680 1 3 2018 Part 1 3 Common components USB Type C Cable and Connector Specification IEC 62680 1 4 2018 Part 1 4 Common components USB Type C Authentication Specification IEC 62680 2 1 2015 Part 2 1 Universal Serial Bus Specification Revision 2 0 IEC 62680 2 2 2015 Part 2 2 Micro USB Cables and Connectors Specification Revision 1 01 IEC 62680 2 3 2015 Part 2 3 Universal Serial Bus Cables and Connectors Class Document Revision 2 0 IEC 62680 3 1 2017 Part 3 1 Universal Serial Bus 3 1 Specification Retrieved from https en wikipedia org w index php title USB amp oldid 1184756555, wikipedia, wiki, book, books, library,

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