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5 janvier 2011 3 05 /01 /janvier /2011 09:33

Universal Serial Bus (USB) is a specification to establish communication between devices and a host controller (usually personal computers), developed and invented by Ajay Bhatt while working for Intel. USB has effectively replaced a variety of interfaces such as serial and parallel ports  (Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .

USB can connect computer peripherals such as mice, keyboards, digital cameras, printers, personal media players, flash drives, Network Adapters, andexternal hard drives. For many of those devices, USB has become the standard connection method (Dell Studio XPS 1340 Battery) .

USB was designed for personal computers, but it has become commonplace on other devices such as smartphones, PDAs and video game consoles, and as a power cord. As of 2008, there are about 2 billion USB devices sold per year, and approximately 6 billion total sold to date (Dell Studio XPS 1640 Battery) .


The USB is a standard for peripheral devices. It began development in 1994 by a group of seven companies: Compaq, DEC, IBM, Intel, Microsoft, NEC andNortel. USB was intended to make it fundamentally easier to connect external devices to PCs by replacing the multitude of connectors at the back of PCs  (Dell Studio XPS 1640 Battery)     ,

addressing the usability issues of existing interfaces, and simplifying software configuration of all devices connected to USB, as well as permitting greater bandwidths for external devices. The first silicon for USB was made by Intel in 1995.

The USB 1.0 specification was introduced in January 1996. The original USB 1.0 specification had a data transfer rate of 12 Mbit/s (Dell Vostro 1710 Battery)   .

The first widely used version of USB was 1.1, which was released in September 1998. It allowed for a 12 Mbit/s data rate for higher-speed devices such as disk drives, and a lower 1.5 Mbit/s rate for low bandwidth devices such as joysticks.

The USB 2.0 specification was released in April 2000 and was standardized by the USB-IF at the end of 2001  (Dell KM958 battery)  .

Hewlett-Packard, Intel, Lucent Technologies(now Alcatel-Lucent following its merger with Alcatel in 2006), NEC and Philips jointly led the initiative to develop a higher data transfer rate, with the resulting specification achieving 480 Mbit/s, a fortyfold increase over 12 Mbit/s for the original USB 1.0  (Sony VGP-BPS13 battery)      .


A USB system has an asymmetric design, consisting of a host, a multitude of downstream USB ports, and multiple peripheral devices connected in a tiered-star topology. Additional USB hubs may be included in the tiers, allowing branching into a tree structure with up to five tier levels   (Sony VGP-BPS13/B battery)   .

A USB host may have multiple host controllers and each host controller may provide one or more USB ports. Up to 127 devices, including hub devices if present, may be connected to a single host controller.

USB devices are linked in series through hubs (Sony VGP-BPS13/S battery)      .

There always exists one hub known as the root hub, which is built into the host controller. So-called sharing hubs, which allow multiple computers to access the same peripheral device(s), also exist and work by switching access between PCs, either automatically or manually   (Sony VGP-BPS13A/B battery)   .

Sharing hubs are popular in small-office environments. In network terms, they converge rather than diverge branches.

A physical USB device may consist of several logical sub-devices that are referred to as device functions. A single device may provide several functions, for example, a webcam (video device function) with a built-in microphone (audio device function)  (Sony VGP-BPS13B/B battery)   .

Such a device is called a compound device in which each logical device is assigned a distinctive address by the host and all logical devices are connected to a built-in hub to which the physical USB wire is connected. A host assigns one and only one device address to a function   (Sony VGP-BPL9 battery)     .

USB endpoints actually 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, found on a device, and named an endpoint   (Sony VGP-BPS13B/B battery)    .

The term endpoint is occasionally incorrectly used for referring to the pipe. However, while an endpoint exists on the device permanently, a pipe is only formed when the host makes a connection to the endpoint. Therefore, when referring to a particular connection between a host and a USB device function, the term pipe should be used   Sony VGP-BPL15 battery .

A USB device can have up to 32 endpoints: 16 into the host controller and 16 out of the host controller. But, as one of the pipes is required to be of a bi-directional type (the default control pipe), and thus uses 2 endpoints, the theoretical maximum number of pipes is 31. USB devices seldom have this many endpoints   Dell Inspiron E1505 battery .

There are two types of pipes: stream and message pipes depending on the type of data transfer.

  • isochronous transfers: at some guaranteed data rate (often, but not necessarily, as fast as possible) but with possible data loss (e.g. realtime audio or video)   Dell Latitude E6400 battery .
  • interrupt transfers: devices that need guaranteed quick responses (bounded latency) (e.g. pointing devices and keyboards).
  • bulk transfers: large sporadic transfers using all remaining available bandwidth, but with no guarantees on bandwidth or latency (e.g. file transfers)   HP Pavilion dv6000 Battery .
  • control transfers: typically used for short, simple commands to the device, and a status response, used, for example, by the bus control pipe number 0         SONY VAIO VGN-FZ Battery .

A stream pipe is a uni-directional pipe connected to a uni-directional endpoint that transfers data using an isochronous, interrupt, or bulk transfer. A message pipe is a bi-directional pipe connected to a bi-directional endpoint that is exclusively used for controldata flow       SONY VAIO VGN-FZ18 Battery .

An endpoint is built into the USB device by the manufacturer and therefore exists permanently. An endpoint of a pipe is addressable with tuple (device_address, endpoint_number) as specified in a TOKEN packet that the host sends when it wants to start a data transfer session     SONY VAIO VGN-FZ180E Battery .

If the direction of the data transfer is from the host to the endpoint, an OUT packet (a specialization of a TOKEN packet) having the desired device address and endpoint number is sent by the host. If the direction of the data transfer is from the device to the host, the host sends an IN packet instead          SONY VAIO VGN-FZ220E Battery .

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 will be ignored         SONY VAIO VGN-FZ340E Battery .

Otherwise, it will be accepted and the data transaction can start. A bi-directional endpoint, on the other hand, accepts both IN and OUT packets.

Endpoints are grouped into interfaces and each interface is associated with a single device function   SONY VAIO VGN-FZ430E Battery .

An exception to this is endpoint zero, which is used for device configuration and which is not associated with any interface. A single device function composed of independently controlled interfaces is called acomposite device. A composite device only has a single device address because the host only assigns a device address to a function   SONY VAIO VGN-FZ460E Battery .

When a USB device is first connected to a USB host, the USB device enumeration process is started. The enumeration starts by sending a reset signal to the USB device. The data rate of the USB device is determined during the reset signaling. After reset, the USB device's information is read by the host and the device is assigned a unique 7-bit address  SONY VAIO VGN-FZ480E Battery .

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  SONY VAIO VGN-FZ4000 Battery .

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 slowest device connected to a controller sets the bandwidth of the interface   SONY VAIO VGN-FZ31E Battery .

For SuperSpeed USB (defined since USB 3.0), connected devices can request service from host. Because there are two separate controllers in each USB 3.0 host, USB 3.0 devices will transmit and receive at USB 3.0 data rates regardless of USB 2.0 or earlier devices connected to that host. Operating data rates for them will be set in the legacy manner  SONY VAIO VGN-FZ31B Battery .

Device classes

USB3 defines class codes used to identify a device’s functionality and to load a device driver based on that functionality. This enables every device driver writer to support devices from different manufacturers that comply with a given class code  SONY VAIO VGN-FZ31J Battery .

USB mass storage

USB implements connections to storage devices using a set of standards called the USB mass storage device class (referred to as MSC or UMS). This was initially intended for traditional magnetic and optical drives, but has been extended to support a wide variety of devices, particularly flash drives  SONY VAIO VGN-FZ31M Battery .

This generality is because many systems can be controlled with the familiar metaphor of file manipulation within directories (the process of making a novel device look like a familiar device is also known as extension)  SONY VAIO VGN-FZ31Z Battery .

The ability to boot a write-locked SD card with a USB adapter is particular advantageous for maintaining the integrity and non-corruptible, pristine state of the booting medium. A live USB OS, resident on a write-locked SD card is impervious to modification by computer viruses or ill-conditioned software   SONY VAIO VGN-FZ38M Battery .

Though most newer computers are capable of booting off USB mass storage devices, USB is not intended to be a primary bus for a computer's internal storage: buses such as Parallel ATA (PATA) (or IDE), Serial ATA (SATA), or SCSI fulfill that role in PC class computers   SONY VGP-BPS8 Battery .

However, USB has one important advantage in that it is possible to install and remove devices without rebooting the computer (hot-swapping), making it useful for mobile peripherals, including drives of various kinds. Originally conceived and still used today for optical storage devices (CD-RW drives, DVD drives, etc.)   SONY VGP-BPS13 Battery ,

several manufacturers offer external portable USBhard drives, or empty enclosures for disk drives, which offer performance comparable to internal drives, limited by the current number and type of attached USB devices and by the upper limit of the USB interface (in practice about 40 MB/s for USB 2.0 and potentially 400 MB/s or more for USB 3.0)  SONY VGP-BPS13/S Battery .

These external drives have typically included a "translating device" that bridges between a drive's interface (IDE, ATA, SATA, PATA, ATAPI, or even SCSI) to a USB interface port. Functionally, the drive appears to the user much like an internal drive   SONY VGP-BPS13A/B Battery .

Other competing standards for external drive connectivity include eSATA, ExpressCard (now at version 2.0), and FireWire (IEEE 1394).

Another use for USB mass storage devices is the portable execution of software applications (such as web browsers and VoIP clients) without requiring installation on the host computer   SONY VGP-BPS13B/B Battery .

Human interface devices (HIDs)

Mice and keyboards usually have USB connectors. These can be used with older computers that have PS/2 connectors with the aid of a small USB-to-PS/2 adapter. Such adaptors contain no logic circuitry: the hardware in the USB keyboard or mouse is designed to detect whether it is connected to a USB or PS/2 port, and communicate using the appropriate protocol  SONY VGP-BPS13A/S Battery.

Joysticks, keypads, tablets and other human-interface devices are also progressively migrating from MIDI, and PC game port connectors to USB      SONY VGP-BPS13AS Battery .

Physical appearance

The connectors specified by the USB committee were designed to support a number of USB's underlying goals, and to reflect lessons learned from the menagerie of connectors which have been used in the computer industry      Dell Inspiron 1320n Battery .

Usability and "upside down" connectors

  • It is by design difficult to attach a USB connector incorrectly. Most connectors cannot be plugged in upside down and it is clear from kinesthetic sensation of making a connection when the plug and socket are correctly mated    Dell Inspiron 1464 Battery .

However, it is not obvious whether the connector should be face up or face down, and thus it is often necessary to try the insertion both ways. The side of the connector on a USB cable or other product with the "USB Icon" (trident logo) should be "visible" to the user during the mating process  Dell Inspiron 1564 Battery .

Some manufacturers do not, however, make the trident logo on USB cables easily visible or detectable by touch. Additionally, Some computers such as the mac mini, have the ports vertically, in many Japanese computers and other devices, the trident logo on the cable or other USB device being inserted must be facing down   Dell Inspiron 1764 Battery ,

while in many American computers and devices the logo must be facing up. However, this is not always the case, such as with some Dell (an American company) computers     Dell Studio 1450 Battery .

Officially, the USB 2.0 specification states that the required USB Icon is to be "embossed" ("engraved" on the accompanying diagram) on the "topside" of the USB plug, which "provides easy user recognition and facilitates alignment during the mating process  Dell Studio 1457 Battery .

" The specification also shows that the "recommended" (optional) "Manufacturer's logo" ("engraved" on the diagram but not specified in the text) is on the opposite side of the USB Icon. The specification further states "The USB Icon is also located adjacent to each receptacle         Dell Latitude D610 Battery .

Receptacles should be oriented to allow the Icon on the plug to be visible during the mating process." However, the specification does not consider the height of the device compared to the eye level height of the user, so the side of the cable that is "visible" when mated to a computer on a desk can depend on whether the user is standing or kneeling    Toshiba NB100 Battery .

Although published eight years later, the USB 3.0 specification has similar wording, stating only "USB 3.0 receptacles should be orientated to allow the Icon on the plug to be visible during the mating process."

  • Only moderate insertion/removal force is needed (by specification)       Toshiba Satellite M65 battery .
  • USB cables and small USB devices are held in place by the gripping force from the receptacle (without need of the screws, clips, or thumbturns other connectors have required). The force needed to make or break a connection is modest, allowing connections to be made in awkward circumstances (i.e., behind a floor-mounted chassis, or from below) or by those with motor disabilities       Toshiba Satellite M60 battery .
  • This has the disadvantage of easily and unintentionally breaking connections that one has intended to be permanent in case of cable accident (e.g., tripping, or inadvertent tugging).
  • The standard connectors were deliberately intended to enforce the directed topology of a USB network: type A connectors on host devices that supply power and type B connectors on target devices that receive power  Dell Latitude D830 Battery .
  • This prevents users from accidentally connecting two USB power supplies to each other, which could lead to dangerously high currents, circuit failures, or even fire. USB does not support cyclical networks and the standard connectors from incompatible USB devices are themselves incompatible   Dell Latitude D620 Battery .
  • Unlike other communications systems (e.g. RJ-45 cabling) gender changers make little sense with USB and are almost never used, though cables with 2 standard type A plugs are commonly found in North American dollar stores  Dell Inspiron Mini 10 Battery .

USB extension cord


  • The standard connectors were designed to be robust. Many previous connector designs were fragile, specifying embedded component pins or other delicate parts which proved vulnerable to bending or breakage, even with the application of modest force   Sony VGN-FW11S Battery .
  • The electrical contacts in a USB connector are protected by an adjacent plastic tongue, and the entire connecting assembly is usually protected by an enclosing metal sheath. Although meant to reinforce the metal connections, the construction and materials used in the plastic tongue can actually decrease the life of the connector  Sony VGN-FW11M Battery ,
  • as some manufacturers use hard, brittle plastics that easily break when bent.
  • The connector construction always ensures that the external sheath on the plug makes contact with its counterpart in the receptacle before any of the four connectors within make electrical contact     Sony VGN-FW139E/H battery .
  • The external metallic sheath is typically connected to system ground, thus dissipating damaging static charges. This enclosure design also provides a (moderate) degree of protection from electromagnetic interference to the USB signal while it travels through the mated connector pair (the only location when the otherwise twisted data pair travels in parallel)  Dell Latitude E5400 Battery .
  • In addition, because of the required sizes of the power and common connections, they are made after the system ground but before the data connections. This type of staged make-break timing allows for electrically safe hot-swapping, a common practice in the design of connectors in the aerospace industry    Dell Latitude E4200 Battery .
  • The newer Micro-USB receptacles are designed for up to 10,000 cycles of insertion and removal between the receptacle and plug, compared to 1500 for the standard USB and 5000 for the Mini-USB receptacle. This is accomplished by adding a locking device and by moving the leaf-spring connector from the jack to the plug   Dell Vostro A840 Battery ,
  • so that the most-stressed part is on the cable side of the connection. This change was made so that the connector on the less expensive cable would bear the most wear instead of the more expensive micro-USB device.


eSATAp (eSATA/USB) combo port is compatible with USB devices     Dell Inspiron 300M Battery

  • The USB standard specifies relatively loose tolerances for compliant USB connectors to minimize physical incompatibilities in connectors from different vendors. To address a weakness present in some other connector standards, the USB specification also defines limits to the size of a connecting device in the area around its plug   Dell Studio 1737 battery .
  • This was done to prevent a device from blocking adjacent ports due to the size of the cable strain relief mechanism (usually molding integral with the cable outer insulation) at the connector. Compliant devices must either fit within the size restrictions or support a compliant extension cable which does       Dell Inspiron E1505 battery .
  • Two-way communication is also possible. In USB 3.0, full-duplex communications are done when using SuperSpeed (USB 3.0) transfer. In previous USB versions (i.e., 1.x or 2.0), all communication is half-duplex and directionally controlled by the hostDell RM791 battery .

In general, cables have only plugs (very few have a receptacle on one end, although extension cables with a standard A plug and jack are sold), and hosts and devices have only receptacles. Hosts almost universally have type-A receptacles, and devices one or another type-B variety        Dell XPS M1530 battery .

Type-A plugs mate only with type-A receptacles, and type-B with type-B; they are deliberately physically incompatible. However, an extension to USB standard specification called USB On-The-Go allows a single port to act as either a host or a device—chosen by which end of the cable plugs into the receptacle on the unit     Dell XPS M2010 battery .

Even after the cable is hooked up and the units are talking, the two units may "swap" ends under program control. This capability is meant for units such as PDAs in which the USB link might connect to a PC's host port as a device in one instance, yet connect as a host itself to a keyboard and mouse device in another instance     Acer Aspire One battery .

  • USB 3.0 receptacles are electrically compatible with USB 2.0 device plugs if they can physically match. Most combinations will work, but there are a few physical incompatibilities. However, only USB 3.0 Standard-A receptacles can accept USB 3.0 Standard-A device plugs    Toshiba Satellite P10 Battery .
  • eSATAp (eSATA/USB) port is also compatible with USB 2.0 devices

Cable plugs (USB 1.x/2.0)

Cables exist with pairs of plugs as indicated in the following table.

In addition to the above cable assemblies comprising two plugs, an "adapter" cable with a Micro-A plug and a Standard-A receptacle is compliant with USB specifications  SONY VGN-FZ210CE Battery .

Other combinations of connectors are not compliant. However, some older devices and cables with Mini-A connectors have been certified by USB-IF. The Mini-A connector has been deprecated: there will be no new certification of assemblies using Mini-A connector    Dell Precision M70 Battery .

Connector types

Types of USB connectors left to right:
• Micro-B plug
• Mini-B plug (8-pin)
• Mini-B plug (5-pin)
• Standard-A receptacle       Toshiba Satellite L305 Battery
• Standard-A plug
• Standard-B plug
(ruler in centimeters)      Toshiba Satellite T4900 Battery

There are several types of USB connectors, including some that have been added while the specification progressed. The original USB specification detailed Standard-A and Standard-B plugs and receptacles. The first engineering change notice to the USB 2.0 specification added Mini-B plugs and receptacles     Toshiba PA3399U-2BRS battery .

The data connectors in the Standard-A plug are actually recessed in the plug as compared to the outside power connectors. This permits the power to connect first which prevents data errors by allowing the device to power up first and then transfer the data. Some devices will operate in different modes depending on whether the data connection is made  Toshiba Satellite A200 Battery .

This difference in connection can be exploited by inserting the connector only partially. For example, some battery-powered MP3 players switch into file transfer mode and cannot play MP3 files while a USB plug is fully inserted, but can be operated in MP3 playback mode using USB power by inserting the plug only part way so that the power slots make contact while the data slots do notToshiba Satellite 1200 Battery .

This enables those devices to be operated in MP3 playback mode while getting power from the cable.

To reliably enable a charge-only feature, modern USB accessory peripherals now include charging cables that provide power connections to the host port but no data connections      Toshiba Satellite M300 Battery ,

and both home and vehicle charging docks are available that supply power from a converter device and do not include a host device and data pins, allowing any capable USB device to be charged and/or operated from a standard USB cable.

USB standard connectors

Standard type A

The Standard-A type of USB plug is a flattened rectangle which inserts into a "downstream-port" receptacle on the USB host, or a hub, and carries both power and data      Sony Vaio PCG-5G2L Battery .

This plug is frequently seen on cables that are permanently attached to a device, such as one connecting a keyboard or mouse to the computer via usb connection.

USB connections eventually wear out as the connection loosens through repeated plugging and unplugging. The lifetime of a USB-A male connector is approximately 1,500 connect/disconnect cycles      Sony Vaio PCG-5G3L Battery .

Standard type B

A Standard-B plug—which has a square shape with bevelled exterior corners—typically plugs into an "upstream receptacle" on a device that uses a removable cable, e.g. a printer. A Type B plug delivers power in addition to carrying data. On some devices, the Type B receptacle has no data connections, being used solely for accepting power from the upstream device Sony Vaio PCG-5J1L Battery .

This two-connector-type scheme (A/B) prevents a user from accidentally creating anelectrical loop.

USB Mini and Micro connectors

Various connectors have been used for smaller devices such as PDAs, mobile phones or digital cameras  Sony Vaio PCG-5K2L Battery.

These include the now-deprecated  (but standardized) Mini-A and the currently standard Mini-B,  Micro-A, and Micro-B connectors. The Mini-A and Mini-B plugs are approximately 3 by 7 mm       Sony Vaio PCG-5J2L Battery .

The micro-USB plugs have a similar width but approximately half the thickness, enabling their integration into thinner portable devices. The micro-A connector is 6.85 by 1.8 mm with a maximum overmold size of 11.7 by 8.5 mm. The micro-B connector is 6.85 by 1.8 mm with a maximum overmold size of 10.6 by 8.5 mm     Sony Vaio PCG-5K1L Battery .

The Micro-USB connector was announced by the USB-IF on January 4, 2007 . The Mini-A connector and the Mini-AB receptacle connector were deprecated on May 23, 2007 . As of February 2009, many currently available devices and cables still use Mini plugs, but the newer Micro connectors are being widely adopted and as of December 2010, the Micro connectors are the most widely usedSony Vaio PCG-5L1L Battery .

The thinner micro connectors are intended to replace the Mini plugs in new devices including smartphones and personal digital assistants. The Micro plug design is rated for at least 10,000 connect-disconnect cycles which is significantly more than the Mini plug design   Sony Vaio PCG-6S2L Battery .

The Universal Serial Bus Micro-USB Cables and Connectors Specification details the mechanical characteristics of Micro-A plugs, Micro-AB receptacles, and Micro-B plugs and receptacles, along with a Standard-A receptacle to Micro-A plug adapter  Sony Vaio PCG-6S3L Battery .

The cellular phone carrier group, Open Mobile Terminal Platform (OMTP) in 2007 have endorsed Micro-USB as the standard connector for data and power on mobile devices. These include various types of battery chargers, allowing Micro-USB to be the single external cable link needed by some devices    Sony Vaio PCG-6V1L Battery .

As of January 30, 2009 Micro-USB has been accepted and is being used by almost all cell phone manufacturers as the standard charging port (including HTC, Motorola, Nokia, LG, Hewlett-Packard, Samsung, Sony Ericsson, Research In Motion) in most of the world     Sony Vaio PCG-6W1L Battery .

On June 29, 2009, following a request from the European Commission and in close co-operation with the Commission services, major producers of mobile phones have agreed in a Memorandum of Understanding (“MoU”) to harmonise chargers for data-enabled mobile phones sold in the European Union     Sony Vaio PCG-6W2L Battery .

Industry commits to provide charger compatibility on the basis of the Micro-USB connector. Consumers will be able to purchase mobile phones without a charger, thus logically reducing their cost.  Following a mandate from the European Commission  Sony Vaio PCG-6W3L Battery ,

the European Standardisation Bodies CEN-CENELEC and ETSI have now made available the harmonised standards needed for the manufacture of data-enabled mobile phones compatible with the new common charger based on micro-USB  Sony Vaio PCG-7111L Battery .

In addition, on October 22, 2009 the International Telecommunication Union (ITU) has also announced that it had embraced micro-USB as the Universal Charger Solution its "energy-efficient one-charger-fits-all new mobile phone solution", and added: "Based on the Micro-USB interface     Sony Vaio PCG-7112L Battery ,

UCS chargers will also include a 4-star or higher efficiency rating—up to three times more energy-efficient than an unrated charger."

USB Micro-AB Socket OTG

An OTG device is required to have one, and only one USB connector: a Micro-AB receptacle as defined in [Micro-USB1.01]. This receptacle is capable of accepting either a Micro-A plug or a Micro-B plug attached to any of the legal cables and adapters defined in [Micro-USB1.01]     Sony Vaio PCG-7113L Battery .

The OTG device with the A-plug inserted is called the A-device and is responsible for powering the USB interface when required and by default assumes the role of host. The OTG device with the B-plug inserted is called the B-device and by default assumes the role of peripheral      Sony Vaio PCG-7133L Battery .

An OTG device with no plug inserted defaults to acting as a B-device. If an application on the B-device requires the role of host, then the HNP protocol is used to temporarily transfer the host role to the B-device        Sony Vaio PCG-7Z2L Battery .

OTG devices attached either to a peripheral-only B-device or a standard/embedded host will have their role fixed by the cable since in these scenarios it is only possible to attach the cable one way around       Sony Vaio PCG-8Y1L Battery .

Proprietary connectors and formats

  • Microsoft's original Xbox game console uses standard USB 1.1 signalling in its controllers and memory cards, but uses proprietary connectors and ports            Sony Vaio PCG-8Y2L Battery .
  • IBM UltraPort uses standard USB signalling, but via a proprietary connection format.
  • American Power Conversion uses USB signalling and HID device class on its uninterruptible power supplies using 10P10C connectors     Sony Vaio PCG-8Z1L Battery .

HTC manufactures Windows Mobile and Android-based Communicators which have a proprietary connector called HTC ExtUSB (Extended USB). ExtUSB combines mini-USB (with which it is backwards-compatible) with audio input as well as audio and video output in an 11-pin connector       Sony Vaio PCG-8Z2L Battery .

Nokia includes a USB connection as part of the Pop-Port connector on some older mobile phone models.

  • Sony Ericsson used a proprietary connector called FastPort from 2005 to 2009    Sony VAIO PCG-5G2L Battery .
  • The second, third, and fourth generation iPod Shuffle uses a TRRS connector to carry USB, audio, or power signals.
  • iriver added a fifth power pin within USB-A plugs for higher power and faster charging, used for the iriver U10 series. A mini-USB version contains a matching extra power pin for the cradle    Sony VAIO PCG-5G3L Battery .
  • Apple has shipped non-standard USB extension cables with some of their computers, for use with the included Apple USB keyboards. The extension cable's socket is keyed with a small protrusion to prevent the insertion of a standard USB plug, while the Apple USB keyboard's plug has a matching indentation      Sony VAIO PCG-5J1L Battery .
  • The indentation on the keyboard's plug does not interfere with insertion into a standard USB socket. Despite the keying, it is still possible to insert standard USB plugs into the extension cord. The protrusion can also be shaved off with an appropriate blade, or crushed with locking pliers       Sony VAIO PCG-5K2L Battery .
  • Apple also uses a proprietary USB 30-pin dock connector on its iPods, iPhones, and the iPad.
  • HP Tablet computers use non-standard connectors to transmit the USB signals between the keyboard/mouse unit and the Computer Tablet Unit            Sony VAIO PCG-5J2L Battery .
  • PDMI (Portable Digital Media Interface) is a 30-pin docking connector for portable devices standardized by ANSI/CEA which includes USB 3.0 "SuperSpeed" and USB 2.0 "High/Standard Speed" with USB-on-the-go, as well as DisplayPort, HDMI CEC, 5 V power, and analog audio     Sony VAIO PCG-5K1L Battery .
  • Some digital cameras have their own USB connectors, like the Panasonic Lumix DMC-FT2.


The maximum length of a standard USB cable (for USB 2.0 or earlier) is 5 metres (16.4 ft). The primary reason for this limit is the maximum allowed round-trip delay of about 1,500 ns        Sony VAIO PCG-5L1L Battery .

If USB host commands are unanswered by the USB device within the allowed time, the host considers the command lost. When adding USB device response time, delays from the maximum number of hubs added to the delays from connecting cables, the maximum acceptable delay per cable amounts to be 26 ns        Sony VAIO PCG-6S2L Battery .

The USB 2.0 specification requires cable delay to be less than 5.2 ns per meter (192,000 km/s, which is close to the maximum achievable transmission speed for standard copper cable). This allows for a five meter cable. The USB 3.0 standard does not directly specify a maximum cable length, requiring only that all cables meet an electrical specification  Sony VAIO PCG-6S3L Battery .

For copper wire cabling, some calculations have suggested a maximum length of perhaps 3 m.

The data cables for USB 1.x and USB 2.x use a twisted pair to reduce noise and crosstalk. They are arranged much as in the diagram below       Sony VAIO PCG-6V1L Battery .

USB 3.0 cables are more complex and employ shielding for some of the added data lines (2 pairs); a shield is added around the pair sketched.

Maximum useful distance

USB 1.1 maximum cable length is 3 metres (9.8 ft) and USB 2.0 maximum cable length is 5 metres (16 ft)  Sony VAIO PCG-6W1L Battery .

Although a single cable is limited to five meters, the USB 2.0 specification permits up to five USB hubs in a long chain of cables and hubs. The USB FAQ recommends using hubs as repeaters, allowing a maximum distance of 30 metres (98 ft) between host and device, using six cables 5 metres (16 ft) long and five hubs    Sony VAIO PCG-6W2L Battery .

In actual use, since some USB devices have built-in cables for connecting to the hub, the maximum achievable distance is 25 metres (82 ft) + the length of the device's cable. For longer lengths, USB extenders that use CAT5 cable can increase the distance between USB devices up to 50 metres (160 ft)      Sony VAIO PCG-6W3L Battery .

A method of extending USB beyond 5 metres (16 ft) is by using low resistance cable. The higher cost of USB 2.0 Cat 5 extenders has urged some manufacturers to use other methods to extend USB, such as using built-in USB hubs, and custom low-resistance USB cable      Sony VAIO PCG-7111L Battery .

It is important to note that devices which use more bus power, such as USB hard drives and scanners, will require a powered USB hub at the end of the extension to deliver the proper voltage to the device. If power and data does not have sufficient power then problems can result, such as no connection at all, or intermittent connections during use      Sony VAIO PCG-7112L Battery

USB 3.0 cable assembly may be of any length as long as all requirements defined in the specification are met. However, maximum bandwidth can be achieved across a maximum cable length of approximately three meters.


The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire from which connected USB devices may draw power  Sony VAIO PCG-7113L Battery .

The specification provides for no more than 5.25 V and no less than 4.75 V (5 V±5%) between the positive and negative bus power lines. For USB 2.0 the voltage supplied by low-powered hub ports is 4.4 V to 5.25 V .

A unit load is defined as 100 mA in USB 2.0, and was raised to 150 mA in USB 3.0     Sony VAIO PCG-7133L Battery .

A maximum of 5 unit loads (500 mA) can be drawn from a port in USB 2.0, which was raised to 6 (900 mA) in USB 3.0. There are two types of devices: low-power and high-power. Low-power devices draw at most 1 unit load, with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0     Sony VAIO PCG-7Z1L Battery .

High-power devices draw the maximum number of unit loads supported by the standard. All devices default as low-power but the device's software may request high-power as long as the power is available on the providing bus    Sony VAIO PCG-7Z2L Battery .

Some devices like high-speed external disk drives may require more than 500 mA of current  and therefore cannot be powered from one USB 2.0 port. Such devices usually come with Y-shaped cable that has two USB connectors to be inserted into a computer. With such a cable a device can draw power from two USB ports simultaneously    Sony VAIO PCG-8Y1L Battery .

A bus-powered hub is initialized at 1 unit load and transitions to maximum unit loads after hub configuration is obtained. Any device connected to the hub will draw 1 unit load regardless of the current draw of devices connected to other ports of the hub (i.e. one device connected on a four-port hub will only draw 1 unit load despite the fact that all unit loads are being supplied to the hub  Sony VAIO PCG-8Y2L Battery ).

A self-powered hub will supply maximum supported unit loads to any device connected to it. A battery-powered hub may supply maximum unit loads to ports. In addition, the VBUS will supply 1 unit load upstream for communication if parts of the Hub are powered down    Sony VAIO PCG-8Z1L Battery .

In Battery Charging Specification,  new powering modes are added to the USB specification. A host or hub Charging Downstream Port can supply a maximum of 1.5 A when communicating at low-bandwidth or full-bandwidth, a maximum of 900 mA when communicating at high-bandwidth  Sony VAIO PCG-8Z2L Battery ,

and as much current as the connector will safely handle when no communication is taking place; USB 2.0 standard-A connectors are rated at 1500 mA by default. A Dedicated Charging Port can supply a maximum of 1.8 A of current at 5.25 V. A portable device can draw up to 1.8 A from a Dedicated Charging Port     Sony VGP-BPS21 Battery .

The Dedicated Charging Port shorts the D+ and D- pins with a resistance of at most 200 ?. The short disables data transfer, but allows devices to detect the Dedicated Charging Port and allows very simple, high current chargers to be manufactured. The increased current (faster, 9 W charging) will occur once both the host/hub and devices support the new charging specification Sony VGP-BPS21A Battery .

Sleep and Charge

Sleep-and-charge USB ports can be used to charge electronic devices even when the computer is switched off. Normally when a computer is powered off the USB ports are powered down. This prevents phones and other devices from being able to charge unless the computer is powered on      Sony VGP-BPS21B Battery .

Sleep-and-charge USB ports remain powered even when the computer is off. On laptops charging devices from the USB port when it is not being powered from AC will drain the laptop battery faster. Desktop machines need to remain plugged into AC power for Sleep-and-charge to work      Sony VGP-BPS21/S Battery .

Mobile device charger standards

As of June 14, 2007, all new mobile phones applying for a license in China are required to use the USB port as a power port. This was the first standard to use the convention of shorting D+ and D-       Sony VGP-BPS21A/B Battery .

In September 2007, the Open Mobile Terminal Platform group—a forum of mobile network operators and manufacturers such as Nokia, Samsung, Motorola,Sony Ericsson and LG—announced that its members had agreed on micro-USB as the future common connector for mobile devices  SONY VGP-BPS21 Battery .

On February 17, 2009, the GSM Association announced  that they had agreed on a standard charger for mobile phones. The standard connector to be adopted by 17 manufacturers including Nokia, Motorola and Samsung is to be the micro-USB connector (several media reports erroneously reported this as the mini-USB)   SONY VGP-BPS21A Battery .

The new chargers will be much more efficient than existing chargers. Having a standard charger for all phones means that manufacturers will no longer have to supply a charger with every new phone. The basis of the GSMA's Universal Charger Solution (UCS) is the technical recommendation from OMTP and the USB-IF battery charging standard  SONY VGP-BPS21B Battery .

On April 22, 2009, this was further endorsed by the CTIA – The Wireless Association.

On June 29, 2009 the European Commission announced an agreement with ten producers that starting in 2010, data-enabled mobile phones sold in theEuropean Union would include a micro-USB connector for recharge   SONY VGP-BPS21/S Battery .

On October 22, 2009 the International Telecommunication Union (ITU) announced that it had embraced the Universal Charger Solution as its "energy-efficient one-charger-fits-all new mobile phone solution", and added: "Based on the Micro-USB interface, UCS chargers will also include a 4-star or higher efficiency rating—up to three times more energy-efficient than an unrated charger  SONY VGP-BPS21A/B Battery ."

Non-standard devices

Some USB devices require more power than is permitted by the specifications for a single port. This is common for external hard and optical disc drives, and generally for devices with motors or lamps  WD passport essential (500GB/640GB) .

Such devices can use an external power supply, which is allowed by the standard, or use a dual-input USB cable, one input of which is used for power and data transfer, the other solely for power, which makes the device a non-standard USB device   WD passport essential (250GB/320GB) .

Some external hubs may, in practice, supply more power to USB devices than required by the specification but a standard-compliant device may not depend on this.

Some non-standard USB devices use the 5 V power supply without participating in a proper USB network which negotiates power draws with the host interface  WD  passport essential SE (750GB/1TB)      .

These are usually referred to as USB decorations. The typical example is a USB-powered keyboard light; fans, mug coolers and heaters, battery chargers, miniature vacuum cleaners, and even miniature lava lamps are available. In most cases, these items contain no digital circuitry, and thus are not Standard compliant USB devices at all  WD  passport elite(250GB/320GB) .

This can theoretically cause problems with some computers, such as drawing too much current and hurting circuitry; prior to the Battery Charging Specification, the USB specification required that devices connect in a low-power mode (100 mA maximum) and communicate their current requirements to the host, which would then permit the device to switch into high-power mode  WD passport elite(500GB/640GB) .

In addition to limiting the total average power used by the device, the USB specification limits the inrush current (i.e., that used to charge decoupling and filter capacitors) when the device is first connected. Otherwise, connecting a device could cause problems with the host's internal power      WD  passport studio for Mac(320GB/500GB) .

Also, USB devices are required to automatically enter ultra low-power suspend mode when the USB host is suspended. Nevertheless, many USB host interfaces do not cut off the power supply to USB devices when they are suspended since resuming from the suspended state would become a lot more complicated if they did    WD  passport studio for Mac(500GB/640GB) .

There are also devices at the host end that do not support negotiation, such as battery packs that can power USB-powered devices; some provide power, while others pass through the data lines to a host PC. USB power adapters convert utility power and/or another power source (e.g., a car's electrical system) to run attached devices    WD  Elements series(250GB/320GB) .

Some of these devices can supply up to 1 A of current. Without negotiation, the powered USB device is unable to inquire if it is allowed to draw 100 mA, 500 mA, or 1 A.

Powered USB

Powered USB uses standard USB signaling with the addition of extra power lines       WD  Elements SE(500GB/640GB) .

It uses four additional pins to supply up to 6 A at either 5 V, 12 V, or 24 V (depending on keying) to peripheral devices. The wires and contacts on the USB portion have been upgraded to support higher current on the 5 V line, as well. This is commonly used in retail systems and provides enough power to operate stationary barcode scanners, printers, pin pads, signature capture devices, etc WD Elements SE(750GB/1TB) .

This modification of the USB interface is proprietary and was developed by IBM, NCR, and FCI/Berg. It is essentially two connectors stacked such that the bottom connector accepts a standard USB plug and the top connector takes a power connector  WD  Elements desktop(500GB/640GB) .


USB supports the following signaling rates:

  • A low-speed rate of 1.5 Mbit/s (~183 kB/s) is defined by USB 1.0. It is very similar to "full-bandwidth" operation except each bit takes 8 times as long to transmit. It is intended primarily to save cost in low-bandwidth human interface devices (HID) such as keyboards, mice, and joysticks      dd-elements-desktop-750GB-1TB.html">WD  Elements desktop(750GB/1TB) .
  • The full-speed rate of 12 Mbit/s (~1.43 MB/s) is the basic USB data rate defined by USB 1.1. All USB hubs support full-bandwidth.
  • A high-speed (USB 2.0) rate of 480 Mbit/s (~57 MB/s) was introduced in 2001. All hi-speed devices are capable of falling back to full-bandwidth operation if necessary; they are backward compatible. Connectors are identical    WD  Elements desktop(1.5 TB/2TB) .
  • A SuperSpeed (USB 3.0) rate of 4800 Mbit/s (~572 MB/s). The written USB 3.0 specification was released by Intel and partners in August 2008. The first USB 3 controller chips were sampled byNEC May 2009   and products using the 3.0 specification arrived beginning in January 2010      WD  passport essential SE (750GB/1TB)--USB 3.0)    .
  • USB 3.0 connectors are generally backwards compatible, but include new wiring and full duplex operation.

USB signals are transmitted on a twisted-pair data cable with 90? ±15% Characteristic impedance, labeled D+ and D?. Prior to USB 3.0, these collectively use half-duplex differential signaling to reduce the effects of electromagnetic noise on longer lines  WD  passport essential (500GB/640GB) .

Transmitted signal levels are 0.0–0.3 volts for low and 2.8–3.6 volts for high in full-bandwidth and low-bandwidth modes, and ?10–10 mV for low and 360–440 mV for high in hi-bandwidth mode. In FS mode the cable wires are not terminated, but the HS mode has termination of 45 ? to ground, or 90 ? differential to match the data cable impedance, reducing interference due to signal reflections      WD  passport for Mac(320GB/500GB) .

USB 3.0 introduces two additional pairs of shielded twisted wire and new, mostly interoperable contacts in USB 3.0 cables, for them. They permit the higher data rate, and full duplex operation.

A USB connection is always between a host or hub at the "A" connector end, and a device or hub's "upstream" port at the other end WD  passport for Mac(640GB/1TB) .

Originally, this was a "B' connector, preventing erroneous loop connections, but additional upstream connectors were specified, and some cable vendors designed and sold cables which permitted erroneous connections (and potential damage to the circuitry). USB interconnections are not as fool-proof or as simple as originally intended    My book essential 4 generation  (640GB/1TB) .

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

A USB device pulls one of the data lines high with a 1.5 k? resistor   WD  My book essential 4 generation( 1.5TB/2TB) .

This overpowers one of the pull-down resistors in the host and leaves the data lines in an idle state called "J". For USB 1.x, the choice of data line indicates a device's bandwidth support; full-bandwidth devices pull D+ high, while low-bandwidth devices pull D? high.

USB data is transmitted by toggling the data lines between the J state and the opposite K state WD  My book elite( 1TB/1.5TB) .

USB encodes data using the NRZI convention; a 0 bit is transmitted by toggling the data lines from J to K or vice-versa, while a 1 bit is transmitted by leaving the data lines as-is. To ensure a minimum density of signal transitions, USB uses bit stuffing; an extra 0 bit is inserted into the data stream after any appearance of six consecutive 1 bits  WD  My book studio(1TB/2TB) .

Seven consecutive 1 bits is always an error. USB 3.0 has introduced additional data transmission encodings.

A USB packet begins with an 8-bit synchronization sequence '00000001'. That is, after the initial idle state J, the data lines toggle KJKJKJKK       WD  My book essential 4 generation( 1.5TB/2TB) .

The final 1 bit (repeated K state) marks the end of the sync pattern and the beginning of the USB frame. For high bandwidth USB, the packet begins with a 32-bit synchronization sequence.

A USB packet's end, called EOP (end-of-packet), is indicated by the transmitter driving 2 bit times of SE0 (D+ and D? both below max) and 1 bit time of J state      WD  My book elite(640GB/2TB) .

After this, the transmitter ceases to drive the D+/D? lines and the aforementioned pull up resistors hold it in the J (idle) state. Sometimes skew due to hubs can add as much as one bit time before the SE0 of the end of packet. This extra bit can also result in a "bit stuff violation" if the six bits before it in the CRC are '1's. This bit should be ignored by receiver  Seagate  expansion portable (320GB/500GB) .

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

USB 2.0 devices use a special protocol during reset, called "chirping", to negotiate the high bandwidth mode with the host/hub. A device that is HS capable first connects as an FS device (D+ pulled high), but upon receiving a USB RESET (both D+ and D? driven LOW by host for 10 to 20 ms) it pulls the D? line high, known as chirp K      Seagate  expansion (1.5TB/2TB) .

This indicates to the host that the device is high bandwidth. If the host/hub is also HS capable, it chirps (returns alternating J and K states on D? and D+ lines) letting the device know that the hub will operate at high bandwidth. The device has to receive at least 3 sets of KJ chirps before it changes to high bandwidth terminations and begins high bandwidth signaling   Seagate  Freeagent Desktop (500GB/1TB) .

Because USB 3.0 uses wiring separate and additional to that used by USB 2.0 and USB 1.x, such bandwidth negotiation is not required.

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

Though high bandwidth devices are commonly referred to as "USB 2.0" and advertised as "up to 480 Mbit/s", not all USB 2.0 devices are high bandwidth       Seagate  Freeagent Go(250GB/320GB) .

The USB-IF certifies devices and provides licenses to use special marketing logos for either "basic bandwidth" (low and full) or high bandwidth after passing a compliance test and paying a licensing fee. All devices are tested according to the latest specification, so recently compliant low bandwidth devices are also 2.0 devices     Seagate  Freeagent Go(500GB/640GB) .


USB 3 released in 2008-11-12 uses tinned copper stranded AWG-28 cables with 90±7 ? impedance for its high-speed differential pairs and linear feedback shift register and 8b/10b encoding sent with a voltage of 1000 mV nominal with a 100 mV receiver threshold, the receiver uses equalization      Seagate  Freeagent Go(750GB/1TB) .

SSC clock and 300 ppm precision is used. Packet headers are protected with CRC-16, while data payload is protected with CRC-32. Power up to 3.6 W may be used. And one unit load in superspeed mode is equal to 150 mA     Seagate  Freeagent Goflex(250GB/320GB) .

USB 3 Pinout

USB 2.0 data rates

The theoretical maximum data rate in USB 2.0 is 480 Mbit/s (60 MB/s) per controller and is shared amongst all attached devicesSeagate Freeagent Goflex(500GB/640GB) .

Some chipset manufacturers overcome this bottleneck by providing multiple USB 2.0 controllers within the southbridge. Big performance gains can be achieved when attaching multiple high bandwidth USB devices such as disk enclosures in different controllers. The following table displays southbridge ICs that have multiple EHCI controllers  Seagate  Freeagent Goflex(750GB/1TB).

Every other AMD, Broadcom, and Intel southbridge supporting USB 2.0 has only one EHCI controller. All SiS southbridge, ULi, and VIA southbridge, single chip northbridge/southbridge supporting USB 2.0 have only one EHCI controller. Also all PCI USB 2.0 ICs used for add-in cards have only one EHCI controller      Seagate  Freeagent Goflex Pro(500GB/750GB) .

In PCIe, the usual design with multiple USB ports per EHCI controller has changed with the introduction of the MosChip MCS9990 IC. MCS9990 has one EHCI controller per port so all its USB ports can operate simultaneously without any performance limitations. Cards with dual ICs have been introduced as well, with two PCI USB 2.0 ICs attached to one PCI to PCIe bridge  Seagate  Freeagent Goflex desktop(1TB/2TB) .

Transfer speeds in practice

As of 2004, the actual throughput of USB 2.0 high bandwidth attained with a hard drive tested on a Mac was about 18 MiB/s, 30% of the maximum theoretical bulk data transfer rate of 60 MiB/s(480 Mbit/s). On Windows, the highest speed observed was 33 MiB/s, or 60% of the theoretical max     Seagate  Freeagent go for Mac(320GB/640GB) .

The drive could reach 58 MiB/s on Firewire, so the drive's speed was not a limiting factor.

According to a USB-IF chairman, "at least 10 to 15 percent of the stated peak 60 MB/s (480 Mbit/s) of Hi-Speed USB goes to overhead — the communication protocol between the card and the peripheral. Overhead is a component of all connectivity standards    Samsung  G2 protable (250gb/320GB) ."

Tables illustrating the transfer limits are shown in Chapter 5 of the USB spec.

Typical high bandwidth USB devices operate at lower data rates, often about 3 MiB/s overall, sometimes up to 10–20 MiB/s[citation needed]. For USB 1.1, an average transfer speed of 880 KiB/s has been observed       Samsung  G2 protable (500GB/640GB) .

For isochronous devices like audio streams, the bandwidth is constant, and reserved exclusively for a given device. The bus bandwidth therefore only has an effect on the number of channels that can be sent at a time, not the "speed" or latency of the transmission     Samsung  S2 protable (320GB/500GB) .

Data packets

USB communication takes the form of 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       Samsung S1 Mini (120GB/160GB) .

After the sync field, all packets are made of 8-bit bytes, transmitted least-significant bit first. The first byte is a packet identifier (PID) byte. The PID is actually 4 bits; the byte consists of the 4-bit PID followed by its bitwise complement. This redundancy helps detect errors      Samsung  S1 Mini (250GB/320GB) .

(Note also that a PID byte contains at most four consecutive 1 bits, and thus will never need bit-stuffing, even when combined with the final 1 bit in the sync byte. However, trailing 1 bits in the PID may require bit-stuffing within the first few bits of the payloadSamsung story station (1TB/1.5TB) .)

Handshake packets

Handshake packets consist of nothing but a PID byte, and are generally sent in response to data packets. The three basic types are ACK, indicating that data was successfully received, NAK, indicating that the data cannot be received at this time and should be retried     Samsung Story station (1.5TB/2TB) ,

and STALL, indicating that the device has an error and will never be able to successfully transfer data until some corrective action (such as device initialization) is performed.

USB 2.0 added two additional handshake packets, NYET which indicates that a split transaction is not yet complete  Samsung story station Esata(1TB/1.5TB) .

A NYET packet is also used to tell the host that the receiver has accepted a data packet, but cannot accept any more due to buffers being full. The host will then send PING packets and will continue with data packets once the device ACK's the PING. The other packet added was the ERR handshake to indicate that a split transaction failed     Samsung  G3 station (1TB/1.5TB) .

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

Token packets

Token packets consist of a PID byte followed by 2 payload bytes: 11 bits of address and a 5-bit CRC. Tokens are only sent by the host, never a device     Maxtor  one touch 4 plus (500GB/750GB) .

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

An IN token expects a response from a device      Maxtor one touch 4 plus (1TB/1.5TB) .

The response may be a NAK or STALL response, or a DATAx frame. In the latter case, the host issues an ACK handshake if appropriate.

An OUT token is followed immediately by a DATAx frame. The device responds with ACK, NAK, NYET, or STALL, as appropriate  Maxtor cool black(640GB/1TB) .

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

Every millisecond (12000 full-bandwidth bit times), the USB host transmits a special SOF (start of frame) token  Maxtor Black diamond (320GB/500GB) ,

containing an 11-bit incrementing frame number in place of a device address. This is used to synchronize isochronous data flows. High-bandwidth USB 2.0 devices receive 7 additional duplicate SOF tokens per frame, each introducing a 125 µs "microframe" (60000 high-bandwidth bit times each)     Hitachi  simple touch (250GB/320GB) .

USB 2.0 added a PING token, which asks a device if it is ready to receive an OUT/DATA packet pair. The device responds with ACK, NAK, or STALL, as appropriate. This avoids the need to send the DATA packet if the device knows that it will just respond with NAK  Hitachi simple touch (320GB/500GB) .

USB 2.0 also added a larger 3-byte SPLIT token with a 7-bit hub number, 12 bits of control flags, and a 5-bit CRC. This is used to perform split transactions. Rather than tie up the high-bandwidth USB bus sending data to a slower USB device, the nearest high-bandwidth capable hub receives a SPLIT token followed by one or two USB packets at high bandwidth  Hitachi  life studio (320GB/500GB) ,

performs the data transfer at full or low bandwidth, and provides the response at high bandwidth when prompted by a second SPLIT token. The details are complex; see the USB specification  Hitachi life studio desk (500GB/1TB) .

Data packets

A data packet consists of the PID followed by 0–1023 bytes of data payload (up to 1024 in high bandwidth, at most 8 at low bandwidth), and a 16-bit CRC        Hitachi life studio plus (320GB/500GB) .

There are two basic data packets, DATA0 and DATA1. They must always be preceded by an address token, and are usually followed by a handshake token from the receiver back to the transmitter. The two packet types provide the 1-bit sequence number required by Stop-and-wait ARQ     Hitachi life studio plus (320GB/500GB) .

If a USB host does not receive a response (such as an ACK) for data it has transmitted, it does not know if the data was received or not; the data might have been lost in transit, or it might have been received but the handshake response was lost.

To solve this problem, the device keeps track of the type of DATAx packet it last accepted      Hitachi X mobile (250GB/320GB) .

If it receives another DATAx packet of the same type, it is acknowledged but ignored as a duplicate. Only a DATAx packet of the opposite type is actually received.

When a device is reset with a SETUP packet, it expects an 8-byte DATA0 packet next      Hitachi X mobile(320GB/500GB) .

USB 2.0 added DATA2 and MDATA packet types as well. They are used only by high-bandwidth devices doing high-bandwidth isochronous transfers which need to transfer more than 1024 bytes per 125 µs "microframe" (8192 kB/s).

PRE "packet"

Low-bandwidth devices are supported with a special PID value, PRE Hitachi  XL (1TB/2TB) .

This marks the beginning of a low-bandwidth packet, and is used by hubs which normally do not send full-bandwidth packets to low-bandwidth devices. Since all PID bytes include four 0 bits, they leave the bus in the full-bandwidth K state, which is the same as the low-bandwidth J state        Toshiba canvio portable(320GB/500GB) .

It is followed by a brief pause during which hubs enable their low-bandwidth outputs, already idling in the J state, then a low-bandwidth packet follows, beginning with a sync sequence and PID byte, and ending with a brief period of SE0. Full-bandwidth devices other than hubs can simply ignore the PRE packet and its low-bandwidth contents, until the final SE0 indicates that a new packet follows        Toshiba  canvio portable(750GB/1TB) .

Protocol analyzers

Due to the complexities of the USB protocol, USB protocol analyzers are invaluable tools to USB device developers. USB analyzers are able to capture the data on USB and display information from low-level bus states to high-level data packets and class-level information       Toshiba  anvio for Mac(500GB/750GB) .

Comparisons with other device connection technologies


USB was originally seen as a complement to FireWire (IEEE 1394), which was designed as a high-bandwidth serial bus which could efficiently interconnect peripherals such as hard disks, audio interfaces, and video equipment. USB originally operated at a far lower data rate and used much simpler hardware, and was suitable for small peripherals such as keyboards and mice  Toshiba  canvio for Mac(750GB/1TB) .

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