USB

"USB" redirects here. For other uses, see USB (disambiguation).

USB

Universal Serial Bus
Original USB Logo
Year created	January 1996
Width in bits	1
No. of devices	127 per host controller
Speed	12 or 480 Mbit/s (1.5 to 60 MByte/s)
Style	Serial
Hotplugging interface	Yes
External interface	Yes
Website	www.usb.org

A USB Series “A” plug, the most common USB plug

The USB "trident" logo

In information technology, Universal Serial Bus (USB) is a serial bus standard to interface devices to a host computer. USB was designed to allow many peripherals to be connected using a single standardized interface socket and to improve the plug-and-play capabilities by allowing hot swapping, that is, by allowing devices to be connected and disconnected without rebooting the computer or turning off the device. Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.

USB is intended to replace many legacy varieties of serial and parallel ports. USB can connect computer peripherals such as mouse, keyboards, PDAs, gamepads and joysticks, scanners, digital cameras, printers, personal media players, and flash drives. For many of those devices USB has become the standard connection method. USB was originally designed for personal computers, but it has become commonplace on other devices such as PDAs and video game consoles, and as a bridging power cord between a device and an AC adapter plugged into a wall plug for charging purposes. As of 2008, there are about 2 billion USB devices in the world.

The design of USB is standardized by the USB Implementers Forum (USB-IF), an industry standards body incorporating leading companies from the computer and electronics industries. Notable members have included Agere (now merged with LSI Corporation), Apple Inc., Hewlett-Packard, Intel, NEC, and Microsoft.

History

The USB 1.0 specification model was introduced in November 1995. USB was created by the Core group of companies that consisted of Intel, Compaq, Microsoft, Digital, IBM, and Northern Telecom. Intel produced the UHCI host controller and open software stack; Microsoft produced a USB software stack for Windows and co-authored the OHCI host controller specification with National Semicondutor and Compaq; Philips produced early USB-Audio; and TI produced the most widely used hub chips. Originally USB was intended to replace the multitude of connectors at the back of PCs, as well as to simplify software configuration of communication devices.

The original Apple "Bondi blue" iMac G3, introduced May 6, 1998, was the first computer to offer USB ports without offering "legacy" ports . USB 1.1 came out in September 1998 to help rectify the adoption problems that occurred with earlier iterations of USB, mostly those relating to hubs.

As of 2008, the USB specification is at version 2.0 (with revisions). Hewlett-Packard, Intel, Lucent (now LSI Corporation since its merger with Lucent spinoff Agere Systems), Microsoft, NEC, and Philips jointly led the initiative to develop a higher data transfer rate than the 1.1 specification. The USB 2.0 specification was released in April 2000 and was standardized by the USB-IF at the end of 2001. Equipment conforming with any version of the standard will also work with devices designed to any previous specification (known as backward compatibility).

A conventional USB hub.

Overview

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

USB devices are linked in series through hubs. There always exists one hub known as the root hub, which is built-in to the host controller. So-called "sharing hubs", which allow multiple computers to access the same peripheral device(s), also exist and work by either switching access between PCs automatically or manually. They 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, because a single device may provide several functions, such as a webcam (video device function) with a built-in microphone (audio device function).

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). Pipes are connections from the host controller to a logical entity on the device named an endpoint. The term endpoint is also occasionally used to refer to the pipe. A USB device can have up to 32 active pipes, 16 into the host controller and 16 out of the controller. Each endpoint can transfer data in one direction only, either into or out of the device, so each pipe is uni-directional. 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 which is not associated with any interface.

When a new USB device is connected to a USB host, the USB device enumeration process is started. The enumeration process first sends a reset signal to the USB device. The speed of the USB device is determined during the reset signaling. After reset, USB device setup information is read from the device by the host and the device is assigned a unique host-controller-specific 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 configured state. If the USB host is restarted, the enumeration process is repeated for all connected devices.

The host controller polls the bus for traffic, usually in a Round-robin fashion, so no USB device can transfer any data on the bus without an explicit request from the host controller.

Host controllers

The computer hardware that contains the host controller and the root hub has an interface geared toward the programmer which is called Host Controller Device (HCD) and is defined by the hardware implementer.

In the version 1.x age, there were two competing HCD implementations, Open Host Controller Interface (OHCI) and Universal Host Controller Interface (UHCI). OHCI was developed by Compaq, Microsoft and National Semiconductor; UHCI was by Intel.

A typical USB connector.

VIA Technologies licensed the UHCI standard from Intel; all other chipset implementers use OHCI. UHCI is more software-driven, making UHCI slightly more processor-intensive than OHCI but cheaper to implement. The dueling implementations forced operating system vendors and hardware vendors to develop and test on both implementations, which increased cost.

HCD standards are out of the USB specification's scope, and the USB specification does not specify any HCD interfaces. In other words, USB defines the format of data transfer through the port, but not the system by which the USB hardware communicates with the computer it sits in.

During the design phase of USB 2.0, the USB-IF insisted on only one implementation. The USB 2.0 HCD implementation is called the Enhanced Host Controller Interface (EHCI). Only EHCI can support hi-speed (480 Mbit/s) transfers. Most of PCI-based EHCI controllers contain other HCD implementations called 'companion host controller' to support Full Speed (12 Mbit/s) and may be used for any device that claims to be a member of a certain class. An operating system is supposed to implement all device classes so as to provide generic drivers for any USB device. Device classes are decided upon by the Device Working Group of the USB Implementers Forum.

Device classes include:

Class	Usage	Description	Examples
00h	Device	UnspecifiedTemplate:Fn	(Device class is unspecified. Interface descriptors are used for determining the required drivers.)
01h	Interface	Audio	Speaker, microphone, sound card
02h	Both	Communications and CDC Control	Ethernet adapter, modem, serial port adapter
03h	Interface	Human Interface Device (HID)	Keyboard, mouse, joystick
05h	Interface	Physical Interface Device (PID)	Force feedback joystick
06h	Interface	Image	Digital camera (Most cameras function as Mass Storage for direct access to storage media).
07h	Interface	Printer	Laser printer, Inkjet printer
08h	Interface	Mass Storage	USB flash drive, memory card reader, digital audio player, external drives
09h	Device	USB hub	Full speed hub, hi-speed hub
0Ah	Interface	CDC-Data	(This class is used together with class 02h - Communications and CDC Control.)
0Bh	Interface	Smart Card	USB smart card reader
0Dh	Interface	Content Security	-
0Eh	Interface	Video	Webcam
0Fh	Interface	Personal Healthcare	-
DCh	Both	Diagnostic Device	USB compliance testing device
E0h	Interface	Wireless Controller	Wi-Fi adapter, Bluetooth adapter
EFh	Both	Miscellaneous	ActiveSync device
FEh	Interface	Application Specific	IrDA Bridge
FFh	Both	Vendor Specific	(This class code indicates that the device needs vendor specific drivers.)

Template:Fnb Use class information in the Interface Descriptors. This base class is defined to be used in Device Descriptors to indicate that class information should be determined from the Interface Descriptors in the device.

USB mass-storage

Main article: USB mass storage device class

A flash drive, a typical USB mass-storage device.

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. This generality is because many systems can be controlled with the familiar idiom of file manipulation within directories (The process of making a novel device look like a familiar device is also known as extension).

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 ATA (IDE), Serial ATA (SATA), and SCSI fulfill that role. However, USB has one important advantage in that it is possible to install and remove devices without opening the computer case, making it useful for external drives. Originally conceived and still used today for optical storage devices (CD-RW drives, DVD drives, etc.), a number of manufacturers offer external portable USB hard drives, or empty enclosures for drives, that offer performance comparable to internal drives. These external drives usually contain a translating device that interfaces a drive of conventional technology (IDE, ATA, SATA, ATAPI, or even SCSI) to a USB port. Functionally, the drive appears to the user just like another internal drive. Other competing standards that allow for external connectivity are eSATA and FireWire.

Human-interface devices (HIDs)

Mice and keyboards are frequently fitted with USB connectors, but because most PC motherboards still retain PS/2 connectors for the keyboard and mouse as of 2007, they are often supplied with a small USB-to-PS/2 adaptor, allowing usage with either USB or PS/2 interface. There is no logic inside these adaptors: they make use of the fact that such HID interfaces are equipped with controllers that are capable of serving both the USB and the PS/2 protocol, and automatically detect which type of port they are plugged into. Joysticks, keypads, tablets and other human-interface devices are also progressively migrating from MIDI, PC game port, and PS/2 connectors to USB.

Apple Macintosh computers have been using USB exclusively for all external wired mice and keyboards since January 1999. The original iMac raised public awareness of USB considerably in August 1998, as it discarded legacy ports to use only USB. PCs had USB ports prior to the iMac's introduction, but they were included with a full complement of traditional ports which slowed down USB's adoption. The iMac's influence can be seen in the number of USB peripherals with matching translucent, colored plastic enclosures that were available in the late '90s and early '00s.

USB signaling

USB supports three data rates:

• The Full Speed rate of 12 Mbit/s (1.5 MB/s) is the basic USB data rate defined by USB 1.0. All USB hubs support Full Speed.
• A Low Speed rate of 1.5 Mbit/s (187.5 kB/s) is also defined by USB 1.0. It is very similar to full speed operation except that 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.
• A High-Speed (USB 2.0) rate of 480 Mbit/s (60 MB/s) was introduced in USB 2.0. All high-speed devices are capable of falling back to full-speed operation if necessary.

Experimental data rate:

• A Super-Speed (USB 3.0) rate of 4.8 Gbit/s (600 MB/s). The USB 3.0 specification was released by Intel and its partners in August 2008, according to early reports from CNET news. According to Intel, bus speeds will be 10 times faster than USB 2.0 due to the inclusion of a fiber-optic link that works with traditional copper connectors. Products using the 3.0 specification are likely to arrive in 2009 or 2010.

USB signals are transmitted on a twisted pair data cable with 90Ω ±15% impedance, labeled D+ and D−. These collectively use half-duplex differential signaling to combat the effects of electromagnetic noise on longer lines. Transmitted signal levels are 0.0–0.3 volts for low and 2.8–3.6 volts for high in Full Speed and Low Speed modes, (and ±400mV in High Speed (HS) mode)^{this phrase is ambiguous}. In FS mode the cable wires are not terminated, but the HS mode has termination of 45Ω to ground, or 90Ω differential to match the data cable impedance.

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

A USB device pulls one of the data lines high with a 1.5 kΩ resistor. This overpowers one of the pull-down resistors in the host and leaves the data lines in an idle state called "J". The choice of data line indicates a device's speed support; full-speed devices pull D+ high, while low-speed devices pull D− high.

USB data is transmitted by toggling the data lines between the J state and the opposite K state. 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. Seven consecutive 1 bits is always an error.

A USB frame begines with an 8-bit synchronization sequence 00000001. That is, after the initial idle state J, the data lines toggle KJKJKJKK. The final 1 bit (repeated K state) marks the end of the sync pattern and the beginning of the USB frame proper.

A USB frame's end is indicated by 2 bit times of SE0 and 1 bit time of J. After this, the transmitter ceases to drive the D+/D− lines and the aforementioned resistors hold it in the J (idle) state. A receiver may take extra time to decode the SE0 state, and will see the first bit time as a repetition of the last data bit. Since USB frames are always a multiple of 8 bits long, this extra "dribble bit" can be detected and ignored.

A USB bus is reset using a prolonged (several milliseconds) SE0 signal.

USB 2.0 uses a special protocol called "chirping" to negotiate the High-Speed mode. In simplified terms, a device that is HS capable always connects as an FS device first, but after receiving a USB RESET (both D+ and D- are driven LOW by host) it tries to pull the D- line high. If the host (or hub) is also HS capable, it returns alternating signals on D- and D+ lines letting the device know that the tier will operate at High Speed.

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

Though Hi-Speed devices are commonly referred to as "USB 2.0" and advertised as "up to 480 Mbit/s", not all USB 2.0 devices are Hi-Speed. The USB-IF certifies devices and provides licenses to use special marketing logos for either "Basic-Speed" (low and full) or Hi-Speed after passing a compliance test and paying a licensing fee. All devices are tested according to the latest spec, so recently-compliant Low-Speed devices are also 2.0 devices.

The actual throughput currently (2006) attained with real devices is about two thirds of the maximum theoretical bulk data transfer rate of 53.248 MB/s. Typical hi-speed USB devices operate at lower speeds, often about 3 MB/s overall, sometimes up to 10–20 MB/s.

USB packets

USB communication takes the form of packets. Initially, all packets are sent from the host, via zero or more hubs, to devices. Some of those packets direct a device to send some packets in reply.

After the sync field described above, 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. (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, the OUT PID byte ends with three consecutive 1 bits, so if the following USB device address begins with three 1 bits, bit-stuffing will be required.)

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

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, 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, and an ERR handshake to indicate that a split transaction failed.

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

Token packets

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

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

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

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

SETUP operates much like an OUT token, but is used for initial device setup.

Every 1 ms (12000 full-speed bit times), the USB host transmits a special SOF (start of frame) token, containing an 11-bit incrementing frame number in place of a device address. This is used to synchronize isochronous data flows. High-speed USB 2.0 devices receive 7 additional duplicate SOF tokens per frame, each introducing a 125 µs "microframe".

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.

USB 2.0 also added a larger 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-speed USB bus sending data to a slower USB device, the nearest high-speed capable hub receives a SPLIT token followed by one or two USB packets at high speed, performs the data transfer at full or low speed, and provides the response at high speed when prompted by a second SPLIT token. The details are complex; see the USB specification.

Data packets

There are two basic data packets, DATA0 and DATA1. Both consist of a DATAx PID field, 0–1023 bytes of data payload (up to 1024 in high speed, at most 8 at low speed), and a 16-bit CRC. 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. If a USB host does not receive a response (such as an ACK) for data it has transmitted, it does not know if the data was received or not; the data might have been lost in transit, or it might have been received but the handshake response was lost.

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

When a device is reset with a SETUP packet, it expects a DATA0 packet next.

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

PRE "packet"

Low-speed devices are supported with a special PID value, PRE. This marks the beginning of a low-speed packet, and is used by hubs which normally do not send full-speed packets to low-speed devices. Since all PID bytes include 4 0 bits, they leave the bus in the full-speed K state, which is the same as the low-speed J state. It is followed by a brief pause during which hubs enable their low-speed outputs, already idling in the J state, then a low-speed packet follows, beginning with a sync sequence and PID byte, and ending with a brief period of SE0. Full-speed devices other than hubs can simply ignore the PRE packet and its low-speed contents, until the final SE0 indicates that a new packet follows.

USB 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.

USB connector properties

Series "A" plug and receptacle.

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 varied menagerie of connectors then in service.

Usability

• It is difficult to incorrectly attach a USB connector. Connectors cannot be plugged-in upside down, and it is clear from the appearance and kinesthetic sensation of making a connection when the plug and socket are correctly mated. However, it is not obvious at a glance to the inexperienced user (or to a user without sight of the installation) which way around the connector goes, so it is often necessary to try both ways. More often than not, however, the side of the connector with the "trident" logo should be on top.
• Only a moderate insertion/removal force is needed (by specification). USB cables and small USB devices are held in place by the gripping force from the receptacle (without the need for the screws, clips, or thumbturns that other connectors require). The force needed to make or break a connection is modest, allowing connections to be made in awkward circumstances or by those with motor disabilities.
• The connectors enforce the directed topology of a USB network. USB does not support cyclical networks, so the connectors from incompatible USB devices are themselves incompatible. Unlike other communications systems (e.g. RJ-45 cabling) gender-changers are almost never used, making it difficult to create a cyclic USB network.

USB extension cord

Durability

• The connectors are designed to be robust. Many previous connector designs were fragile, with pins or other delicate components prone to bending or breaking, even with the application of only very modest force. The electrical contacts in a USB connector are protected by an adjacent plastic tongue, and the entire connecting assembly is usually further protected by an enclosing metal sheath. As a result USB connectors can safely be handled, inserted, and removed, even by a small child.
• The connector construction always ensures that the external sheath on the plug contacts with its counterpart in the receptacle before the four connectors within are connected. This sheath is typically connected to the system ground, allowing otherwise damaging static charges to be safely discharged by this route (rather than via delicate electronic components). This means of enclosure also means that there is a (moderate) degree of protection from electromagnetic interference afforded to the USB signal while it travels through the mated connector pair (this is the only location when the otherwise twisted data pair must travel a distance in parallel). In addition, the power and common connections are made after the system ground but before the data connections. This type of staged make-break timing allows for safe hot-swapping and has long been common practice in the design of connectors in the aerospace industry.
• The newer USB Micro receptacles are designed to allow up to 10,000 cycles of insertion and exertion between the receptacle and plug, compared to 500 for the standard USB and Mini-USB receptacle. This is accomplished by adding a locking device and by moving the 'lief spring' connections from the jack to the plug, i.e. if you are no longer getting a good connection, replace your cable. This move was done to allow for the fact that the devices using the new micro USB ports will probably cost more to replace than the cables to connect it.

Compatibility

• The USB standard specifies relatively loose tolerances for compliant USB connectors, intending to minimize incompatibilities in connectors produced by different vendors (a goal that has been very successfully achieved). Unlike most other connector standards, the USB specification also defines limits to the size of a connecting device in the area around its plug. This was done to prevent a device from blocking adjacent ports due to its size. Compliant devices must either fit within the size restrictions or support a compliant extension cable which does.
• Two-way communication is also possible. In general, cables have only plugs, and hosts and devices have only receptacles: hosts having type-A receptacles and devices type-B. Type-A plugs only mate with type-A receptacles, and type-B with type-B. However, an extension to USB 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 socket on the unit. Even after the cable is hooked up and the units are talking, the two units may "swap" ends under program control. This facility targets units such as PDAs where 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.

Interface

Receptacles \ Plugs	USB-A	USB-B	Mini-B	Micro-A	Micro-B
USB-A	OK	X	X	X	X
USB-B	X	OK	X	X	X
Mini-B	X	X	OK	X	X
Micro-AB	X	X	X	OK	OK
Micro-B	X	X	X	X	OK

Cables

Plug \ Plug	Micro-B	Micro-A	Mini-B	USB-B	USB-A
USB-A	OK	NS	OK	OK	NS
USB-B	X	NS	X	X
Mini-B	X	NS	X
Micro-A	OK	X
Micro-B	X

In addition to these cable assemblies also a cable with Micro-A and Standard-A receptacle is compliant with USB specifications. Other combinations of connectors are not compliant. However, some older devices and cables with Mini-A connector have been certified by USB-IF; the Mini-A connector has been deprecated, and no new certification for assemblies using Mini-A connector will be allowed.

Note: NS is non-standard, existing for specific proprietary purposes not at the guidance of the USB-IF.

Types of USB connector

Micro-USB Connectors

USB Connectors

Different types of USB connectors from left to right
• 8-pin mystery plug
• Mini-B plug
• B-type plug
• A-type receptacle
• A-type plug

Pin configuration of the USB connectors Standard A/B, viewed from face of plug

Micro-USB Receptacles

There are several types of USB connectors, including some that have been added as the specification has 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. The data slots in the A - Plug are actually farther in the plug than the outside power wires to prevent data errors by powering the device first, then transferring data.

The Standard-A type of USB connectors takes on the appearance of flattened rectangles that plugs into downstream-port sockets on the USB host or a hub. This kind of connector is most frequently seen on cables that are permanently attached to a device, such as one on a cable that connects a keyboard or mouse to the computer. Standard-B connectors looks square with beveled corners, and plugs into upstream sockets on devices and hubs. The Standard-B connector is mainly used only for the device end of a removable cable, such as between a hub and a printer. This two-connector scheme prevents a user from accidentally creating a loop.

The non-standard Mini-USB's, official Mini-B, Micro-A, and Micro-B connectors are used for smaller devices such as PDAs, mobile phones or digital cameras. The Standard-A plug is approximately 4 by 12 mm, the Standard-B approximately 7 by 8 mm, and the Mini-A and Mini-B plugs approximately 2 by 7 mm.

The Micro-USB connector, was announced by the USB-IF on January 4, 2007. It is intended to replace the Mini-USB plugs used in many new smartphones and Personal digital assistants. This Micro-USB plug is rated for 10,000 connect-disconnect cycles. It is about half the height of the mini-USB connector, but features a similar width. In the Universal Serial Bus Micro-USB Cables and Connectors Specification, details have been laid down for Micro-A plugs, Micro-AB receptacles, and Micro-B plugs and receptacles, along with a Standard-A receptacle to Micro-A plug adapter. The carrier led group OMTP have recently endorsed micro-USB as the standard connector for data and power on mobile devices.

Proprietary connectors and formats

Microsoft's original Xbox game console uses standard USB 1.1 signaling in its controllers and memory cards, but features proprietary connectors and ports. Similarly, IBM UltraPort uses standard USB signaling, but via a proprietary connection format. American Power Conversion uses USB signaling and HID device class on its uninterruptible power supplies using 10P10C connectors. HTC, a company which makes Windows Mobile-based Communicators, has a proprietary connector called HTC ExtUSB, which combines mini-USB with audio input and output. Nokia includes a USB connection as part of the Pop-Port connector on their mobile phones. The second-generation iPod Shuffle uses a TRS connector to carry USB, audio, or power signals. Many digital cameras have a tiny 8 pin connector that combines USB with video and audio out. There are at least 10 different types of non-standard Mini-USB receptacles and jacks currently in use. Because of this, the USB-IF created the Micro series of receptacles and jacks, so as to standardize on one jack that delivers both power and data.

USB cables

Pin	Name	Cable color	Description
1	VCC	Red	+5V
2	D−	White	Data −
3	D+	Green	Data +
4	GND	Black	Ground

The maximum length of a standard USB cable is 5.0 meters (16.4 ft). The primary reason for this limit is the maximum allowed round-trip delay of about 1500 ns. If a USB device does not answer to host commands within the allowed time, the host considers the command to be lost. When adding up the USB device response time, delays from using the maximum number of hubs, and delays from the cables connecting, the maximum delay caused by a single cable turns out to be 26 ns. The USB 2.0 specification states that the cable delay must be less than 5.2 ns per meter, which means that maximum length USB cable is 5 meters long (which is close to the maximum possible length when using a standard copper cable).

Miniplug/Microplug

Pin	Name	Color	Description
1	VCC	Red	+5 V
2	D−	White	Data −
3	D+	Green	Data +
4	ID	none	permits distinction of Micro-A- and Micro-B-Plug Type A: connected to Ground Type B: not connected
5	GND	Black	Signal Ground

The data cables are a twisted pair to reduce noise and crosstalk.

Maximum Useful Signalling Distance

Although a single cable is limited to 5 meters, the USB specification permits up to five USB hubs in a long chain of cables and hubs. This allows for a maximum signalling distance of 30 meters, using six 5-meter cables and five hubs. In actual use, the last hub is a more convenient endpoint since some USB devices include built-in cables intended to directly connect to a hub, setting the maximum useful signalling distance at 25 meters.

Since USB can provide power for additional devices connected to the bus, a special type of USB extender cable was created which consists of a miniature one-port USB hub molded into one end of a 5 meter cable. These mini-hubs are fully self-contained within the cable, requiring no separate bulky hub device, and are as simple to use as plugging cables together, with each hub drawing power through all the previous single-port hubs in the chain. The bus power is limited however, so the most practical application is to use four single-port hub extender cables, one plain 5 meter cable, and then a powered multiport hub at the very end to support multiple additional USB devices.

Power

The USB specification provides a 5 V supply on a single wire from which connected USB devices may draw power. 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. Initially, a device is only allowed to draw 100 mA. It may request more current from the upstream device in units of 2 mA up to a maximum of 500 mA.

If a bus-powered hub is used, the devices downstream may only use a total of four units — 400 mA — of current. This limits compliant bus-powered hubs to 4 ports. The host operating system typically keeps track of the power requirements of the USB network and may issue a warning when a given segment requires more power than is available.

On-The-Go and Battery Charging Specification both add new powering modes to the USB specification. The latter specification allows USB devices to draw up to 1.5 A (low and full speed or 900mA in Hi-Speed mode) from hubs and hosts or up to 1.8A for dedicated chargers that follow the Battery Charging Specification. The dedicated charger shorts the D+ and D- pins together and will not send or receive any information on those lines, allowing for the creation of very simple, high current chargers to be manufactured. The increased current (faster charging) will occur once the host/hub and devices both support the new charging specification.

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.

In September, 2007 the Open Mobile Terminal Platform—a forum dominated by mobile network operators but including 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.

Non-standard devices

A USB vacuum cleaner

A number of USB devices require more power than is permitted by the specifications for a single port. This is a common requirement of external hard and optical disc drives and other devices with motors or lamps. Such devices can be used with an external power supply of adequate rating, which is allowed by the standard, or by means of a dual input USB cable, one input of which is used for power and data transfer, the other solely for power, which makes the device a non-standard USB device. Some external hubs may, in practice, supply more power to USB devices than required by the specification but a standard compliant device must not depend on this.

Some non-standard USB devices use the 5 V power supply without participating in a proper USB network. These are usually referred to as USB decorations. The typical example is a USB-powered reading light; fans, mug heaters (though some may include USB hubs), battery chargers (particularly for mobile telephones), miniature vacuum cleaners, a miniature Lava Lamp, and even toy missile launchers are available. In most cases, these items contain no digitally based circuitry, and thus are not proper USB devices at all. This can theoretically cause problems with some computers — the USB specification requires that devices connect in a low-power mode (100 mA maximum) and state how much current they need, before switching, with the host's permission, into high-power mode.

In addition to limiting the total average power used by the device, the USB specification limits the inrush current (to charge decoupling and bulk capacitors) when the device is first connected; otherwise, connecting a device could cause glitches in the host's internal power. Also, USB devices are required to automatically enter ultra low-power suspend mode when the USB host is suspended; many USB hosts 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.

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 power from a car's electrical system to run attached devices. 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.

The Apple SuperDrive uses a non-standard extension to USB to negotiate with the MacBook Air to draw 1.5 A from the USB port. Due to the proprietary protocol, the SuperDrive only functions when connected directly to the Air, and cannot be operated with an external supply or through a USB hub, even if the hub can source the current specified on the package.

PoweredUSB

Main article: PoweredUSB

PoweredUSB uses standard USB signaling with the addition of extra power lines. It uses 4 additional pins to supply up to 6A at either 5V, 12V, or 24V (depending on keying) to peripheral devices. The wires and contacts on the USB portion have been upgraded to support higher current on the 5V 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. This standard 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.

USB compared with FireWire

USB was originally seen as a complement to FireWire (IEEE 1394), which was designed as a high-speed 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.

The most significant technical differences between FireWire and USB include the following:

• USB networks use a tiered-star topology, while FireWire networks use a repeater-based topology.
• USB uses a "speak-when-spoken-to" protocol; peripherals cannot communicate with the host unless the host specifically requests communication. A FireWire device can communicate with any other node at any time, subject to network conditions.
• A USB network relies on a single host at the top of the tree to control the network. In a FireWire network, any capable node can control the network.
• USB runs with a 5 V power line, whereas Firewire can supply up to 30 V.

These and other differences reflect the differing design goals of the two buses: USB was designed for simplicity and low cost, while FireWire was designed for high performance, particularly in time-sensitive applications such as audio and video. Although similar in theoretical maximum transfer rate, FireWire 400 tends to have the performance edge over USB 2.0 Hi-Speed in real-world uses, especially in high-bandwidth use such as external hard-drives. The newer FireWire 800 standard is twice as fast as FireWire 400 and outperforms USB 2.0 Hi-Speed both theoretically and practically. 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. Audio peripherals in particular are affected by the USB driver implementation.

Initially, cost was significant in USB being more widespread than FireWire. Over time, USB benefited from network effect.

Version history

Prereleases

Hi-Speed USB Logo

File:Otg usb.gif

USB OTG Logo

• USB 0.7: Released in November 1994.
• USB 0.8: Released in December 1994.
• USB 0.9: Released in April 1995.
• USB 0.99: Released in August 1995.
• USB 1.0 Release Candidate: Released in November 1995.

USB 1.0

• USB 1.0: Released in January 1996.
  Specified data rates of 1.5 Mbit/s (Low-Speed) and 12 Mbit/s (Full-Speed). Did not anticipate or pass-through monitors. Few such devices actually made it to market.
• USB 1.1: Released in September 1998.
  Fixed problems identified in 1.0, mostly relating to hubs. Earliest revision to be widely adopted.

USB 2.0

• USB 2.0: Released in April 2000.
  Added higher maximum speed of 480 Mbit/s (now called Hi-Speed). Further modifications to the USB specification have been done via Engineering Change Notices (ECN). The most important of these ECNs are included into the USB 2.0 specification package available from USB.org:
  • Mini-B Connector ECN: Released in October 2000.
    Specifications for Mini-B plug and receptacle. These should not be confused with Micro-B plug and receptacle.
  • Errata as of December 2000: Released in December 2000.
  • Pull-up/Pull-down Resistors ECN: Released in May 2002.
  • Errata as of May 2002: Released in May 2002.
  • Interface Associations ECN: Released in May 2003.
    New standard descriptor was added that allows multiple interfaces to be associated with a single device function.
  • Rounded Chamfer ECN: Released in October 2003.
    A recommended, compatible change to Mini-B plugs that results in longer lasting connectors.
  • Unicode ECN: Released in February 2005.
    This ECN specifies that strings are encoded using UTF-16LE. USB 2.0 did specify that Unicode is to be used but it did not specify the encoding.
  • Inter-Chip USB Supplement: Released in March 2006.
  • On-The-Go Supplement 1.3: Released in December 2006.
    USB On-The-Go makes it possible for two USB devices to communicate with each other without requiring a separate USB host. In practice, one of the USB devices acts as a host for the other device.
  • Battery Charging Specification 1.0: Released in March 2007.
    Adds support for dedicated chargers (power supplies with USB connectors), host chargers (USB hosts that can act as chargers) and the No Dead Battery provision which allows devices to temporarily draw 100 mA current after they have been attached. If a USB device is connected to dedicated charger or host charger, maximum current drawn by the device may be as high as 1.5 A. (Note that this document is not distributed with USB 2.0 specification package.)
  • Micro-USB Cables and Connectors Specification 1.01: Released in April 2007.
  • Link Power Management Addendum ECN: Released in July 2007.
    This adds a new power state between enabled and suspended states. Device in this state is not required to reduce its power consumption. However, switching between enabled and sleep states is much faster than switching between enabled and suspended states, which allows devices to sleep while idle.
  • High-Speed Inter-Chip USB Electrical Specification Revision 1.0: Released in September 2007.

USB 3.0

On September 18, 2007, Pat Gelsinger demonstrated USB 3.0 at the Intel Developer Forum. USB 3.0 is targeted at ten times the current bitrate, reaching roughly 4.8 Gbit/s (600MB/s) by utilizing two additional high-speed differential pairs for "Superspeed" mode add a clock frequency of 2.5GHz , and with the possibility for optical interconnect. The two new differential pairs make the cable about as thick as an ethernet cable and provide full-duplex transfers. The USB 3.0 specification was 90% complete as of August 13, 2008 and commercial products are expected to arrive in 2009 or 2010. USB 3.0 is designed to be backwards-compatible with USB 2.0 only and employs more efficient protocols such as 8b/10b encoding to conserve power while increasing the maximum power available for connected devices.

Related technologies

The PictBridge standard allows for interconnecting consumer imaging devices. It typically uses USB as the underlying communication layer.

The USB Implementers Forum is working on a wireless networking standard based on the USB protocol. Wireless USB is intended as a cable-replacement technology, and will use ultra-wideband wireless technology for data rates of up to 480 Mbit/s. Wireless USB is well suited to wireless connection of PC centric devices, just as Bluetooth is now widely used for mobile phone centric personal networks (at much lower data rates).

See also

Serial cable (Bidirectional Communication)

• Serial ATA
• RS-232
• Serial Null Modem Cable
• FireWire
• HP-IL
• Sync cable
• Enhanced mini-USB
• USB On-The-Go (master / slave)
• Ethernet over USB
• Wireless USB
• USB streaming
• HCI (UHCI, EHCI {xHCI 0.9 SuperSpeed - USB 3.0}, WHCI 1.0,)
• CEA-936-A
• U3
• UGA (USB Graphic Adapter)
• List of device bandwidths
  • ACCESS.bus
  • Apple Desktop Bus
• PS/2 connector
• DE-9 connector

External links

• Overview comparison between FW and USB
• Home of USB Implementers Forum, Inc.
  • USB Specification
• Intel Universal Host Controller Interface (UHCI)

USB 3.0

• USB Promoter Group - Mission Statement at USB.org
• Intel announces, demonstrates USB 3.0 Ars Technica 2007-09-18
• USB 3.0 guns for Firewire eeTimes 2007-09-18