Universal Serial Bus (USB) is an industry standard developed in the mid-1990s that defines the cables, connectors and communications protocols used in a bus for connection, communication and power supply between computers and electronic devices.
USB is not a true bus, meaning only the root hub sees the entire electrical communications. Or, there is no method to monitor upstream communications from a down stream device.
USB was designed to standardize the connection of computer peripherals, such as keyboards, pointing devices, digital cameras, printers, portable media players, disk drives and network adapters to personal computers, both to communicate and to supply electric power. It has become commonplace on other devices, such as smartphones, PDAs and video game consoles. USB has effectively replaced a variety of earlier interfaces, such as serial and parallel ports, as well as separate power chargers for portable devices.
As of 2008, approximately 6 billion USB ports and interfaces were in the global marketplace, and about 2 billion were being sold each year.
A group of seven companies began development on USB in 1994: Compaq, DEC, IBM, Intel, Microsoft, NEC and Nortel. The goal was to make it fundamentally easier to connect external devices to PCs by replacing the multitude of connectors at the back of PCs, addressing the usability issues of existing interfaces, and simplifying software configuration of all devices connected to USB, as well as permitting greater data rates for external devices. The first silicon for USB was made by Intel in 1995.
The original USB 1.0 specification, which was introduced in January 1996, defined data transfer rates of 1.5 Mbit/s "Low Speed" and 12 Mbit/s "Full Speed". The first widely used version of USB was 1.1, which was released in September 1998. The 12 Mbit/s data rate was intended for higher-speed devices such as disk drives, and the lower 1.5 Mbit/s rate for low data rate devices such as joysticks.
The USB 2.0 specification was released in April 2000 and was ratified by the USB Implementers Forum (USB-IF) at the end of 2001. Hewlett-Packard, Intel, Lucent Technologies (now Alcatel-Lucent), 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 the original USB 1.1 specification.
The USB 3.0 specification was published on 12 November 2008. Its main goals were to increase the data transfer rate (up to 5 Gbit/s), to decrease power consumption, to increase power output, and to be backwards-compatible with USB 2.0. USB 3.0 includes a new, higher speed bus called SuperSpeed in parallel with the USB 2.0 bus. For this reason, the new version is also called SuperSpeed.The first USB 3.0 equipped devices were presented in January 2010.
USB devices are linked in series through hubs. One hub is known as the root hub which is built into the host controller.
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). This kind of device is called composite device. An alternative for this is 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.
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. Because pipes correspond 1-to-1 to endpoints, the terms are sometimes used interchangeably. A USB device can have up to 32 endpoints: 16 into the host controller and 16 out of the host controller. The USB standard reserves one endpoint of each type, leaving a theoretical maximum of 30 for normal use. USB devices seldom have this many endpoints.
There are two types of pipes: stream and message pipes depending on the type of data transfer.
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. A single device function composed of independently controlled interfaces is called a composite device. A composite device only has a single device address because the host only assigns a device address to a function.
When a USB device is first connected to a USB host, the USB device enumeration process is started. The enumeration starts by sending a reset signal to the USB device. The data rate of the USB device is determined during the reset signaling. After reset, the USB device's information is read by the host and the device is assigned a unique 7-bit address. If the device is supported by the host, the device drivers needed for communicating with the device are loaded and the device is set to a configured state. If the USB host is restarted, the enumeration process is repeated for all connected devices.
The host controller directs traffic flow to devices, so no USB device can transfer any data on the bus without an explicit request from the host controller. In USB 2.0, the host controller polls the bus for traffic, usually in a round-robin fashion. The throughput of each USB port is determined by the slower speed of either the USB port or the USB device connected to the port.
High-speed USB 2.0 hubs contain devices called transaction translators that convert between high-speed USB 2.0 buses and full and low speed buses. When a high-speed USB 2.0 hub is plugged into a high-speed USB host or hub, it will operate in high-speed mode. The USB hub will then either use one transaction translator per hub to create a full/low-speed bus that is routed to all full and low speed devices on the hub, or will use one transaction translator per port to create an isolated full/low-speed bus per port on the hub.
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.
Device classes include:
USB implements connections to storage devices using a set of standards called the USB mass storage device class (MSC or UMS). This was at first intended for traditional magnetic and optical drives, but has been extended to support a wide variety of devices, particularly flash drives, 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. The ability to boot a write-locked SD card with a USB adapter is particularly advantageous for maintaining the integrity and non-corruptible, pristine state of the booting medium.
Though most post-2005 computers are capable of booting from 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. 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 and so on), several manufacturers offer external portable USB hard disk 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 30 MB/s for USB 2.0 and potentially 400 MB/s or more for USB 3.0). These external drives have typically included a "translating device" that bridges between a drive's interface to a USB interface port. Functionally, the drive appears to the user much like an internal drive. Other competing standards for external drive connectivity include eSATA, ExpressCard (now at version 2.0), FireWire (IEEE 1394), and most recently Thunderbolt.
Another use for USB mass storage devices is the portable execution of software applications (such as web browsers and VoIP clients) with no need to install them on the host computer.
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. The connector mounted on the host or device is called the receptacle, and the connector attached to the cable is called the plug. In the case of an extension cable, the connector on one end is a receptacle. The official USB specification documents periodically define the term male to represent the plug, and female to represent the receptacle.
By design, it is difficult to insert a USB plug into its receptacle incorrectly. Connectors cannot be plugged in upside down. The USB specification states that the required USB icon is to be embossed on the "topside" of the USB plug, which "provides easy user recognition and facilitates alignment during the mating process". The specification also shows that the "recommended" "Manufacturer's logo" ("engraved" on the diagram but not specified in the text) is on the opposite side of the USB icon. The specification further states "the USB Icon is also located adjacent to each receptacle. Receptacles should be oriented to allow the icon on the plug to be visible during the mating process". However, the specification does not consider the height of the device compared to the eye level height of the user, so the side of the cable that is "visible" when mated to a computer on a desk can depend on whether the user is standing or kneeling. Despite the specification there are many products on the market which have the USB icon on the wrong side of the plug.
Only moderate insertion/removal force is needed. USB cables and small USB devices are held in place by the gripping force from the receptacle (without need of the screws, clips, or thumb-turns other connectors have required). 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.[citation needed]
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. 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 cyclic networks and the standard connectors from incompatible USB devices are themselves incompatible. Unlike other communications systems (e.g. network cabling) gender changers make little sense with USB and are almost never used.[citation needed]
The connector construction always ensures that the external sheath on the plug makes contact with its counterpart in the receptacle before any of the four connectors within make electrical contact. The external metallic sheath is typically connected to system ground, thus dissipating damaging static charges. This enclosure design also provides a degree of protection from electromagnetic interference to the USB signal while it travels through the mated connector pair (the only location when the otherwise twisted data pair travels in parallel). In addition, because of the required sizes of the power and common connections, they are made after the system ground but before the data connections. This type of staged make-break timing allows for electrically safe hot-swapping.
The newer Micro-USB receptacles are designed for up to 10,000 cycles of insertion and removal between the receptacle and plug, compared to 1,500 for the standard USB and 5,000 for the Mini-USB receptacle. This is accomplished by adding a locking device and by moving the leaf-spring connector from the jack to the plug, so that the most-stressed part is on the cable side of the connection. This change was made so that the connector on the less expensive cable would bear the most wear instead of the more expensive micro-USB device.
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. 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. Even after the cable is hooked up and the units are communicating, the two units may "swap" ends under program control. This capability is meant for units such as 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.
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 B connector was necessary so that cabling could be plug ended at both ends and still prevent users from connecting one computer receptacle to another. The first engineering change notice to the USB 2.0 specification added Mini-B plugs and receptacles.
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. This difference in connection can be exploited by inserting the connector only partially. For example, some battery-powered MP3 players switch into file transfer mode and cannot play MP3 files while a USB plug is fully inserted, but can be operated in MP3 playback mode using USB power by inserting the plug only part way so that the power slots make contact while the data slots do not. This enables those devices to be operated in MP3 playback mode while getting power from the cable.[original research?]
To reliably enable a charge-only feature, modern USB accessory peripherals now include charging cables that provide power connections to the host port but no data connections, and both home and vehicle charging docks are available that supply power from a converter device and do not include a host device and data pins, allowing any capable USB device to be charged or operated from a standard USB cable.
The USB 2.0 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. 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.
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. This two-connector-type scheme (A/B) prevents a user from accidentally creating an electrical loop.
Various connectors have been used for smaller devices such as PDAs, mobile phones or digital cameras. These include the now-deprecated (but standardized) Mini-A and Mini-AB connectors (Mini-B are still supported but not OTG compliant), Micro-A, and Micro-B connectors.
The Mini-A and Mini-B plugs are approximately 3 by 7 mm. The micro-USB plugs have a similar width and 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.
The Micro-USB connector was announced by the USB-IF on 4 January 2007. The Mini-A connector and the Mini-AB receptacle connector were deprecated on 23 May 2007. While many currently available devices and cables still use Mini plugs, the newer Micro connectors are being widely adopted and as of December 2010, they are the most widely used. 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.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,[32] along with a Standard-A receptacle to Micro-A plug adapter.
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.[33] These include various types of battery chargers, allowing Micro-USB to be the single external cable link needed by some devices.
As of 30 January 2009 Micro-USB has been accepted and is being used by almost all cell phone manufacturers as the standard charging port (including Hewlett-Packard, HTC, LG, Motorola, Nokia, Research In Motion, Samsung, Sony Ericsson) in most of the world.[citation needed]
On 29 June 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. 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.[34] Following a mandate from the European Commission, 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 External Power Supply (EPS) based on micro-USB.[35]
In addition, on 22 October 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, UCS chargers will also include a 4-star or higher efficiency rating—up to three times more energy-efficient than an unrated charger".[36]
A USB On-The-Go device is required to have one, and only one USB connector: a Mini-AB or Micro-AB receptacle. This receptacle is capable of accepting both Mini-A and Mini-B plugs, and alternatively, Micro-A and Micro-B plugs, attached to any of the legal cables and adapters as defined in Micro-USB1.01.
The OTG device with the A-plug inserted is called the A-device and is responsible for powering the USB interface when required and by default assumes the role of host. The OTG device with the B-plug inserted is called the B-device and by default assumes the role of peripheral. An OTG device with no plug inserted defaults to acting as a B-device. If an application on the B-device requires the role of host, then the HNP protocol is used to temporarily transfer the host role to the B-device.
OTG devices attached either to a peripheral-only B-device or a standard/embedded host will have their role fixed by the cable since in these scenarios it is only possible to attach the cable one way around.[citation needed]
NS: non-standard, existing for specific proprietary purposes, and not interoperable with USB-IF compliant equipment.
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.[23] 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.[28]
The data cables for USB 1.x and USB 2.x use a twisted pair to reduce noise and crosstalk. USB 3.0 cables contain twice as many wires as USB 2.x to support SuperSpeed data transmission, and are thus larger in diameter.[39]
The USB 1.1 Standard specifies that a standard cable can have a maximum length of 3 meters with devices operating at Low Speed (1.5 Mbit/s), and a maximum length of 5 meters with devices operating at Full Speed (12 Mbit/s).[citation needed]
USB 2.0 provides for a maximum cable length of 5 meters for devices running at Hi Speed (480 Mbit/s). The primary reason for this limit is the maximum allowed round-trip delay of about 1.5 μs. If USB host commands are unanswered by the USB device within the allowed time, the host considers the command lost. When adding USB device response time, delays from the maximum number of hubs added to the delays from connecting cables, the maximum acceptable delay per cable amounts to 26 ns.[40] 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 wire).
The USB 3.0 standard does not directly specify a
USB is not a true bus, meaning only the root hub sees the entire electrical communications. Or, there is no method to monitor upstream communications from a down stream device.
USB was designed to standardize the connection of computer peripherals, such as keyboards, pointing devices, digital cameras, printers, portable media players, disk drives and network adapters to personal computers, both to communicate and to supply electric power. It has become commonplace on other devices, such as smartphones, PDAs and video game consoles. USB has effectively replaced a variety of earlier interfaces, such as serial and parallel ports, as well as separate power chargers for portable devices.
As of 2008, approximately 6 billion USB ports and interfaces were in the global marketplace, and about 2 billion were being sold each year.
History
The basic USB trident logo; each released version has a specific logo variant[clarification needed]
The original USB 1.0 specification, which was introduced in January 1996, defined data transfer rates of 1.5 Mbit/s "Low Speed" and 12 Mbit/s "Full Speed". The first widely used version of USB was 1.1, which was released in September 1998. The 12 Mbit/s data rate was intended for higher-speed devices such as disk drives, and the lower 1.5 Mbit/s rate for low data rate devices such as joysticks.
A USB Standard Type A plug, the most common USB plug
The USB 3.0 specification was published on 12 November 2008. Its main goals were to increase the data transfer rate (up to 5 Gbit/s), to decrease power consumption, to increase power output, and to be backwards-compatible with USB 2.0. USB 3.0 includes a new, higher speed bus called SuperSpeed in parallel with the USB 2.0 bus. For this reason, the new version is also called SuperSpeed.The first USB 3.0 equipped devices were presented in January 2010.
Version history
A PCI USB 2.0 card for a computer motherboard
Prereleases
The USB standard evolved through several versions before its official release in 1995:- 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-Bandwidth) and 12 Mbit/s (Full-Bandwidth). Does not allow for extension cables or pass-through monitors (due to timing and power limitations). Few such devices actually made it to market. - USB 1.1: Released in August 1998.
Fixed problems identified in 1.0, mostly relating to hubs. Earliest revision to be widely adopted.
USB 2.0
The Hi-Speed USB Logo
- USB 2.0: Released in April 2000. Added higher maximum bandwidth of 480 Mbit/s (60 MB/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-A and Mini-B Connector ECN: Released in October 2000.
Specifications for Mini-A and B plug and receptacle. Also receptacle that accepts both plugs for On-The-Go. These should not be confused with Micro-B plug and receptacle. - Errata as of December 2000: Released in December 2000.
- Pull-up/Pull-down Resistors ECN: Released in May 2002.
- Errata as of May 2002: Released in May 2002.
- Interface Associations ECN: Released in May 2003.
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 1.1: Released in March 2007 (Updated 15 Apr 2009).
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, maximum current drawn by the device may be as high as 1.8 A. (Note that this document is not distributed with USB 2.0 specification package only USB 3.0 and USB On-The-Go.) - Micro-USB Cables and Connectors Specification 1.01: Released in April 2007.
- Link Power Management Addendum ECN: Released in July 2007.
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. - Battery Charging Specification 1.2. Released in December 2010.
Several changes and increasing limits including allowing 1.5A on charging ports for unconfigured devices, allowing High Speed communication while having a current up to 1.5A and allowing a maximum current of 5A.
- Mini-A and Mini-B Connector ECN: Released in October 2000.
USB 3.0
The Super-Speed USB Logo
Main article: USB 3.0
- USB 3.0 was released in November 2008. The standard specifies a maximum transmission speed of up to 5 Gbit/s (625 MB/s), which is more than 10 times as fast as USB 2.0 (480 Mbit/s, or 60 MB/s), although this speed is typically only achieved using powerful professional grade or developmental equipment. USB 3.0 reduces the time required for data transmission, reduces power consumption, and is backward compatible with USB 2.0. The USB 3.0 Promoter Group announced on 17 November 2008 that the specification of version 3.0 had been completed and had made the transition to the USB Implementers Forum (USB-IF), the managing body of USB specifications.This move effectively opened the specification to hardware developers for implementation in products. The new "SuperSpeed" bus provides a fourth transfer mode at 5.0 Gbit/s, in addition to the modes supported by earlier versions. The raw throughput is 4 Gbit/s (using 8b/10b encoding), and the specification considers it reasonable to achieve around 3.2 Gbit/s (0.4 GB/s or 400 MB/s), which should increase with future hardware advances. Communication is full-duplex during SuperSpeed; in the modes supported previously, by 1.x and 2.0), communication is half-duplex, with direction controlled by the host.
- Battery Charging Specification 1.2. released in December 2010. Several changes and increasing limits including allowing 1.5A on charging ports for unconfigured devices, allowing High Speed communication while supplying a current up to 1.5A, and allowing a maximum current of 5A.
System design
The design architecture of USB is asymmetrical in its topology, 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 implement multiple host controllers and each host controller may provide one or more USB ports. Up to 127 devices, including hub devices if present, may be connected to a single host controller.USB devices are linked in series through hubs. One hub is known as the root hub which is built into the host controller.
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). This kind of device is called composite device. An alternative for this is 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.
USB endpoints actually reside on the connected device: the channels to the host are referred to as pipes
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).
- 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).
- 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.
Two USB standard A receptacles on the front of a computer
When a USB device is first connected to a USB host, the USB device enumeration process is started. The enumeration starts by sending a reset signal to the USB device. The data rate of the USB device is determined during the reset signaling. After reset, the USB device's information is read by the host and the device is assigned a unique 7-bit address. If the device is supported by the host, the device drivers needed for communicating with the device are loaded and the device is set to a configured state. If the USB host is restarted, the enumeration process is repeated for all connected devices.
The host controller directs traffic flow to devices, so no USB device can transfer any data on the bus without an explicit request from the host controller. In USB 2.0, the host controller polls the bus for traffic, usually in a round-robin fashion. The throughput of each USB port is determined by the slower speed of either the USB port or the USB device connected to the port.
High-speed USB 2.0 hubs contain devices called transaction translators that convert between high-speed USB 2.0 buses and full and low speed buses. When a high-speed USB 2.0 hub is plugged into a high-speed USB host or hub, it will operate in high-speed mode. The USB hub will then either use one transaction translator per hub to create a full/low-speed bus that is routed to all full and low speed devices on the hub, or will use one transaction translator per port to create an isolated full/low-speed bus per port on the hub.
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.
Device classes
The functionality of USB devices is defined by class codes, communicated to the USB host to effect the loading of suitable software driver modules for each connected device. This provides for adaptability and device independence of the host to support new devices from different manufacturers.Device classes include:
| Class | Usage | Description | Examples, or exception |
|---|---|---|---|
| 00h | Device | Unspecified | Device class is unspecified, interface descriptors are used to determine needed drivers |
| 01h | Interface | Audio | Speaker, microphone, sound card, MIDI |
| 02h | Both | Communications and CDC Control | Modem, Ethernet adapter, Wi-Fi adapter |
| 03h | Interface | Human interface device (HID) | Keyboard, mouse, joystick |
| 05h | Interface | Physical Interface Device (PID) | Force feedback joystick |
| 06h | Interface | Image | Webcam, scanner |
| 07h | Interface | Printer | Laser printer, inkjet printer, CNC machine |
| 08h | Interface | Mass storage | USB flash drive, memory card reader, digital audio player, digital camera, external drive |
| 09h | Device | USB hub | Full bandwidth hub |
| 0Ah | Interface | CDC-Data | Used together with class 02h: communications and CDC control |
| 0Bh | Interface | Smart Card | USB smart card reader |
| 0Dh | Interface | Content security | Fingerprint reader |
| 0Eh | Interface | Video | Webcam |
| 0Fh | Interface | Personal Healthcare | Pulse monitor (watch) |
| DCh | Both | Diagnostic Device | USB compliance testing device |
| E0h | Interface | Wireless Controller | Bluetooth adapter, Microsoft RNDIS |
| EFh | Both | Miscellaneous | ActiveSync device |
| FEh | Interface | Application-specific | IrDA Bridge, Test & Measurement Class (USBTMC),USB DFU (Direct Firmware update) |
| FFh | Both | Vendor-specific | Indicates that a device needs vendor specific drivers |
USB mass storage
A flash drive, a typical USB mass-storage device
Though most post-2005 computers are capable of booting from 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. 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 and so on), several manufacturers offer external portable USB hard disk 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 30 MB/s for USB 2.0 and potentially 400 MB/s or more for USB 3.0). These external drives have typically included a "translating device" that bridges between a drive's interface to a USB interface port. Functionally, the drive appears to the user much like an internal drive. Other competing standards for external drive connectivity include eSATA, ExpressCard (now at version 2.0), FireWire (IEEE 1394), and most recently Thunderbolt.
Another use for USB mass storage devices is the portable execution of software applications (such as web browsers and VoIP clients) with no need to install them on the host computer.
Human interface devices (HIDs)
Main article: USB human interface device class
Joysticks, keypads, tablets and other human-interface devices are also progressively migrating from MIDI, and PC game port connectors to USB.[citation needed] USB mice and keyboards can usually 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. Converters also exist to allow PS/2 keyboards and mice (usually one of each) to be connected to a USB port. These devices present two HID endpoints to the system and use a microcontroller to perform bidirectional translation of data between the two standards.[citation needed]Physical appearance
Pinouts of Standard, Mini, and Micro USB plugs. The white areas in these drawings represent hollow spaces. As the plugs are shown here, the USB logo (with optional letter A or B) is on the top of the overmold in all cases.
Micro-B USB 3.0 compatible (cable/male end)
USB 2.0 connector on the side of the specification standard micro USB 3.0 connector are aligned pin-minute increase in the standard.
No.1: power (VBUS)
No.2: USB 2.0 differential pair (D−)
No.3: USB 2.0 differential pair (D+)
No.4: USB OTG ID for identifying lines
No.5: GND
No.6: USB 3.0 signal transmission line (−)
No.7: USB 3.0 signal transmission line (+)
No.8: GND
No.9: USB 3.0 signal receiving line (−)
No.10: USB 3.0 signal receiving line (+)
USB 2.0 connector on the side of the specification standard micro USB 3.0 connector are aligned pin-minute increase in the standard.
No.1: power (VBUS)
No.2: USB 2.0 differential pair (D−)
No.3: USB 2.0 differential pair (D+)
No.4: USB OTG ID for identifying lines
No.5: GND
No.6: USB 3.0 signal transmission line (−)
No.7: USB 3.0 signal transmission line (+)
No.8: GND
No.9: USB 3.0 signal receiving line (−)
No.10: USB 3.0 signal receiving line (+)
| Pin | Name | Cable color | Description |
|---|---|---|---|
| 1 | VBUS | Red | +5 V |
| 2 | D− | White | Data − |
| 3 | D+ | Green | Data + |
| 4 | GND | Black | Ground |
| Pin | Name | Cable color | Description |
|---|---|---|---|
| 1 | VBUS | Red | +5 V |
| 2 | D− | White | Data − |
| 3 | D+ | Green | Data + |
| 4 | ID | None | Permits distinction of host connection from slave connection * host: connected to Signal ground * slave: not connected |
| 5 | GND | Black | Signal ground |
Connector properties
Standard type A plug and receptacle
Usability and "upside down" connectors
USB extension cord
Only moderate insertion/removal force is needed. USB cables and small USB devices are held in place by the gripping force from the receptacle (without need of the screws, clips, or thumb-turns other connectors have required). 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.[citation needed]
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. 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 cyclic networks and the standard connectors from incompatible USB devices are themselves incompatible. Unlike other communications systems (e.g. network cabling) gender changers make little sense with USB and are almost never used.[citation needed]
Durability
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. 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.The connector construction always ensures that the external sheath on the plug makes contact with its counterpart in the receptacle before any of the four connectors within make electrical contact. The external metallic sheath is typically connected to system ground, thus dissipating damaging static charges. This enclosure design also provides a degree of protection from electromagnetic interference to the USB signal while it travels through the mated connector pair (the only location when the otherwise twisted data pair travels in parallel). In addition, because of the required sizes of the power and common connections, they are made after the system ground but before the data connections. This type of staged make-break timing allows for electrically safe hot-swapping.
The newer Micro-USB receptacles are designed for up to 10,000 cycles of insertion and removal between the receptacle and plug, compared to 1,500 for the standard USB and 5,000 for the Mini-USB receptacle. This is accomplished by adding a locking device and by moving the leaf-spring connector from the jack to the plug, so that the most-stressed part is on the cable side of the connection. This change was made so that the connector on the less expensive cable would bear the most wear instead of the more expensive micro-USB device.
Compatibility
The USB standard specifies relatively loose tolerances for compliant USB connectors to minimize physical incompatibilities in connectors from different vendors. To address a weakness present in some other connector standards, the USB specification also defines limits to the size of a connecting device in the area around its plug. This was done to prevent a device from blocking adjacent ports due to the size of the cable strain relief mechanism (usually molding integral with the cable outer insulation) at the connector. Compliant devices must either fit within the size restrictions or support a compliant extension cable which does.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. 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. Even after the cable is hooked up and the units are communicating, the two units may "swap" ends under program control. This capability is meant for units such as 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.
Compatibility of USB 3.0 connectors
- TypeA plugs and receptacles from both USB 3.0 and USB 2.0 are designed to interoperate.
- TypeB receptacles in USB 3.0 are somewhat larger than would be required for a TypeB plug in USB 2.0 and earlier. This is intended to allow connecting an older TypeB plug into a newer USB 3.0 TypeB receptacle. Accordingly, a USB 3.0 TypeB receptacle on a peripheral device can be connected using the corresponding plug end of a USB 2.0 TypeB cable.
- TypeB plugs in USB 3.0 are somewhat larger; therefore, a USB 3.0 TypeB plug cannot enter a USB 2.0 or earlier TypeB receptacle. Accordingly, normal USB 3.0 TypeB plugs cannot be inserted into normal USB 2.0 TypeB receptacles found on peripheral devices (and connect them to a computer).
- A receptacle for eSATAp (eSATA/USB Combo) is designed to accept USB TypeA plugs from USB2.0 and USB3.0, but transfer speeds are restricted to USB2.0 limits.
Connector types
Types of USB connectors left to right (ruler in centimeters): Micro-B plug, a non-USB proprietary plug, Mini-B plug (5-pin, upside down), Standard-A receptacle (upside down), Standard-A plug, Standard-B plug
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. This difference in connection can be exploited by inserting the connector only partially. For example, some battery-powered MP3 players switch into file transfer mode and cannot play MP3 files while a USB plug is fully inserted, but can be operated in MP3 playback mode using USB power by inserting the plug only part way so that the power slots make contact while the data slots do not. This enables those devices to be operated in MP3 playback mode while getting power from the cable.[original research?]
To reliably enable a charge-only feature, modern USB accessory peripherals now include charging cables that provide power connections to the host port but no data connections, and both home and vehicle charging docks are available that supply power from a converter device and do not include a host device and data pins, allowing any capable USB device to be charged or operated from a standard USB cable.
USB standard connectors
Pin configuration of the USB connectors Standard A/B, viewed looking into face/end of plug
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.
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. This two-connector-type scheme (A/B) prevents a user from accidentally creating an electrical loop.
Mini and Micro connectors
Micro B USB
USB Mini A (left) and USB Mini B (right) plugs
The Mini-A and Mini-B plugs are approximately 3 by 7 mm. The micro-USB plugs have a similar width and 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.
The Micro-USB connector was announced by the USB-IF on 4 January 2007. The Mini-A connector and the Mini-AB receptacle connector were deprecated on 23 May 2007. While many currently available devices and cables still use Mini plugs, the newer Micro connectors are being widely adopted and as of December 2010, they are the most widely used. 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.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,[32] along with a Standard-A receptacle to Micro-A plug adapter.
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.[33] These include various types of battery chargers, allowing Micro-USB to be the single external cable link needed by some devices.
As of 30 January 2009 Micro-USB has been accepted and is being used by almost all cell phone manufacturers as the standard charging port (including Hewlett-Packard, HTC, LG, Motorola, Nokia, Research In Motion, Samsung, Sony Ericsson) in most of the world.[citation needed]
On 29 June 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. 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.[34] Following a mandate from the European Commission, 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 External Power Supply (EPS) based on micro-USB.[35]
In addition, on 22 October 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, UCS chargers will also include a 4-star or higher efficiency rating—up to three times more energy-efficient than an unrated charger".[36]
A USB On-The-Go device is required to have one, and only one USB connector: a Mini-AB or Micro-AB receptacle. This receptacle is capable of accepting both Mini-A and Mini-B plugs, and alternatively, Micro-A and Micro-B plugs, attached to any of the legal cables and adapters as defined in Micro-USB1.01.
The OTG device with the A-plug inserted is called the A-device and is responsible for powering the USB interface when required and by default assumes the role of host. The OTG device with the B-plug inserted is called the B-device and by default assumes the role of peripheral. An OTG device with no plug inserted defaults to acting as a B-device. If an application on the B-device requires the role of host, then the HNP protocol is used to temporarily transfer the host role to the B-device.
OTG devices attached either to a peripheral-only B-device or a standard/embedded host will have their role fixed by the cable since in these scenarios it is only possible to attach the cable one way around.[citation needed]
Host interface receptacles
The following receptacles accept the following plugs:| Receptacle | Plug | ||||
|---|---|---|---|---|---|
| Yes | No | No | No | No | |
| No | Yes | No | No | No | |
| No | No | Yes | No | No | |
| No | No | No | Yes | Yes | |
| No | No | No | No | Yes | |
Cable plugs (USB 1.x/2.0)
Cables exist with pairs of plugs:| Plug | Plug | ||||
|---|---|---|---|---|---|
| Yes | NS | Yes | Yes | NS | |
| No | NS | No | No | ||
| No | NS | No | |||
| Yes | No | ||||
| No | |||||
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.[23] 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.[28]
Proprietary connectors and formats
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HTC ExtUSB
HTC ExtMicro USB port and connector
Nokia Pop-Port connector
- Microsoft's original Xbox game console uses standard USB 1.1 signalling in its controllers and memory cards, but uses proprietary connectors and ports. The Xbox 360 (pre Xbox 360 S) has two Memory Unit ports which use USB signalling, but proprietary connectors and 3.3v power.[37]
- 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.
- HTC manufactured Windows Mobile and Android-based devices 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.
- HTC introduced devices (e.g. Flyer tablet, Droid Incredible, Amaze and Rezound Android phones) in 2010 featuring a 12-pin ExtMicro USB variant, backwards-compatible with Micro-USB.[38]
- Nokia included 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.
- 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.
- 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. The indentation on the keyboard's plug does not interfere with insertion into a standard USB socket. Apple's proprietary 30-pin dock connector on its iPod, iPhone, and iPad serves purposes in addition to USB (such as analog audio, IEEE-1394, eSATA and HDMI). The second, third, and fourth generation iPod Shuffle uses a TRRS connector to carry USB, audio, or power signals.
- HP Tablet computers use non-standard connectors to transmit the USB signals between the keyboard/mouse unit and the Computer Tablet Unit.
- A number of digital cameras use an 8-pin variant of Micro USB.
- The United States Army's Land Warrior system uses standard USB signaling with 15.6 V power using a ruggedized connector from Glenair, Inc.
Cabling
A USB twisted pair, where the "Data +" and "Data −" conductors are twisted together in a double helix. The wires are enclosed in a further layer of shielding.
The USB 1.1 Standard specifies that a standard cable can have a maximum length of 3 meters with devices operating at Low Speed (1.5 Mbit/s), and a maximum length of 5 meters with devices operating at Full Speed (12 Mbit/s).[citation needed]
USB 2.0 provides for a maximum cable length of 5 meters for devices running at Hi Speed (480 Mbit/s). The primary reason for this limit is the maximum allowed round-trip delay of about 1.5 μs. If USB host commands are unanswered by the USB device within the allowed time, the host considers the command lost. When adding USB device response time, delays from the maximum number of hubs added to the delays from connecting cables, the maximum acceptable delay per cable amounts to 26 ns.[40] 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 wire).
The USB 3.0 standard does not directly specify a
