Universal Serial Bus
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 was designed to standardize the connection of computer peripherals (including 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 six billion USB ports and interfaces were in the global marketplace, and about 2 billion were being sold each year.
USB was designed to standardize the connection of computer peripherals (including 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 six billion USB ports and interfaces were in the global marketplace, and about 2 billion were being sold each year.
History
A group of seven companies began the development of 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. A team
including Ajay Bhatt worked on the standard at Intel; the first integrated circuits supporting USB were produced 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.
A USB Standard Type A plug, the most common USB plug
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(hp), 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 forty times 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), decrease power consumption, increase power output, and 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.
A PCI USB 2.0 card for a computer motherboard
The Hi-Speed USB Logo
The Super-Speed USB Logo
USB 3.0 was released in November 2008. The standard defines a
new "SuperSpeed" mode with a signalling speed of 5 Gbit/s and a usable
data rate of up to 4 Gbit/s. USB 3.0 reduces the time required for data
transmission, thereby reducing power consumption, and is backwards
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 (raw data rate), in addition to the
modes supported by earlier versions. The payload 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.
Device classes include:
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
(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-Summer 2004 computers are capable of booting from USB mass storage devices, USB is not intended as 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.
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. For mice and keyboards with dual-protocol support, an adaptor that contain no logic circuitry may be used: the hardware in the USB keyboard or mouse is designed to detect whether it is connected to a USB or PS/2 port, and communicate using the appropriate protocol. Converters also exist that connect PS/2 keyboards and mice (usually one of each) to a USB port. These devices present two HID endpoints to the system and use a microcontroller to perform bidirectional data translation between the two standards.
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.
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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(hp), 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 forty times 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), decrease power consumption, increase power output, and 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
Prereleases
The USB standard evolved through several versions before its official release in 1996:- 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 (Full Speed)
Released in January 1996, USB 1 specified data rates of 1.5 Mb/s (Low-Bandwidth) and 12 Mb/s (Full-Bandwidth). It did not allow for extension cables or pass-through monitors (due to timing and power limitations). Few USB devices made it to market until USB 1.1, released in August 1998, which fixed problems identified in 1.0, mostly relating to hubs. 1.1 was the earliest revision that was widely adopted. USB 1 is usually colored white, but some white USB ports are "USB 2 Compatible"USB 2.0 (High Speed)
- USB 2.0: Released in April 2000. Added higher maximum signaling rate of 480 Mbit/s (effective throughput up to 35 MB/s or 280 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
USB 3.0 (Super Speed)
Main article: USB 3.0
- Power Handling Capabilities: As with previous USB versions, USB 3.0 ports come in low-power and high-power variants, providing 150 mA and 900 mA respectively while simultaneously transmitting data at SuperSpeed rates. Additionally, there is a Battery Charging Specification (Version 1.2 – December 2010), which increases the power handling capability to 1.5 A but does not allow concurrent data transmission.The Battery Charging Specification requires that the physical ports themselves be capable of handling 5 A of current but the specification limits the maximum current drawn to 1.5 A.
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—built into the host controller—is the root hub.Device classes
The functionality of USB devices is defined by class codes, communicated to the USB host to affect the loading of suitable software driver modules for each connected device. This provides for adaptability and device independence of the host to support new devices from different manufacturers.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 (MSC or UMS) | 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) |
| 10h | Interface | Audio/Video (AV) | Webcam, TV |
| 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 / USB key
Though most post-Summer 2004 computers are capable of booting from USB mass storage devices, USB is not intended as 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.
Media Transfer Protocol
Media Transfer Protocol (MTP) was designed by Microsoft to give higher-level access to a device's filesystem than USB mass storage, at the level of files rather than disk blocks. It also has optional DRM features. MTP was designed for use with portable media players, but it has since been adopted as the primary storage access protocol of the Android operating system.Human interface devices
Main article: USB human interface device class
Joysticks, keypads, tablets and other human-interface devices (HIDs) are also progressively migrating from MIDI, and PC game port connectors to USB.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. For mice and keyboards with dual-protocol support, an adaptor that contain no logic circuitry may be used: the hardware in the USB keyboard or mouse is designed to detect whether it is connected to a USB or PS/2 port, and communicate using the appropriate protocol. Converters also exist that connect PS/2 keyboards and mice (usually one of each) to a USB port. These devices present two HID endpoints to the system and use a microcontroller to perform bidirectional data translation between the two standards.
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