USB, Standard

Universal Serial Bus (USB) is a serial bus standard to interface devices. more...

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It was originally designed for computers, but its popularity has prompted it to also become commonplace on video game consoles, PDAs, portable DVD and media players, cellphones; and even devices such as televisions, home stereo equipment (e.g., mp3 players), car stereos and portable memory devices.

The radio spectrum based USB implementation is known as Wireless USB.

Overview

The Universal Serial Bus (USB) was devised as a major component in the transition towards a legacy-free PC. The intention was to let go of all older serial and parallel ports on personal computers since these were not properly standardized, and required a multitude of device drivers to be developed and maintained.

A USB system has an asymmetric design, consisting of a host controller and multiple daisy-chained devices. Additional USB hubs may be included in the chain, allowing branching into a tree structure, subject to a limit of 5 levels of branching per controller. No more than 127 devices, including the bus devices, may be connected to a single host controller. Modern computers often have several host controllers, allowing a very large number of USB devices to be connected. USB cables do not need to be terminated.

Because of the capability of daisy-chaining USB devices, early USB announcements predicted that each USB device would include a USB port to allow for long chains of devices. In this model, computers would not need many USB ports, and computers shipped at this time typically had only two. However, for economical and technical reasons, daisy chaining never became widespread. To reduce the necessity of USB hubs, computers now come with a large number of USB ports, typically six. Most modern desktop computers have up to half of their total complement of USB ports on the front panel, to facilitate temporary connection of portable devices.

USB was designed to allow peripherals to be connected without the need to plug expansion cards into the computer's ISA, EISA, or PCI bus, and to improve plug-and-play capabilities by allowing devices to be hot-swapped (connected or disconnected without powering down or rebooting the computer). When a device is first connected, the host enumerates and recognizes it, and loads the device driver it needs.

USB can connect peripherals such as mouse devices, keyboards, gamepads and joysticks, scanners, digital cameras, printers, external storage, networking components, etc. For many devices such as scanners and digital cameras, USB has become the standard connection method. USB is also used extensively to connect non-networked printers, replacing the parallel ports which were widely used; USB simplifies connecting several printers to one computer. As of 2004 there were about 1 billion USB devices in the world. As of 2005, the only large classes of peripherals that cannot use USB are high data rate devices such as displays and monitors, and high-quality digital video components. USB cannot provide the data rates necessary for these devices.

Read more at Wikipedia.org