An external peripheral interface standard for communication between a computer and external peripherals over low-cost cable using serial transmission. USB is replacing existing serial ports and parallel ports and is used with keyboards, mice, printers, scanners, and removable hard drives. It is now available on virtually all personal computers and many printers, scanners, etc. It is supported by all the popular operating systems providing plug-and-play.

USB 1.1 works at 12 Mbps with specific consideration for low-cost peripherals, supports up to 127 devices over cables up to five metres long, and includes power distribution for low-power devices. It supports daisy chaining through a tiered star multidrop topology. A USB cable has a rectangular ‘Type A’ plug at the computer end and a square ‘Type B’ plug at the peripheral end. A mini version of the plug is used for very small devices such as digital cameras. USB provides a 5-volt supply at up to 500 mA for powering low-power devices.

The USB 2.0 specification, released in 2000, allows USB to compete with FireWire, etc. USB 2.0 is backward-compatible with USB 1.1 but works at 480 Mbps. The next version, USB 3.0, is expected to work at about 5 Gbps. Most, if not all, modern systems support hot swapping of USB devices.

http://www.usb.org/developers/docs The USB (version 2) specification