(Universal Serial Bus) The most widely used hardware interface for attaching peripherals to a computer. There are typically at least two USB ports on laptops and four on desktop computers, while USB "hubs" allow many more connections (see below). After appearing on PCs in 1997, USB quickly became popular for connecting keyboards, mice, printers and hard drives, eventually replacing the PC's serial and parallel ports.|
USB devices are "hot swappable;" they can be plugged in and unplugged while the computer is on. This feature, combined with easy-to-reach ports on the front of the computer case, gave rise to the ubiquitous flash drive for backup and data transport (see USB drive).
Just for Electricity
Many devices plug into a USB socket, not to transfer data, but to use the USB's power lines to obtain DC current. For example, a set of speakers may use USB for power and plug another wire into the computer's audio jack for sound. USB is used to recharge batteries and power devices such as a keyboard lamp, coffee cup warmer and mini-fridge.
USB 1.0, 2.0 and 3.0
Supporting up to 127 devices, USB 1.0 (1996) and USB 1.1 (1998) provide a Low-Speed 1.5 Mbps subchannel for keyboards and mice and a Full-Speed channel at 12 Mbps.
Hi-Speed USB 2.0 (2001) jumps the top rate to 480 Mbps, while SuperSpeed USB 3.0 (2008) provides a huge 10x increase to 4.8 Gbps (see USB 3.0).
Hubs Add Extra Ports
A USB hub plugs into one USB port on the computer and provides four or more shared ports for peripherals. Monitors and keyboards may also have built-in hubs, which offer convenient sockets on top of the desk for connecting devices. For more details, see USB hub. See PoweredUSB, USB device class, USB OTG, USB drive, USB printer, USB switch and USB toy.
All USB hosts and devices have sockets, and all USB cables have plugs at both ends. The sockets on a host are Type A, and the sockets on peripheral devices are Type B or Mini-B (see below). Hubs have both Type A and Type B sockets because they function as a host to their devices as well as a device to the computer.
Type A sockets are found on every computer, and USB cables with a Type A plug on one end have a Type B, Mini-B or Micro-B plug on the other. There are also Type A to Type A extension cables. The Micro connectors are used for USB On the Go (see
This Belkin hub, which plugs into a wall outlet for power, is connected to the computer by a Mini-B/Type A cable. It provides four Type A ports for peripherals, netting out to only three extra, because one Type A port on the computer is used to connect the hub.
Not your ordinary computer, the Land Warrior, developed by Pacific Consultants, is a body-worn system that uses USB as its "personal network." (Image courtesy of the U.S. Department of Defense.)