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(Universal Serial Bus) A widely used hardware interface for attaching a maximum of 127 peripheral devices to a computer. There are usually at least two USB ports on laptops and four USB ports on desktop computers. After appearing on PCs in 1997, USB quickly became popular for connecting keyboards, mice, printers and external drives and eventually replaced 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 USB 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 power lines in the USB bus to obtain DC current. For example, the USB bus is used to recharge batteries and power devices such as a keyboard lamp and ultra-small data projector. They are also used to power gadgets such as a coffe cup warmer, mini-fridge and mini-microwave.

USB 1.0, 2.0 and 3.0
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. SuperSpeed USB 3.0 (2008) provides a huge 10x increase over USB 2.0 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.




Type A and Type B
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.





USB Plugs and Sockets
Type B sockets are on printers and external devices such as disk drives, while Mini-B sockets are on PDAs, cameras and other portables. The two versions of Mini-B are distinguishable by their outer shells. Mini-A is the small counterpart of Type A, but not widely used.





Sockets on a Hub
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.






USB-based Land Warrior
Not your ordinary computer, the Land Warrior is a body-worn system that uses USB as its "personal network." Developed by Pacific Consultants, its USB controller was created by TransDimension. (Image courtesy of the U.S. Department of Defense.)