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Universal Serial Bus (USB)
USB is a serial bus standard to interface devices. A major component in the legacy-free PC, USB was designed to allow peripherals to be connected using a single standardised interface socket, to improve plug-and-play capabilities by allowing devices to be connected and disconnected without rebooting the computer (hot swapping). Other convenient features include powering low-consumption devices without the need for an external power supply and allowing some devices to be used without requiring individual device drivers to be installed.
USB is intended to help retire all legacy serial and parallel ports. USB can connect computer peripherals such as mouse devices, keyboards, PDAs, gamepads and joysticks, scanners, digital cameras and printers. For many of those devices USB has become the standard connection method. USB is also used extensively to connect non-networked printers; USB simplifies connecting several printers to one computer. USB was originally designed for personal computers, but it has become commonplace on other devices such as PDAs and video game consoles.
USB signals are transmitted on a twisted pair of data cables, labelled D+ and D−. These collectively use half-duplex differential signaling to combat the effects of electromagnetic noise on longer lines. D+ and D− usually operate together; they are not separate simplex connections. Transmitted signal levels are 0.0–0.3 volts for low and 2.8–3.6 volts for high.
USB supports three data rates:
A Low Speed (1.0) rate of 1.5 Mbit/s (192 KB/s) that is mostly used for Human Interface Devices (HID) such as keyboards, mice, and joysticks.
A Full Speed (1.1) rate of 12 Mbit/s (1.5 MB/s). Full Speed was the fastest rate before the USB 2.0 specification and many devices fall back to Full Speed. Full Speed devices divide the USB bandwidth between them in a first-come first-served basis and it is not uncommon to run out of bandwidth with several isochronous devices..
A Hi-Speed (2.0) rate of 480 Mbit/s (60 MB/s).
Though Hi-Speed devices are commonly referred to as "USB 2.0" and advertised as "up to 480 Mbit/s", not all USB 2.0 devices are Hi-Speed. The actual throughput currently (2006) attained with real devices is about half of the full theoretical (60 MB/s) data throughput rate. Most hi-speed USB devices typically operate at much slower speeds, often about 3 MB/s overall, sometimes up to 10-20 MB/s. The USB-IF certifies devices and provides licenses to use special marketing logos for either "Basic-Speed" (low and full) or Hi-Speed after passing a compliancy test and paying a licensing fee. All devices are tested according to the latest spec, so recently-compliant Low Speed devices are also 2.0.
Cables
The maximum length of a USB cable is 5 meters; greater lengths active extension cables up to 4 active extension and one 5m Cable give maximum of 25m unless a USB extender is used to 100m over CAT5 or up to 10 km over fiber by using special USB extender products developed by various manufacturers.
Conectors
TYPE A
TYPE B
TYPE Mini B
TYPE MINI A
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