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Universal Serial Bus

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Universal Serial Bus (USB) is a serial bus standard for connecting devices to a computer (usually a PC). Over a hundred devices can be connected to a single port in a tree-like fashion. Devices can be attached and removed whilst the computer is still powered on ("hot plugging and swapping"), and they can be supplied with power through the USB connection. Care is called for, however, when choosing a power supply for a USB system. It is very easy to overdraw a system's wattage when many USB devices are in use. USB hubs, both powered and unpowered are used to increase the number of attached devices. By using powered hubs (hub contains a separate power supply) power management concerns are minimized. Powered hubs supply power to downstream devices (within prescribed limits) without draining power from the upstream connection.

USB is used to connect peripherals such as mice, keyboards, scanners, digital cameras, printers, hard drives, and networking components to the main computer. For multimedia devices such as scanners and digital cameras, USB is the most common interconnect method. In printers, USB is also growing in popularity and displacing parallel ports because USB makes it simple to add more than one printer to a computer. Another popular item is the USB memory stick, a little device that can contain from 16 MB to 1 GB (medium 2003) of flash memory.

In the case of hard drives, USB is unlikely to completely replace buses such as ATA (IDE) and SCSI because USB is somewhat slower than those standards. The new Serial ATA standard allows transfer rates to approximately 150 MB (mega bytes) per second. However, USB, and especially USB 2.0 has the important advantage that it is possible to install and remove devices without opening the computer case, making it useful for external hard disks. Today a number of manufacturers offer portable USB 2.0 hard drives that offer performance nearly indistinguishable from conventional ATA (IDE) drives.

USB has not completely replaced AT keyboard connections and PS/2 mouse connections, but virtually all PC motherboards today have one or more USB ports. As of 2003 most new motherboards have multiple USB 2.0 high-speed ports.

USB 1.1 has two data rates. 1.5 Mbps for keyboards, mice, joysticks, and the like, and full speed at 12 Mbps (12 million bits per second). The USB 2.0 standard supports high speed at 480 Mbps along with operation at the full speed signalling rate of 12 Mbps. At this highest speed USB 2.0 is in direct competition with firewire.

USB 1.1 has been renamed to USB 2.0 Full speed by the USB Forum, and USB 2.0 has been renamed USB 2.0 High speed.

See also: ACCESS.bus

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