USB
Universal Serial Bus (USB) is a serial bus standard for connecting devices to a computer (usually a PC).
Over one hundred devices can be connected to a single port in a tree-like fashion. Devices can be attached and removed while 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 (hubs containing 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.
While USB defines four types of connectors for the attachment of devices to the bus, there are some examples where the mechanical layer has been changed. For example, the IBM UltraPort is a proprietary USB connector located on the top of IBM's notebook LCDs. It uses a different mechanical connector while preserving the USB signaling and protocol.
USB is used to connect peripherals such as mice, keyboards, scanners, digital cameras, printers, hard drives, and networking components. For multimedia devices such as scanners and digital cameras, USB has become the standard connection 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.
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 up to approximately 150 MB per second. However, USB has one important advantage in 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 Mbit/s for keyboards, mice, joysticks, and the like, and full speed at 12 Mbit/s.
The USB 2.0 standard supports HiSpeed at 480 Mbit/s along with operation at the full speed signalling rate of 12 Mbit/s. 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