Universal Serial Bus (USB) is a serial port bus standard for connecting devices to a computer (usually a personal computer).
A USB system is asymmetric, consisting of a single host and multiple devices connected in a tree-like fashion using special hub devices. Up to 127 devices may be connected to a single host, but the count must include the hub devices as well, so the total useful number of connected devices is somewhat less.
The standard includes provision for power to the connected device. Some devices draw minimal power, so several may be connected without needing extra power sources. Most hubs include power supplies which will power devices connected through them, but some devices draw enough that they need their own power. Powered hubs supply power to downstream devices (within prescribed limits) without draining power from the upstream connection.
USB was designed to remove the need for adding separate cards into the computer's ISA or PCI bus, and improve things by allowing devices to be hot swapped or added to the system without rebooting the computer. The new device is enumerated the first time it is plugged in and the host adds the software driver necessary to run the new device.
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 Hi Speed 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 Fire Wire (except in the areas of digital camcorders, USB has techonological limitations that prevent it from being viable in this area).
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.
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 Ultra Port 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.
An extenstion to USB 2.0 called USB-on-the-go allows a single port to act as either a host or a device - chosen by which end of the cable is plugged into the socket on the unit. Even after the cable is hooked up and the units are talking, the two units may "swap" ends under program control. This facility is aimed at units such as PDAs where the USB link might be used to connect to a PC's host port as a device in one instance, yet connect as a host itself to a keyboard and mouse device in another instance.
-  USB home, including documentation.