Universal Serial Bus





Universal Serial Bus (USB) is a serial bus standard for connecting devices
to a computer (usually a PC).

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 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
(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 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.

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.