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Universal
Serial Bus
Universal Serial Bus (USB) provides a
serial
bus
standard for connecting devices to a computer
(usually a PC).
A USB system has an asymmetric
design, 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 diminishes somewhat.
The standard includes provision for power
to the connected device. Some devices draw minimal power, so
several may connect 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.
The design of USB aimed to remove the need for adding separate
cards into the computer's ISA
or PCI
bus, and improve plug-and-play
capabilities by allowing devices to be hot
swapped or added to the system without rebooting the
computer. When the new device first plugs in, the host enumerates
it and adds the software driver necessary to run it.
USB can 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. For
printers, USB has also grown in popularity and started
displacing parallel
ports because USB makes it simple to add more than one
printer to a computer.
In the case of hard
drives, USB seems unlikely to completely replace buses such
as ATA
(IDE) and SCSI
because USB performs somewhat more slowly 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 making
it 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 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.
The USB 1.1 standard had two data
rates: 1.5 Mbit/s for keyboards, mice, joysticks, and the
like, and full speed at 12 Mbit/s. The major
feature of the USB 2.0 standard is the addition of a Hi-Speed
rate of 480 Mbit/s. It also clarifies minor technical errata. At
its highest speed USB competes directly with FireWire
(except in the areas of digital camcorders, USB has
techonological limitations that prevent it from being viable in
this area).
While USB defines four types of connectors for the attachment of
devices to the bus, the mechanical layer has changed in some
examples. 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 extension to USB 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 plugs 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
targets units such as PDAs
where the USB link might 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.
See
also: ACCESS.bus,
how
to transfer data between computers using USB.
External
links
This
content from Wikipedia
is licensed under the GNU
Free Documentation License.
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