Universal Serial Bus (USB) provides a serial
bus standard for connecting devices, usually to computers such as
PCs, video game consoles such as Sony's PlayStation 2, Microsoft's
Xbox 360, Nintendo's Revolution, and PDAs, but it also is in use on
other devices such as set-top boxes.
A USB system has an asymmetric design, consisting of a host
controller and multiple devices connected in a tree-like fashion
using special hub devices, called USB hubs. There is a limit of 5
levels of branching hubs per controller. Up to 127 devices may be
connected to a single host controller, but the count must include
the hub devices as well. A modern computer likely has several host
controllers so the total useful number of connected devices is
beyond what could reasonably be connected to a single controller.
There is no need for a terminator on any USB bus, as there is for
SPI-SCSI and some others.
The design of USB aimed to remove the need for adding separate
expansion 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 loads the device
driver necessary to run it.
USB can connect peripherals such as mice, keyboards, gamepads and
joysticks, scanners, digital cameras, printers, hard disks, 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. As of 2004 there were about 1 billion USB
devices in the world. As of 2005, the only large classes of
peripherals that cannot use USB (because they need a higher data
rate than USB can provide) are displays and monitors, data
acquisition devices that use firewire ports, and high-quality
digital video components.
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