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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