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Universal Serial Bus (USB) provides a serial bus standard for connecting devices, usually to a computer, but it also is in use on other devices such as set-top boxes, game consoles and PDAs.
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. There is no need for a terminator on any USB bus, as there is for SCSI and some others.
The standard includes provision for power (5 volts) 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, usually 500 mAIn physics, the ampere (symbol: A often informally abbreviated to amp is the SI base unit used to measure electrical currents. The present definition, adopted by the 9th CGPM in 1948 is: "one ampere is that constant current which, if maintained in two str) without draining power from the upstream connection.
The design of USB aimed to remove the need for adding separate expansion cardAn expansion card in computing is a printed circuit board that can be inserted into an expansion slot of a computer motherboard to add additional functionality to a computer system. One edge of the expansion card holds the contacts that fit exactly into ts into the computer's ISAIndustry Standard Architecture (in practice almost always shortened to ISA is a bus standard for IBM compatibles introduced in 1984 that extends the XT bus architecture to 16 bits. It is designed to connect peripheral cards to the motherboard. The protoco or PCIFor other meanings of PCI see PCI (disambiguation). motherboard The Peripheral Component Interconnect standard (in practice almost always shortened to PCI specifies a computer bus for attaching peripheral devices to a computer motherboard. These devices c bus, and improve plug-and-playPlug-and-play is a term used in the computer field to describe a computer's ability to have new devices, normally peripherals, added to it without having to restart the computer. There are a number of terms or variations that describe similar abilities, i capabilities by allowing devices to be hot swapHot swap is the ability to remove and replace components of a machine while the machine is operating. Once the appropriate software is installed on your computer, one can plug and unplug the device without re booting your computer. Historically, only expeped or added to the system without rebooting the computer. When the new device first plugs in, the host enumeratesEnumeration is the name given to the generic field of mathematics which deals with counting objects. The counting is abstracted as far as possible from the objects in question to come up with counting techniques that are generic and do not rely on the pro it and loads the device 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. As of 2004 there were about 1 billion USB devices in the world.