Universal Serial Bus (USB) is a serial bus standard to interface devices. A major component in the legacy-free PC, USB was designed to allow peripherals to be connected using a single standardized interface socket and to improve plug-and-play capabilities by allowing devices to be connected and disconnected without rebooting the computer (hot swapping). Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.
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The USB 1.0 specififcation was introduced in November 1995. USB was promoted by Intel (UHCI and open software stack), Microsoft (Windows software stack), Philips (Hub, USB-Audio), US-robotics. Originally USB was intended to replace the multitude of connectors at the back of PCs, as well as to simplify software configurartion of commmunication devices. USB 1.1 came out in September 1998 to help rectify the adoption problems that occurred with earlier iterations of USB.
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A USB system has an asymmetric design, consisting of a host, a multitude of downstream USB ports, and multiple peripheral devices connected in a tiered-star topology. Additional USB hubs may be included in the tiers, allowing branching into a tree structure, subject to a limit of 5 levels of tiers. USB host may have multiple host controllers and each host controller may provide one or more USB ports. Up to 127 devices, including the hub devices, may be connected to a single host controller.
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The computer hardware that contains the host controller and the root hub has an interface geared toward the programmer which is called Host Controller Device (HCD) and is defined by the hardware implementer.
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Devices that attach to the bus can be full-custom devices requiring a full-custom device driver to be used, or may belong to a device class. These classes define an expected behavior in terms of device and interface descriptors so that the same device driver may be used for any device that claims to be a member of a certain class. An operating system is supposed to implement all device classes so as to provide generic drivers for any USB device. Device classes are decided upon by the Device Working Group of the USB Implementers Forum.
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Cables
