LYCOS RETRIEVER 
Universal Serial Bus: Devices
built 631 days ago
USB is the Universal Serial Bus. As its name indicates, it's a serial bus, meant for replacing RS232? ports (under DOS known as COM1?: - COM4?: ) and for connecting all kinds of external user devices (mice, keyboards, joysticks, gamepads, usb flash drives). USB ports are available in 3 variants, supporting up to 3 different speeds\\
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Compared to serial or parallel ports, the USB is a complex bus. For example, each device that is connected to this bus receives an address, and packets of data are exchanged between the host computer and the addressed device. Additionally, devices can operate at multiple speeds, and devices can be temporarily disconnected from the bus if they do not understand high speed transfers. In general, data transfer on USBs is much more complex and relies on a well-defined protocol that is similar, in some respects, to a low-level network protocol.
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USB is a new 4-wire bus that provides a better way to plug peripherals into a computer. "Universal" is the key word in this new interface's name since it replaces many different kinds of serial and parallel port connectors with one standardized plug and port combination. There are a wide variety of computer peripherals with USB connections. These include devices such as keyboards, digital cameras, floppy drives, tape drives, CD-ROM drives, mice, scanners, printers, telephones, modems and more. As of early 1999 there are more that 100 USB compatible peripheral products on the market with hundreds more expected to arrive throughout the year.
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USB defines the ports and the bus topology with data transfer rates up to 12 Mbits/sec. A single cable up to 5 meters in length is used to daisy-chain devices to a single port. Up to 63 devices can be added to the port at any time without rebooting the system. The topology is a tiered star, which allows peripherals to be connected to repeater hubs that provide multiple port connectors. In some cases, hubs may need power. USB ... supplies power to some devices so they will not need power cords or batteries. USB even allows devices to communicate their power requirement needs with the USB host controller.
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The USB Implementers Forum home page contains the complete USB specification, Universal Serial Bus Specification, Revision 1.0. Anyone considering building a USB compatible device or designing a hardware platform that supports USB should read this specification.
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The USB bus is a [Differential] Bi-directional serial interface cable bus. Differential NRZI data is transmitted Isochronous or Asynchronous between devices. Data is transferred at three different rates over a maximum cable length of 4 meters ~ over 4 wires, 2 of which carry data on a balanced twisted pair.
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