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The Universal Serial Bus  (USB) is a new device communication standard developed by the computer and telecommunication industries.  It is aimed at replacing most of the different kind of special interfaces such as conventional serial and parallel ports. Most of the new PCs and Macs have USB connectors at this point, and the number of peripherals is increasing exponentially.  USB provides power to the devices connected to it.  USB 1.1  provides 12 Mbit/s and 1.5 Mbit/s data transmission speeds. USB can't be used directly as a fast connection between PCs, but there are special products which allow small scale networking using USB bus.  USB provides a powerful, hot-plug-capable, "true plug-and-play" interface between a host computer and add-on peripheral devices.

NOTE - "Hot-plug-capable" means that you can plug in, use, and unplug devices without having to shut down the host computer.

These devices can be just about anything: keyboards, mice, joysticks, telephones, scanners, printers, security dongles, microphones, speakers, floppy drives, cameras, modems, CD-ROM drives, etc.

USB was designed to improve on earlier peripheral connection technologies.   USB allows end-users to connect peripheral devices to a host computer on the fly, without having the user hassle with:

  • Shutting down/restarting the computer (a big plus!)
  • Opening the computer case
  • Installing a card
  • ID conflicts
  • Termination
  • Loading device drivers (although some devices may require a one-time driver installation).

In addition, USB provides:

  • A "tiered-star hub" network topology, allowing for the connection of up to 127 devices per host computer
  • Two simple, standardized plugs for the broad range of devices
  • Power for some devices directly through the USB cable
  • Support for two data rates: 1.5 and 12 megabits per second (Mbps)
  • Up to 12 Mbps performance
  • Automatic loading and unloading of drivers as needed

USB devices use two types of plugs, Type A and Type B:

USB cables
  • The Type A plug (left): connects to the host computer or an upstream hub.
  • The Type B plug (right): connects downstream to the USB device. It's "kind of house-shaped."  Some devices (like a mouse) have the cable soldered into the device, and therefore don't use this connector.

    NOTE - "Upstream" means moving up the device chain, in the direction of the host computer. "Downstream" means moving down the device chain, away from the host computer, as shown in the following figure.

    USB network.  Picture courtesy Apple

  • High speed connections (12 Mbps) use shielded cabling to reduce interference at higher connection rates, while low speed connections (1.5 Mbps) may not. 
  • Each connector has four pins, two for power and two for data.


Here are a few USB links:

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