USB Devices

The Universal Serial Bus (USB) was designed by a consortium of hardware and software vendors—Compaq, Intel, Microsoft and NEC—beginning in 1994. It was conceived to provide a standardized way of connecting a wide range of peripheral devices to a computer (read “personal computer”) and to correct some of the limitations of traditional serial and parallel lines.

USB has the following advantages:

  • Up to 127 devices can be connected.

  • Devices can be added and removed while the system is running.

  • Connectors have been standardized across all device types.

  • It is much faster. The theoretical bandwidth of a USB bus is 12 Mbs/sec; however, actual throughput is more like 8-8.5 Mbs/sec, and devices seldom achieve more than about 2 Mbs/sec.

USB cables contain only four wires: power, ground, send, and receive. Communication is handled in a hierarchical manner, under the control of a master; attached devices all function as slaves, thereby eliminating issues such as avoiding collisions. USB cable connectors are illustrated on the far left in Figure 12-2, and Figure 12-5 illustrates their corresponding pinouts.

USB connectors

Figure 12-5. USB connectors

B-type connectors are used for the USB port on the device, and A-type connectors are used for the port on the hub or computer system.

FreeBSD, HP-UX, Linux, Solaris, and Tru64 all support USB devices to some extent, although the support under HP-UX and Tru64 ...

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