USB Universal Serial Bus

by Wayne Maples [Published on 20 April 2004 / Last Updated on 20 April 2004]

Win9x, Windows ME, Windows 2000 and Windows XP all support USB devices. Its real plug and play. The really niffty feature of USB devices is that you can plug them in while the PC is running and they will work, that is they are hot pluggable . You do not need to reboot for the operating system to recognize and use the devices. The required device drivers are dynamic. You can now get USB printers, monitors, keyboards, scanners, modems, mice, cd-rom, and tape drives.

There is one gotcha! that tends to cause problems. Some USB devices pull their power from the USB port while others get their power from the wall. If you plug in a USB device that gets its power from the bus, the required amount of power will flow to the device on the bus. Now the gotcha! USB devices can be chained together. You can plug a USB device into a USB device. Now back to our scenario. If you plug another USB device into the first USB device, no problem is IF gets its power from the wall. If it gets its power from the bus, you have problems. The power flow to the first device is just enough for it. The second device in the chain will not get enough power and things will be unreliable.

USB manufacturers are aware of this and few unpowered USB hubs are now manufactured. Just be aware.

There are three parts that make up USB:

  • Host
    The host is built into the motherboard. It is also called the root or root hub . If you motherboard is not USB enabled, you can purchase an USB adapter card which will function as the hub. Given its physical location and name, it should be no surprise that it controls all traffic on and between the USB bus and the PCs bus.
  • Hub
    The host has a hub and most powered USB devices have hubs. A hub provides USB ports to connect devices to the bus. Hubs can detect whether any USB devices plugged into the hub need power provided by the hub. Thus hubs have to be powered.
  • Device
    dah! those things you plug into the hub like the keyboard I am writing this tip with or the monitor I am using.
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