USB Universal Serial Bus


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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