August 13, 2008

Understand and exploit USB topology in Windows XP

Date: July 9th, 2008
Author: Greg Shultz

The number of USB devices you can connect to a PC running Microsoft Windows XP is likely more than you could use in any practical manner.

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As long as you have enough power, attaching many USB devices to your Microsoft Windows XP system can’t cause degradation in performance — even if you attach as many as 127 USB devices at one time.

While it’s unlikely for that many devices to be connected at a time, that number is made possible by Windows XP’s seven-tiered USB topology. The top, or tier number one, consists of the host controller or root hub, which is the USB hub built in to the computer’s motherboard. Tiers two through six are equipped to consist of a series of USB hubs (two or more at each tier) daisy chained together. Tier seven consists of any devices attached to the USB hub(s) at tier six.

While USB hubs can draw power from the root hub, the amount of power is limited to 100 milliamperes per port, and the hub can have only four ports. However, most USB hubs have their own external AC adapter and can provide up to 500 milliamperes of power per port on more than four ports.

Follow these steps to learn more about the root hub and the USB hubs attached to your system in Device Manager:

Go to Start, right-click My Computer, and select Manage.
Click Device Manager in the left pane.
Click Universal Serial Bus Controllers in Device Manager. (Figure A)
Double-click each root and USB hub and check the information on the tabs. (Figure B)

Note: This tip applies to both Windows XP Home and Windows XP Professional.