While it’s unlikely for 127 USB 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 into 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 only have 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.
- Double-click each root and USB hub and check the information on the tabs.