User Guide

Basics New Technologies - 27
USB Connectivity Rules
Following are the rules for USB connectivity:
• The maximum number of devices that can be connected to
a single USB hub is 127 devices. (Note: A keyboard with a
built-in trackball counts as two devices.)
• The maximum number of tiers allowed is six. This can be
tricky to track sometimes, since hubs can be hidden in
CPU cases, displays, keyboards, etc.
• Bus-powered hubs can't be connected to bus-powered
hubs due to power limitations. A keyboard with an
internal hub is a good example of a bus-powered hub.
• The maximum cable length allowed for 12 Mbps devices
is 5m, and the cable must be shielded. The maximum
cable length allowed for 1.5 Mbps devices is 3m, and the
cable is typically unshielded.