Apple IMAC User Manual - Page 43
New Technologies - 27, internal hub is a good example of a bus-powered hub.
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Basics New Technologies - 2 7 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.
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