Basics USB Support - 20
• Hubs, which serve as connection points for devices. Hubs
also provide device detection, speed matching, and power
control. Hubs connect to the host using the 12 Mbps
connection speed. Hubs may be “bus-powered” by the USB,
or be “self-powered” by an external power source (like an AC
adapter). Only hubs can connect to the host. Hubs can also
connect to other hubs, providing expansion to the 127
device limit (see the example tree diagram below). Hubs can
be freestanding, or embedded in other items (displays,
keyboards, etc.). Most freestanding external hubs provide 4
Type A ports, but this is not a always the case.
Note:
Remember when troubleshooting power errors that
bus-powered hubs can't be connected to bus-powered
hubs, since not enough power is provided to power the
device ports. Instead, to extend the USB properly, users
should alternate self-powered and bus-powered hubs as
necessary.
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