Hardware Reference
In-Depth Information
the hub is listed by its category and power required (see Figure 14.3 ).
Figure 14.3. Examples of USB hub and device power usage in Windows 7/8/Vista (left) and
Windows XP (right).
Because of the variance in the power requirements of different USB devices and the possibility of
damage, I recommend using only self-powered USB hubs. Keep in mind that some USB hubs on the
market do not include an AC adapter or the ability to attach one, automatically making them bus-
powered.
Devices that use more than 100mA, such as the webcam and USB mass storage device listed in
Figure 14.3 , must be connected to a root hub or a self-powered generic hub. Devices that use 100mA
or less can be connected to bus-powered hubs, such as those without separate AC adapters or those
built in to some keyboards and monitors.
Tip
If a device plugged into a self-powered hub stops working, check the power source for the
self-powered hub—it might have failed or been disconnected. In such cases, a self-powered
hub becomes a bus-powered hub, providing only 100mA per port instead of the 500mA per
port available in self-powered mode.
A newly attached hub is assigned a unique address, and hubs can be cascaded up to five levels deep
(see Figure 14.4 ) . A hub operates as a bidirectional repeater, and it repeats USB signals as required
both upstream (toward the PC) and downstream (toward the device). A hub also monitors these
signals and handles transactions addressed to itself. All other transactions are repeated to attached
devices.
 
 
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