Hardware Reference
In-Depth Information
To determine the power requirements of a particular USB device before you purchase it,
checktheproduct'sspecificationsorcontactthevendor.Todeterminethepoweravailable
per port and the power usage of the devices plugged in to the port on a system running
Windows, open the properties sheet for each root or generic hub in the Device Manager,
and click the Power tab. The Power tab lists the hub type (self-powered or bus-powered)
and the amount of power per port in the
Hub information
section of the properties sheet.
In the
Attached devices
section, each device connected to 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/Vista (left) and Windows XP
(right).
Because of the variance in the power requirements of different USB devices and the pos-
sibility 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 (automatically making them
bus-powered),andtherearestillanumberofUSB1.1hubsonthemarket.AUSB1.1hub
runs any attached USB 2.0 or USB 3.0 devices at USB 1.1 speeds.
Devicesthatusemorethan100mA,suchasthewebcamandUSBmassstoragedevicelis-
tedin
Figure14.3
,
mustbeconnectedtoaroothuboraself-poweredgenerichub.Devices
