Hardware Reference
In-Depth Information
type topology they allow the attachment of multiple devices. Each attachment point is referred to as a
port
. Most hubs have either four or eight ports, but more are possible. For further expandability, you
can connect additional hubs to the ports on an existing hub. The hub controls both the connection and
distribution of power to each of the connected functions. A typical hub is shown in
Figure 14.2
.
Figure 14.2. A typical USB hub with four ports.
Besides providing additional sockets for connecting USB peripherals, a hub can provide power to
any attached peripherals. A hub recognizes the dynamic attachment of a peripheral and provides at
least 0.5W of power per peripheral during initialization. Under control of the host driver software,
the hub can provide more device power (up to a maximum of 2.5W) for peripheral operation.
Different types of USB devices require different amounts of power, measured in milliamps (mA).
Bus-powered USB devices (devices that draw power from the USB port) might require as much as
500mA (the maximum amount of power available through a USB port) or as little as 100mA or less.
Self-powered devices also draw power from the USB port, but they might draw as little as 2mA.
The PnP aspects of USB enable the system to query the attached peripherals as to their power
requirements and issue a warning if available power levels are exceeded. This is especially
important for USB when it is used in laptops or other portable systems because the battery power that
is allocated to run the external peripherals might be limited. It is also important because of the
differences in hubs.
Root hubs and self-powered hubs (hubs connected to an AC adapter) provide a full 500mA of power
to each USB port. However, bus-powered hubs (hubs without an AC adapter) provide only 100mA
per port because they use power provided by the upstream USB port and subdivide it among the
ports. If you connect a USB device that requires more power than the port provides, the device will
not work. In a worst-case scenario, you might damage the device. For example, the contents of some
USB flash memory drives (which typically require 200mA-500mA) can be destroyed by being
inserted into a bus-powered hub (which provides only 100mA per port). Some vendors are now
including overcurrent protection in their latest designs, but because of the risk factors, I don't
recommend plugging a USB flash memory drive or a USB card reader into a bus-powered hub.
To determine the power requirements of a particular USB device before you purchase it, check the
product's specifications or contact the vendor. To determine the power available 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
