Hardware Reference
In-Depth Information
that use 100mA or less can be connected to bus-powered hubs, such as those without sep-
arate 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).Ahubalsomonitorsthesesignalsandhandlestransactionsaddressedtoitself.All
other transactions are repeated to attached devices.
Figure 14.4 A typical PC with USB devices can use multiple USB hubs to support a variety of peripherals,
connected to whichever hub is most convenient.
Note
A USB 1.1 hub supports both 12Mbps (Full-Speed) and 1.5Mbps (Low-Speed) peripherals.
AUSB2.0hubsupportsUSB1.112Mbpsand1.5Mbpsspeedsandthe480Mbps(Hi-Speed)
 
Search WWH ::




Custom Search