The USB interface supports up to 127 devices on a single channel. Most PCs have only
one or two USB connectors, as illustrated in Figure 19-10, and not all 127 can directly con-
nect to these ports. These devices connect both to the PC directly or into one or more USB
hubs, as shown in Figure 19-11. The fact that each USB port carries .5 amps of electrical
power, which is enough to power most low-power devices such as a mouse or keyboard,
provides a great deal of flexibility for adding additional devices to the system regardless
ofitslocation.USBdevicesthatrequirehigherpowerusuallyusetheirownACadapters.(^494) PC Hardware: A Beginner’s Guide
Speeds of Common Interface Types
(in Mbps)
Ultra 3 SCSI
FireWire
Wide Ultra2 SCSI
USB 2.0
Ultra SCSI
Fast SCSI
SCSI
USB (high rate)
USB (low rate)
Serial
Port Standards
0 500 1,000 1,500
1,280.00
800.00
640.00
480.00
160.00
80.00
40.00
12.00
0.23
1.50
Figure 19-8. A comparison of port speeds showing how the USB standards compare to other
port types
Figure 19-9. USB connectors and ports