The USB Interface
When the PC had only a printer or a modem connected to it, one or two serial and parallel
ports were enough to provide the support required. However, today’s PC world has
scanners, portable hard disks, Zip and Jazz drives, and no-serious-PC-gamer-should-
be-without-one force-feedback joysticks. As a result, there are situations where there are
not enough serial and parallel ports for everything you wish to connect to your PC. An-
other problem is that for many of the newer peripheral devices, standard serial and parallel
ports aren’t fast enough anyway.
Attempts were made to provide systems with all of the ports a user could possibly
use. PCs were configured with as many as eight serial ports, but the problem was that not
every new device used a serial port. The next great interface was to be the SCSI (Small
Computer System Interface) standard (covered later in this chapter), but it is expensive
and the lack of an early standard hurt its chances for global acceptance. Into the void
came two new high-speed serial data interconnection standards, USB (Universal Serial
Bus) and the FireWire (more on this later).
The Universal Serial Bus (USB) is a newer hardware interface standard that supports
low-speed devices such as keyboards, mice, and scanners as well as higher speed devices
such as digital cameras. USB, which is a serial interface, provides data transfer speeds of
upto12Mbpsforfasterdevicesanda1.5Mbpssubchannelspeedforlowerspeeddevices.
A newer version of the USB standard, USB 2.0, supports up to 480Mbps for data transfer
speeds. Figure 19-8 shows a comparison of the data transfer speeds for the more common
interface types.
A USB port offers the following features: The flexibility of Plug-and-Play devices.
Standard connectors and cables with a wide variety of devices available,
including keyboards, mice, floppy drives, hard disk drives, Zip and Jazz
drives, inkjet printers, laser printers, scanners, digital cameras, modems,
and hubs.
Automatic configuration of USB devices when they are connected.
Hot swapping—USB devices can be connected and disconnected while
the PC is powered on.
The capability to support up to 127 devices on one channel.Connecting with USB
USBusesauniquepairofconnectorsandports,asshowninFigure19-9.USBTypeAcon-
nectors are used to connect devices directly to a PC or USB hub. You’ll find USB Type A
connectors on devices with permanently attached cables. USB Type B connectors are
found on those devices that have a detachable cable. The cable uses a squarish Type B
port on the device and connects to either a Type A or Type B socket (the cable usually has
both on the other end) on the PC or hub.Chapter 19: Ports and Connectors^493