Hardware Reference
In-Depth Information
With a serial bus, the data is sent one bit at a time, one after the other. Because there is no
worryabout having multiple bits arrive simultaneously,the clocking rate can be increased
significantly.
At high clock rates, parallel signals tend to interfere with each other. Serial again has the
advantage because with only one or two signal wires, crosstalk and interference between
the wires in the cable are negligible.
In general, parallel routing or cabling is far more expensive to implement than serial rout-
ing or cabling. In addition to the many extra traces or wires needed to carry the multiple
bits in parallel, the circuit must be specially constructed to prevent crosstalk and interfer-
ence between adjacent data lines. This is one reason external SCSI cables are so expens-
ive.Serialconnections,bycomparison,areinexpensive.Foronething,theyhavesignific-
antlyfewersignalsorwires.Furthermore,theshieldingrequirementsarefarsimpler,even
at high speeds. Because of this, transmitting serial data reliably over longer distances is
alsoeasier,whichiswhyparallelinterfaceshaveshorterrecommendedtracerunsorcable
lengths than do serial interfaces.
Forthesereasons—inadditiontotheneedfornewPlugandPlay(PnP)externalperipheral
interfacesandtheeliminationofthephysicalportcrowdingonportablecomputers—high-
performance serial buses were developed. USB is a standard feature on virtually all PCs
today and is used for most general-purpose, high-speed external interfacing. It's also the
most compatible, widely available, and fastest general-purpose external interface. IEEE
1394, although mainly used in certain niche markets (such as connecting digital video
camcorders), is also spreading into other high-bandwidth uses, such as high-resolution
scanners and external hard drives.
Universal Serial Bus (USB)
Universal serial bus (USB)
is an external peripheral bus standard designed to bring PnP
capability for attaching peripherals externally to the PC. USB eliminates the need for
special-purposeports,reducestheneedtousespecial-purposeI/OcardsorPCCards(thus
reducing the need to reconfigure the system with each new device added), and saves im-
portant system resources such as interrupt requests (IRQs). Regardless of the number of
devices attached to a system's USB ports, only one IRQ is required. PCs equipped with
USBenableperipheralstobeautomaticallyrecognizedandconfiguredassoonastheyare
physically attached, without the need to reboot or run a setup program. USB allows up to
127devicestorunsimultaneouslyonasinglebus,withsomeperipheralssuchasmonitors
and keyboards acting as additional plug-in sites, or
hubs
. USB devices can be identified
by the icons shown in
Figure 14.1
.
Figure 14.1
This icon identifies USB cables, connectors, hubs, and peripherals.
