Hardware Reference
In-Depth Information
circuit must be specially constructed to prevent crosstalk and interference between adjacent data
lines. This is one reason external SCSI cables are so expensive. Serial connections, by comparison,
are inexpensive. For one thing, they have significantly fewer signals or wires. Furthermore, the
shielding requirements are far simpler, even at high speeds. Because of this, transmitting serial data
reliably over longer distances is also easier, which is why parallel interfaces have shorter
recommended trace runs or cable lengths than do serial interfaces. For these reasons—in addition to
the need for new Plug and Play (PnP) external peripheral interfaces and the elimination of the
physical port crowding on portable computers—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 is typically used in camcorders and digital video editing equipment;
however; it is beginning to be replaced in that market by Thunderbolt Technology.
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-purpose ports,
reduces the need to use special-purpose I/O cards or PC Cards (thus reducing the need to reconfigure
the system with each new device added), and saves important 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 USB enable peripherals to be automatically recognized and
configured as soon as they are physically attached, without the need to reboot or run a setup program.
USB allows up to 127 devices to run simultaneously on a single bus, with some peripherals such as
monitors 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.
USB was initially developed by Intel engineers in the early 1990s, after which the design was
transferred to the USB Implementers Forum (USB-IF) to further develop, support, and promote USB
architecture. The USB-IF initially consisted of Intel plus six other companies including Compaq,
Digital, IBM, Microsoft, NEC, and Northern Telecom.
The USB-IF has formally released USB versions as follows:
•
USB 1.0
—January 1996
•
USB 1.1
—September 1998
•
USB 2.0
—April 2000
•
USB 3.0
—November 2008
The 1.1 revision was mostly a clarification of some issues related to hubs and other areas of the
specification. Most devices and hubs should be 1.1 compliant, even if they were manufactured before
the release of the 1.1 specification. A bigger change came with the introduction of USB 2.0, which
was 40 times faster than the original USB yet fully backward compatible. USB ports can be
