Hardware Reference
In-Depth Information
ment for FireWire than USB. Intel has indicated that future Thunderbolt implementations
willsupportopticalcableconnectionsofuptotensofmetersinlengthandatspeedsofup
to 100Gbps.
Low-Speed External Connections
Traditionally, the most basic communication ports in any PC system have been the serial
and parallel ports; however, the prevalence of USB has largely supplanted these ports in
modern systems.
Serial ports (also known as
communication
or
COM
ports) were originally used for
devices that had to communicate bidirectionally with the system. Such devices include
modems, mice, scanners, digitizers, and any other devices that “talk to” and receive in-
formation from the system. Some parallel port standards also allow the parallel port to
perform bidirectional communications; however, it was initially designed as a unidirec-
tional communications format.
The tasks traditionally performed by both serial and parallel ports are performed by USB
and IEEE 1394 (FireWire), but, for some time to come, legacy port types will continue to
be used in certain applications, for example HTPCs (Home Theater PCs) commonly in-
clude serial (COM) ports for infrared remote control connections.
Serial Ports
The asynchronous serial interface was designed as a system-to-system communications
port.
Asynchronous
means that no synchronization or clocking signal is present, so char-
acters can be sent with arbitrary time spacing.
Each character that is sent over a serial connection is framed by a standard start-and-stop
signal. A single 0 bit, called the
start bit
, precedes each character to tell the receiving
system that the next eight bits constitute a byte of data. One or two stop bits follow the
character tosignalthatthecharacter hasbeensent.Atthereceivingendofthecommunic-
ation, characters are recognized by the start-and-stop signals instead of by the timing of
theirarrival.Theasynchronousinterfaceischaracter-orientedandhasapproximately20%
overhead for the extra information needed to identify each character.
Serial
refers to data that is sent over a single wire, with each bit lining up in a series as
the bits are sent. This type of communication is used over the phone system because it
provides one wire for data in each direction.
Typical Locations for Serial Ports
Built-in serial ports are controlled by either a Super I/O or South Bridge chip on the
motherboard.
