Hardware Reference
In-Depth Information
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 information from the system. Some parallel
port standards also allow the parallel port to perform bidirectional communications; however, it was
initially designed as a unidirectional 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 include 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 characters 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 to signal that the character has
been sent. At the receiving end of the communication, characters are recognized by the start-and-stop
signals instead of by the timing of their arrival. The asynchronous interface is character-oriented and
has approximately 20% 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.
If you need more serial ports than your system has as standard, or if your system did not include a
serial port, you can purchase single-port or multiport serial port adapter cards.
Note that card-based modems may also incorporate a built-in serial port on the card as part of the
modem circuitry (except controller-less software modems).
Figure 14.23
shows the standard 9-pin
connector used with most modern external serial ports.
Figure 14.24
shows the original standard 25-
pin version.
