(^484) PC Hardware: A Beginner’s Guide
ports, and communications are based on the principle that serial data is transmitted one
bit at a time. To transmit a single byte of data through a serial port, eight separate one-bit
transmissions are needed. Serial transmissions are somewhat like a single-lane country
road with all traffic lining up to travel over the road single-file.
SerialdevicesareexternaldevicesthatconnecttothePCviaaserialport,whichisalso
referred to as a COM port or an RS-232 port. The term COM originated from early desig-
nations of serial ports as communications ports. Before network adapters and other con-
nector types that can be used to connect a PC to a communications link, only the serial
port was available for this purpose. On many systems, the serial ports are designated as
COM ports, with the first serial port being COM1 and subsequent serial ports designated
as COM2, COM3, etc. RS-232 is an abbreviation for “reference standard number two hun-
dred and thirty-two,” which was so named by the Institute of Electrical and Electronic
Engineers, Inc. (IEEE—pronounced “I triple E”). This is the designation for a wiring pat-
tern used for communications lines, ports, and connectors used to transmit standard serial
data communications.
Originally, serial ports were added to the PC through an expansion board that added
one to four serial ports. Most newer PCs have one serial/COM port mounted directly on the
motherboard. Serial ports are easy to recognize on the back panel of the PC because they are
either a 9- or 25-pin male D-type connector. These connectors are designated as DB-9 and
DB-25 connectors. There are two versions of whatDBmeans. One version is that it means
data buswith the number representing the number of pins in the connector. The other ver-
sion is that the first D-shaped connectors were designated as a series that included DA-15,
DB-25, DC-37, DD-50, and DE-9 connectors. A male version of the DB-25 connector, shown
earlier in Figure 19-4 as a male parallel port, was first used as a serial connector on early PCs.
Eventually all D-shaped connectors were designated with the DB prefix. Either way, all se-
rial connectors are DB-type D-shaped connectors, but not all DB-type connectors are used
for serial connections.
Pinouts and Cable Connections
A serial transmission requires only nine pins and wires to communicate between the de-
vice and the PC serial port adapter, which is why many PCs now use the DB-9 port in
place of the DB-25. The DB-9 connector is smaller and has fewer pins, which reduces the
potential for damaged or bent pins. Older PC models usually included a single serial
DB-25 port on a multipurpose card that also included a second serial port, typically a
DB-9 port, a parallel port, or game port. The DB-25 connector is also popular on external
modems and serial printers.
Table 19-2 shows the pinouts for the DB-25 and DB-9 serial connection. Notice that
there is a difference in the pin assignments between the two connectors; if a cable has a
DB-25 connector at one end and a DB-9 at the other end, care must be taken to match up
the pins at each end.