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PC Serial Ports
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Background
Serial ports (also sometimes referred to as communications ports or COM
ports) are hardware interfaces that permit your PC's microprocessor to
communicate with peripheral devices using a communications standard
called RS-232 (hence, serial ports are also sometimes referred to as RS-232
ports). Many common computer accessories make use of serial ports,
including serial mice, modems, and serial printers.
Under DOS (and Windows, which works cooperatively with DOS), the serial
port interfaces in a PC are uniquely identified by specific device names:
COM1 ("serial communications port 1"), COM2 ("serial communications port
2"), and so on, usually up through COM4. A particular PC might have none of
these devices, some of them, or all of them installed. For example, most PCs
currently on the market arrive from the manufacturer with two serial ports
already installed (COM1 and COM2), often integrated onto the computer's
main system board. Installing additional serial ports (for example, COM3 or
COM4) is usually accomplished by purchasing an add-on card and installing
it into a free expansion slot.
For most purposes (such as configuring software), the generic description of
serial ports provided by their device names is sufficient. For example, during
the PassageWay Service Provider installation procedure, you are asked to
provide the device name of the serial port to which you have connected the
telephone (for example, COM2). Unfortunately, this abstract view of serial
ports is not sufficient for other purposes, notably for troubleshooting
problems: To be able to do this effectively, a basic understanding of serial
port hardware is required. In particular, it is essential to understand the
mechanics by which the computer's microprocessor communicates with serial
port hardware.