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Serial Ports Under Windows
Unlike the DOS-only world of yesterday, today's multitasking environments
like Windows permit the microprocessor to communicate with up to four active
serial port devices at the same time (COM1 through COM4). For example,
under Windows, if you are using a serial mouse (on COM1) within a terminal
emulator program that operates a data modem (on COM2), while using a fax
board (on COM3) to transmit or receive a fax "in the background," you are
using three serial port devices simultaneously. You might even wish to make
a phone call using the PassageWay Service Provider (on COM4) at the same
time, bringing the total up to four simultaneously active serial port devices.
The fact that Windows permits this kind of powerful multitasking does not
guarantee that the underlying PC hardware can support this level of
operation, at least without some customizing at the hardware level. Since
some PCs can support it by default (for example, those that support IRQ
sharing), Windows does not prohibit you from configuring your system and
attempting tasks like the one in the previous paragraph. Unfortunately, most
PCs cannot support this operation by default, and the most likely result of
attempting the above scenario is "hanging" the PC due to an IRQ conflict. On
such systems, using COM1 along with COM2 is generally fine (recall that
these devices have unique IRQs by default), but the addition of COM3 or
COM4 causes the system to fail.
Fortunately, Windows 3.1 permits complete customization of all parameters
involving serial ports through the Control Panel, including configuring
nonstandard I/O port addresses and IRQs (that is, values different from those
in the table -- these parameters can be viewed and/or modified by selecting
the desired port in the Control Panel's Ports icon, selecting the Settings...
button, and then selecting the Advanced... button). This flexibility offers the
opportunity of salvation for owners of PCs that do not support IRQ sharing
who require the use of three or more COM ports simultaneously.