Lucent Technologies 560-201-113 manual Serial Ports Under Windows

PC Serial Ports

Serial Ports Under Windows 3.1

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.

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