2 Feb 1995

Configuring COM3 or COM4

This is a problem that is best avoided. If you bought a machine with existing COM1 and COM2 ports based on an inadequate 16450 UART, and you are not using them, then the best choice is to disable and replace them. On some machines (such as IBM Valuepoints or DEC PC’s) the built-in COM ports can be disabled with the Configuration Utility that comes up after power-on when you press some special key. On other machines, it may be necessary to remove the cover and change a switch or remove one of the tiny metal "jumpers" that connect pins on one of the computer boards. The vendor should supply documentation on the process, or may provide instructions over the phone on an 800-number.

The first two COM lines are rigorously standard. COM1 is always assigned I/O address 3F8 and interrupt level 4. COM2 is always assigned address 2F8 and interrupt level 3. There is no comparable universal standard for COM3 and COM4. A widely used convention (at least for ISA bus machines) is to assign COM3 the I/O address of 3E8 and put COM4 at address 2E8. There are no entirely acceptable interrupt levels. In the original PC architecture, interrupt level 5 was reserved for a second printer. Since there is almost never a second printer installed, it can be reassigned. However, if you already have a LAN adapter or a multimedia sound card in the machine, one of them is probably already using interrupt level 5. For technical reasons, Interrupt levels 2 and 9 are combined together and actually form one circuit. Some manuals call it 2, and others call it 9, but it is the same thing. This is the other widely available value. Some DOS users try to assign devices to use interrupt level 7. Technically this is assigned to the printer, but it does not matter because DOS does not manage the printer with interrupts. Use any more advanced operating system (such as "Chicago" or OS/2) and a device assigned to use level 7 will fail when a file is printed.

In the near future, a standard called "Plug and Play" will make this process automatic. Today, however, the current process is "Plug and Pray." First, read through the system manual to discover which interrupt levels are used by the main system components. Then research and note all the interrupts used by add-in cards. If any interrupt appears to be available, it may be possible to assign it to COM3 or COM4. At this point it should be very clear why the best option is to replace COM1 or COM2 and use their clearly assigned interrupt levels (4 and 3 respectively).

If COM3 and COM4 are used, they should be explicitly defined to Windows 3.1. It may be that Windows would find them anyway, but explicit configuration makes sure.

Click on the Main Group
Click on Control Panel
Click on Ports
Click on COM3 or COM4)
Click on Advanced Settings

Fill in the I/O Port address and IRQ value in the boxes. Pressing the dropdown arrow will provide a list of suggestions. This configuration will be used automatically by any program that uses the standard Windows services to access the COM ports.

COM Port Technical Information

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