The problem is that you are trying to assign two devices to the same interrupt--COM 1 and COM 3 share an IRQ, as do COM 2 and COM 4. In general, you can assign two COM ports with the same interrupt as long as you don't use the two ports at the same time. However, if you are running a single program that uses two COM ports, it may not allow you to assign COM 1 and COM 3. In this case, you could try COM 1 and COM 4.

When you have different programs using the COM ports, you usually won't have a problem as long as you don't try to use them simultaneously. For example, if you have a modem on COM 1 and then try to use COM 3 while the modem is active, you will create a conflict.