Whenever you are adding hardware to a system (whether it be more COM ports or a network card ), you need to be concerned about IRQ line conflicts. IRQ lines are used to pass hardware interrupts which are used to notify the CPU that the device has some input/output that needs to be taken care of. If two devices use the same IRQ line, you will run into some serious conflicts. Multitasking and networking setups will have different common scenarios which are described in the next two sections.

If you are running three or more standard COM ports in a single machine, it is quite likely you have already experienced a COM port conflict. Below is a table which lists typical defaults for 4 COM ports:

Right away you should notice a problem. COM1 and COM3 are set to share IRQ 4. Likewise, COM2 and COM4 share the same IRQ. This will DEFINITELY not work. Therefore, you need to do find two different IRQs to use for COM3 and COM4. At this point, you are probably asking yourself “What IRQs can I use?”
IRQs 2 and 5 are usually available in a system and a wide variety of serial port boards support IRQs 2-5. Below is a table listing base addresses and IRQs which might be used in a computer with four serial ports:

If you decide to use IRQ 2, you need to realize that it is special. IRQ 2 is used to cascade the interrupts from the upper IRQs (8-15). Therefore, if you have any devices using IRQs 8-15 make sure that by using IRQ 2 you will not cause problems for other devices because their interrupts cannot be properly cascaded through IRQ 2. If you do run into a problem, try using IRQ 7 instead. This IRQ is used for LPT1 but it is usually safe to use.

If you have two COM ports or a COM port and another device which conflict with one another, you can get modem reset errors when you load PCBoard. This is because two or more devices are trying to get exclusive use of one IRQ.
Another thing that can happen is that the second device, which was contending for the IRQ, will get it making the other device appear as if it has ceased functioning. The solution to these symptoms is to resolve the IRQ conflict.
Double check your configuration and make sure that no two devices conflict.

One of the reasons external modems are recommended instead of internal modems is that internal modems rarely let you select a large range of IRQs for the COM ports. If you are using external modems and run into an IRQ conflict, you can change the settings on the serial port card rather than a modem card.
If your serial port card does not allow you to select a wide range of IRQs, you can buy a new serial port card that will.

Like COM ports, LAN cards must communicate with the computer through using an IRQ. Therefore, make sure the IRQ settings you use for your COM port(s) and your network card do not conflict with one another. If they do conflict, you could experience lockups, excessive network time-outs, or while PCBoard is running you may see constant Network Delay… messages.