|Com ports have unique I/O address that nothing else uses, except possibly an internal dial-up modem. By default Windows assigns a certain I/O address to Com 1, another one to Com 2, etc., although sometimes you can change that in the mboard's bios, or you can choose a different I/O address in Device Manager in the properties for the Com port. |
Two devices that use a Com port, or that use a Com port's resources, cannot use the same I/O address.
(Only one device on a computer can use a particular I/O address.)
If they use different I/O address, they can use the same IRQ only if you don't use the devices at the same time; otherwise they must each use a different IRQ.
Nothing else should use the same IRQ as a device that uses Com port resources.
If you have a dial-up modem in a mboard slot, you often must disable one Com port in the mboard's bios in order to free up an IRQ and an I/O address (resources) that a Com port normally uses, for the modem's use.