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What is a COM port? What is an IRQ?

A communications (COM) port is a serial data port on the computer, used to communicate with other devices. It may be a physical port on the back of the computer, through which an external modem or a mouse would connect. External COM (serial) ports has a 9- or 25-pin mail connector, as opposed to an LPT (parallel) port, which has a 25-pin female connector. More often now, you will encounter "COM ports" included on an internal modem card. The modem is designed so that data from the PC bus hits the card as parallel data and is converted by the COM port chipset to serial data, fed to the true modem chipset. Regardless of how they are physically set up, all COM ports have a reserved memory I/O Address (actually a range) and an Interrupt ReQuest (IRQ).

Peripheral devices talk to the computer via interrupts, signals that data has arrived or requests have been serviced, or when errors are encountered. Physically, this occurs on one of 15 wires routed to the CPU (the IRQ bus). Each one must be assigned to a unique peripheral. The system timer is IRQ 0, the keyboard is IRQ 1, and the serial ports typically get IRQ's 3 and 4. If there is an IRQ conflict, two devices will address the CPU at the same time and create a short on the IRQ bus. This often happens when the modem and a serial mouse conflict, both set up to use COM 1 (or the modem may be on COM 3). If you are using a serial mouse (as opposed to a PS/2 interface mouse) and it is connected to COM1, the modem cannot be set to use COM1 or COM3. It must be set up for COM2 or COM4. There are four standard COM ports and addresses, but you may see devices going higher. They are set to use the same IRQs as COM1 and COM2, respectively.

Port I/O Address IRQ
COM1 03F8 4
COM2 02F8 3
COM3 03E8 4
COM4 02E8 3


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