Computer Science - Computer Hardware - Discussion

COM port is short for communications port. COM ports are also called serialports and on PCs are sometimes called asynchronous communications ports. PCs can normally have up to 4 COM ports. Most PCs have 2 COM ports either built in to the motherboard or on an add-in I/O card. COM ports built in to a motherboard are usually configured with options in CMOS setup but may be configured with jumpers on the motherboard. COM ports on add-in cards are normally configured with jumpers on the card.

External modems plug into an existing COM port on the back of the system. Internal modems have their own COM ports built-in. The COM port on an internal modem is normally configured with jumpers or DIP switches on the modem. Some modems may be software configurable, including Plug-and-Play modems.

COM ports are configured to use a particular I/O port address and a particular IRQ (Interrupt Request Line).
All COM ports in a system must be configured so that their I/O addresses and IRQs do not conflict.
Here are some standard address and IRQ settings:

COM1 - 3F8 IRQ4
COM2 - 2F8 IRQ3
COM3 - 3E8 IRQ4
COM4 - 2E8 IRQ3

IRQ 0--timer.
IRQ1--keyboard interrupt.
IRQ 2 --cadcade interrupt.
IRQ 3 ---second serial port.
IRQ 4--first serial port.
IRQ 5---sound card.
IRQ 6--flopy disk.
IRQ 7--first parallel port.
IRQ 8 -- System.
IRQ 12-- Mouse.
IRQ13-- floating point unit/coprocessor.
IRQ 14-- IDE Hard drive Primary.
IRQ 15-- IDE Hard drive Secondary.

Irq 6 -- Floppy
IRQ 7 -- Printer
IRQ 8 -- System
IRQ 12-- Mouse
IRQ 14-- IDE Hard drive Primary
IRQ 15-- IDE Hard drive Secondary