IRQ conflicts involving serial and parallel ports

An interrupt is used when a peripheral communicates with the computer. Device interrupt conflicts occur when two or more devices attempt to use the same interrupt simultaneously.

There are 4 COM ports, but only 2 can be used simultaneously because they share interrupts. COM1 and COM3 share interrupt 4; COM2 and COM4 share interrupt 3.

The following are Industry Standard Port Configurations for both serial and parallel ports:
NOTE: IRQ stands for interrupt request.

SERIAL PORTS

Port Address Interrupt

COM1 03F8h IRQ4

COM2 02F8h IRQ3

COM3 * 03E8h IRQ4

COM4 * 02E8h IRQ3

* Available only with DOS 3.3 and later.

PARALLEL PORTS

Port Address Interrupt

LPT1 3BCh IRQ7

LPT2 378h IRQ5

LPT3 278h None