IRQ conflicts involving serial and parallel ports

IRQ is an abbreviation for Interrupt Request Lines. According to the Microsoft Press Computer Dictionary, IRQs are 'hardware lines over which devices such as input/output ports, the keyboard, and disk drives can send interrupts (requests for service) to the microprocessor.' Device IRQ 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. COM1 and COM3 share interrupt 4; COM 2 and COM 4 share interrupt 3.

The following are Industry Standard Port Configurations:
SERIAL PORT
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