COM PORT AND INTERRUPT NOTES

For IBM PC compatible computers since the early 1980s, here are the default or most common interrupt (IRQ) values or settings and COM Port default settings. * Indicates IRQs that can't be used for general devices.

Common Interrupt Settings

COM Port-IRQ-Base Port Address

IRQ0=System Timer *
IRQ1=Keyboard *
IRQ2=PIC *
IRQ3=COM port
IRQ4=COM port
IRQ5 =LPT2/Sound card
IRQ7=LPT1 Printer
IRQ8=RTC *
IRQ10=Varies/Video card/PCI
IRQ11=Varies/SCSI Cont./PCI
IRQ12=Varies/PS2 Mouse
IRQ13=Numeric Data Processor *
IRQ14= Primary IDE Controller
IRQ15= Secondary IDE Controller

COM1 IRQ4 3F8H
COM2 IRQ3 2F8H
COM3 IRQ4 3E8H
COM4 IRO3 2E8H

 

Notes:
1. LPT2 (second printer port) is usually not used thus it is usually used for the soundcard.
2. Some newer modems will auto-setup on COM5/IRQ10/11 depending on which is free.
3. Some sound cards will auto-setup on IRQ10/11.
4. If you are short of resources, especially with laptops, go into Device Manager and/or the BIOS and disable any Infrared (IR) devices (COM , LPT or other ports, etc.)-most folks don't use them.