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IRQ Device Assignments | Tech Info Home Page | Hardware Assignments |

Device IRQ  
System Timer IRQ 0  
Keyboard IRQ 1  
Interrupt controller IRQ 2 ...cascaded with IRQ 9
Com2, Com4 IRQ 3  
Com1, Com3, Network Cards IRQ 4  
...available / Sound Cards / LPT2 IRQ 5  
Floppy Drive controller IRQ 6  
LPT1 Parallel printer port IRQ 7  
...system use (real time clock) IRQ 8  
Cascade with IRQ 2 IRQ 9 Also shared with PCI cards
...available IRQ 10  
...available IRQ 11 ( some Network Cards)
...available or Bus Mouse IRQ 12  
Math co-processor IRQ 13  
Primary Hard Drive controller IRQ 14  
...available / 2nd hard drive IRQ 15  

Note: IRQs 3 and 4 are usually reserved for the COM1 and COM2 ports
When a computer has a modem installed it will want to use one of these 2 settings. Windows 95 and Plug-and-play will try to set the COM port to COM3 or COM4 so it does not conflict with the other internal com ports, but in either case COM3 and COM4 will still have to share the interrupt lines IRQ3 and IRQ4. This will not work. The only safe way is to "disconnect" one of the internal com ports (if you have them) prior to installing the modem. This is usually done by changing a setting in the CMOS/BIOS of the computer.




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