Windows NT does not allow the use of COM3 or COM4 by default. This Windows NT registry will allow you to override: add a string value called COM3 if you want to add COM3, or COM4 if you want to add COM4 support. The text string is the same as the value name(com3=com3).

Hive: HKEY_LOCAL_MACHINE
Key: hardware\devicemap\serialcom
Name: COM3
Name: COM4
Type: REG_SZ
Value: COM3
Value: COM4

Where you can have a problem is getting com1 and com3 to work together or com2 and com4 to work together. com1 and com3 share an interrupt as does com2 and com4. You can assign two COM ports with the same interrupt as long as you don't use the two ports at the same time. For example, com1 to serial port used by a digital camera and com3 to a serial port used by a scanner. As long as the applications are run separately and do not attempt to access the ports simulataneously, they will both work. Try to download images from your camera and use your scanner at the same time, NO. Then you run into a resource clash.

I have seen four ports in use simultaneously using only 2 IRQ's. You will probably have to set

Hive: HKEY_LOCAL_MACHINE
Key: SYSTEM\CurrentControlSet\Services\Serial
Name: ShareInterrupt
Name: PermitShare
Type: REG_DWORD
Value: 1

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