COM.SYS will support COM3 and COM4 ports on AT bus machines. This is in addition to the published support for COM1 and COM2. In order to use COM3 and/or COM4 on AT bus machines it is necessary to include parameters on the DEVICE=COM.SYS command in the CONFIG.SYS. The format of the statement is:
DEVICE=COM.SYS (N,XXX,I)
where N is the COM port number ( 3 or 4 )
XXX is the I/O port address ( 3E8, 2E8, etc )
I is the IRQ (interrupt) level ( from 1 to 15 )
The I/O port address will be found in the specifications for the adapter card. The port address must be unique for each COM port. The port address for COM1 is 3F8 and for COM2 it is 2F8.
The IRQ level must be chosen in such a way that it does not conflict with another adapter card in the system. COM1 uses IRQ 4 and COM2 uses IRQ 3. It is possible that the IRQ level of the COM3 or COM4 adapter cards is not settable and will default to either IRQ 3 or IRQ 4. In this case it is still possible to have up to four COM ports installed. However, if two or more ports use the same IRQ level, it is only possible to have one of those ports active at any one time. If the IRQ level is settable on the adapter card, choose an IRQ level not used in the system. Levels that are usually available include 5, 10 and 15.