Switches 3 and 4 (serial port)-tell the computer how to access the built-in serial port.
The built-in serial port functions as either the primary or secondary serial port. However, if you install any option card with its own serial port you must set these two DIP switches so there is no conflict between the builtin serial port and the added card. Table 1-3 shows how to set the DIP switches.
If you install an option card with a serial port preset as primary by the manufacturer, you must designate it as the primary port and designate the built-in port as the secondary port.
If you install an option card or peripheral with a serial port not pre-set you must designate it as the secondary serial port and the built-in port as the primary serial port.
If you install two option cards with serial ports, one is designated as the primary port and the other as the secondary port. In this case you need to set switch 4 on to disable the built-in port.
If MS-DOS searches the system for a serial port and finds only one, it names it COM1:. If there are two serial ports, it names the primary port COMl: and the secondary COM2:.