To start off with, the RS-232 is a very old standard that most engineers and software developers would like to see go away. First - it is slow, second - it is point to point only (can physically connect to one device at a time) and third - it has limited distance.

Windows API supports com port access by a function called CreateFile. Microsoft Visual Basic uses MSComm. I’m not sure what Mac uses to access its com ports.

This is not an easy thing to do. You might consider using a ActiveX component or simply using a terminal application such as Hyper Terminal.

Good Luck!