What you ask for is exactly what is implemented in the OSAX (AppleScript extension) suggested by Bunsen or in the XFCN from the Dartmouth HyperCard stack. The source code is written in C or Pascal but might be difficult to obtain. Consider to get a copy of the Mac Programmer's Cookbook. This book helped me to gather valuable information I needed to set up my first programme to remotely control an electronic balance via RS422 <-> RS232 communication.

Have a terminal programme available to independently check for proper wiring of your setup, before trying to debug your own code.