Testing/Repairing Serial Controller

[ProfessorC]

I have an old Xantech multi-zone amplifier that is supposed to accept ASCII serial commands via a DB9 RS232 COM port. Needless to say, it does not.

The COM port is mounted to it's own PCB that I've replicated on a breadboard with a new DB9 connector, and jumpered to the main control board for the amp. Still doesn't work.

What's the next step. What's the best way to test the controller board to track down the cause of the issue?

 

[ArmRules]

Oscilloscope and trace the communication path from to the microcontroller.
Are you getting any replys to your text strings? If its the same protocol as below link
http://www.xantech.com/files/manuals/433_ISM4.pdf
You should get OK or ERROR as a reply to your coamands. If you are getting replys then it indicates that the controller is getting and processing the commands correctly but is unable to carry out the intended function due to some other hardware fault.