Thanks for the reply. Much tea has been brewed and processed. Troubleshooting results from your suggestions below:
1) The following is from the ComLog.txt in the main program when trying to do any type of communication through the RS232 port (unfortunately, I don't think it's anything too interesting - but I could be missing something obvious):
15-12-16 21:20:44.65 : < 4,9600,E,7,1> 4, 9600, 7, Even, 1, 10, 12000 > Retransmitting
15-12-16 21:20:56.66 < 4,9600,E,7,1> > Switching Com Port
15-12-16 21:20:56.89 < 4,9600,E,7,1> > Synchronizing Com Ports
2) Playing with the timeouts didn't seem to do anything. Different combinations of baud rates, stop bits, parity, etc had no effect. Manually setting the RS232 parameters on the OPLC to match the PC's didn't work either.
3) When using hyperterminal, no response is received from the OPLC to any PC commands. I did check the serial cable output by putting a jumper between TX and RX pins to get an echo back in hyperterminal and that seems to work fine. No response from the OPLC though.
4) I'll take a look at probing the pins tomorrow. On visual inspection of the port and the board, the pins themselves and board look fine from what I see (comparing it to another M91 that does establish communication fine). But if there is any damage that I'm not seeing, I probably don't have the capability to do anything about it as my surface mount soldering capabilities are non existent.
Do Unitronics distributors typically repair M91's? or would it not be worth their time typically to go through the trouble?