I feel your pain. I have had to decipher lots of serial interfaces over the years.
Try and get hold of a breakout box. Get one that has LED indication of line status and switches to enable/disable comm. lines. some have provision for patching and swithing TD/RD lines.
Then the fun starts. Try different settings for word length, stop bits and baud rate. This is an arduous task. Take it methodically (7,1,2400) until you can get any output to your device. Any output lets you know that you are on the right track. It takes some time and hair pulling but you'll get it eventually.
Good luck.
A+,N+,S+,L+,I+,HTI+,CET,CST,CNST,CNCT,CFOT,CCNT,ACSP,ISA CCST3