If this is an RS232C connection, then unless someone knows the cable pin out for the handshaking the easiest thing to do is get an RS232 break out board.
Pins 2 and 3 on the DB25 will be Transmit/Receive and Pin 7 will be ground. There should be a Terminal Ready pin...don't recall which pin that is. Google RS232C.
You say the other side is a 3M connector....not a Centronics style printer cable?