There is a tech bulletin on communications by Bridgeport. I need your email address.
Pins one through 4 are for 20 MA current loop. You needd four wires for transmit and receive.
Pin 7 is RS232 signal ground. Pins 6 and 8 are the signals. On the DB25 side: pins 2 and 3 are the signals. pin 7 signal ground. pins 4 to 5 are jumpered, pins 6, 8 and 20 are another jumpered set.