This doesn't appear to be a NI serial interface card as our pin 1 is ground and pin 3 is RTS+. Thus I do not know what "A" wire and "B" wire are referring to (TX? RX? etc.). You may need to contact the manufacturer of your card for help on decoding their schematic.
In general, 2 wire devices have TX+ and RX+ connected to each other as well as TX- and RX-, and the transceivers are controlled to prevent double driving.
How many connections does your device have, and how are they labeled?
