The NMEA input consists of an optocoupler. See it as a LED shining on a transistor to get galvanic isolation.
The Signal return is simply the cathode of the LED and NOT gnd or B-. So you connect that to the TX-.
The A and B thing often creates misunderstandings, as the A/B from the RS422 point is opposite to the A/B of NMEA0183.
The pair is always eachothers compliment, so both of them can be +5V and 0V but never at the same time. If you dont get it working, first thing is to reverse polarization. The LED inside the optocoupler will flash either way, but only one is readabl