The way you connect external devices to the multiplexer depends on their type. Output devices can be of 2 types: Single Ended Output devices and Differential Output devices. All the inputs of the multiplexer use opto-couplers which facilitates the connections and offer extended noise immunity. Input devices to which we connect the multiplexer output can also exist in 2 types: Single Ended Input devices and Differential Input devices. We now discuss all the possible cases.
The connections of output devices (also referred to as Nmea 0183 talkers) to the multiplexers are shown in the next picture. If your Nmea data is available though a Differential Output device, the preferred way is to connect its A (or Nmea+) terminal to the multiplexer A and its B (or Nmea-) terminal to the multiplexer B. You could also connect GND to B. If your Nmea data is available through a Single Ended Output device (such as RS-232) you connect its Output terminal to the multiplexer A and its Ground (or common) terminal to the multiplexer B.
We now consider the connection of the output serial port which uses a 3-pin (screw or spring) connector with labels A B and GND. If the receiving device has an opto-isolated port, you connect A to the A (or Nmea+) terminal of the receiving device and B to the B (or Nmea-) terminal of the receiving device. You could also connect GND to B (or Nmea-). If the input port of the receiving device is not opto-isolated you should take special attention to the connections. You should connect A to the input terminal of the receiving device and not use B. In addition, both GNDs (multiplexer and receiving device) should be connected together.
If you connect B to the GND of the receiving device, you may destroy the output driver of the multiplexer. In order to avoid this possibility, the output port of recent multiplexers have only a 2-pin connector with labels A and B instead of A, B and GND. However the B pin can be configured through a jumper. In position 1 (default) B is connected to GND making the output port of the multiplexer a Single Ended Output. In position 2, B is connected to the negative driver output, making the output port of the multiplexer a Differential Output.