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NMEA 0183, until version 2.0 (1992), used a “single ended” RS-232 interface with one TX line
and one RX signal line (grey at Figure 1.a) and ground line (dotted at Figure 1.a) used as reference for TX
and RX signals. Therefore, old devices have only three wires.
Since version 2.0, NMEA 0183 has been based on a “differential” RS-422 interface, which has two RX lines
RX+ (can also be marked as “A”) and RX- (or “B”), two TX lines TX+ (or “A”, grey at Figure 1.b) and TX-
(or “B”, dotted at Figure 1) and ground (not shown at Figure 1.b). Modern devices use five wires.
Devices of different versions can be connected, but with one caution. TX- (“B”) is not equal to a ground line.
Voltage on the TX- line (dotted on Figure 1.b) changes from 0 to 5V, and connecting this line to ground line
can cause a short circuit.
The right connection schemes are shown in Figure 2.
Figure 2. Connection of NMEA 0183 devices