Connecting NMEA 0183 equipment

Interconnecting NMEA instruments can be a real hassle if one is not familiar with the different types of connections. The NMEA 0183 standard specifies the talker ports (outputs) and listener ports (inputs) to be differential. This means that the data is transported by means of voltage levels over two wires, separated from ground. Roughly, the voltage levels swing between 0 and 5 Volt, and both wires are in opposite phase. When one is at 5V, the other is 0V and vice versa. Such a system has two major advantages:
  1. It is less susceptible for interference since an induced voltage has the same polarity on both wires and therefore not changing the ratio between the voltages on both wires. A differential listener only detects the voltage difference between both wires and not the absolute value so interference has no influence on this.

  2. A differential system produces less interference on HF systems. When a current flows through a wire, a magnetic field is generated around that wire. This magnetic field can induce currents in other wires, like the power or antenna wiring to a HF or VHF radio. This results in interference. In a differential system, both wires carry the same signal (=current) but in opposite phase. The magnetic fields around both wires are also in opposite phase and therefore canceling each other.

The NMEA world could be a perfect world if every manufacturer would adhere to the standard. You could simply connect the 'A' terminal on a talker to the 'A terminal of a listener and do the same with the 'B' terminal, as depicted in figure 1.

Fig.1: Differential

Fig.2: Single-ended

However, manufacturers like to save money where they can, and many of them found that they could do so by leaving out proper drivers and even the optocoupler that is required for the mandatory galvanic isolation. So many equipment has single-ended inputs and outputs, where the signal is transported over a single wire while the common ground connection serves as the signal return path, as shown in figure 2. It might seem similar to the setup in figure 1, but the main difference is that the ground connection also carries the supply current of the equipment, together with all power spikes and surges that occur in an electrical system.

From a pure wiring viewpoint, a single ended setup is still easy to connect. But in the real world, we have both differential and single-ended equipment. This results in four possible connections:

  1. Differential -> differential (fig.1 & 3)
  2. Single-ended -> single-ended (fig.2)
  3. Single-ended -> differential (fig.4)
  4. Differential -> single-ended (fig.5)

The following examples will show how to make different types of connections. In each example, a single-ended or differential talker or listener is connected to our multiplexer, which features differential NMEA inputs and outputs.

Differential -> differential

This setup is very straightforward. The talkers' A and B terminals are simply connected to the corresponding A and B terminals of a listener port on the multiplexer.
The same method is used to connect the multiplexers' talker port to an instrument.

Fig.3: Differential

Single-ended -> differential

The output of a single-ended instrument can be connected to the A terminal of a listener port. To close the circuit, the B terminal of the listener port is connected to the instrument ground. Do not connect the B terminal to a power ground close to the multiplexer but feed a ground wire to the instrument and connect it there to the instrument ground. This prevents any supply currents to disturb the data. No ground loops are created since the listener ports of the multiplexer are galvanically isolated and thus floating from ground.

Fig.4: Single-ended -> differential

Differential -> single-ended

When connecting a differential talker to a single-ended listener, only the A terminal of the talker is connected to the input of the listener. The B terminal of the talker is left open. The return path of the signal is the common ground of the talker and the listener. Since there is no galvanic isolation on a talker port and certainly not on a single-ended input, it is best to keep the ground wires as short as possible and preferably on a single bus to minimize the chance of interference.
Note: Never connect the B terminal of a talker to ground! A B terminal on a talker is also supplying signal, which would be short-circuited to ground.

Fig.5: Differential -> single-ended


Multiple listeners

When multiple listeners of different nature (single-ended vs. differential) are to be connected to a talker port, the same connection rules apply. In the example in figure 6, two differential and one single-ended listener are connected to the differential output of a multiplexer.

Fig.6: Multiple listeners