NMEA and Installation


NMEA0183 is the long established communications standard for marine electronics.Whilst new installations on small craft are now going to be based on the newer NMEA2000 standard, this does not mean that NMEA0183 has come to an end. Commercial vessels still follow NMEA0183; almost all NMEA2000 systems also have a NMEA0183 interface; and almost all apps and software expect NMEA0183, due to the high costs and licence restrictions imposed by the NMEA. Also, of course, instruments last a long time, so there will be a user base for NMEA0183 for many years ahead.

What is NMEA0183?

NMEA0183 is a serial communications protocol, with data going in one direction down a pair of wires labelled A and B (or +/- or Signal and Ground). To get data going in both directions, you need two pairs of wires. Electrically, it is very similar to RS422, but the standard initially covered a greater voltage range, so it is still necessary for receivers to cover this bigger range - this is why just using an RS422 converter may not work.Some manufacturers, most notably Garmin, ignore the standard's requirement to use a pair of wires, and output data on just a single wire. which is less resistant to interference problems.

NMEA0183 also requires the receiver to be optically isolated. This does two things: it helps eliminate interference problems, and it also avoids ground loops forming, where a current can flow along the data lines because the instruments at each end have slightly different ground voltages.

NMEA0183 as standard runs at a speed of 4800bps. There is also NMEA0183HS, which runs at 38400bps, and is primarily used for AIS. Some devices will also run at other speeds, but will also support the standard ones.

Understanding NMEA0183 Connections

The key thing to remember is that data flows in just one way down a pair of wires from the talker to the listener. This means that you cannot connect more than one device's talker to a listener, as the data from the two talkers will clash and be unintelligible. But you can split the output froma talker to multiple listeners. This means, for example, that you can split the output from your GPS to your chart plotter, VHF radio and NMEA to Wi-Fi device. There is no hard and fast rule on how many devices can connect to a talker, but 3 or 4 are generally OK. If you add too many, nothing will receive data.

On most boats the chart plotter takes input from the GPS and other sensors and combines all of the data into a single, combined NMEA output (some autopilots can do this as well). Without this, or in more complex installations, you may need to use an NMEA multiplexer from manufacturers such as Brookhouse or ShipModul.

When connecting the wires, always connect A to A and B to B. A and B are sometimes labelled + and - or Signal and Ground. If you have a device that outputs data down a single wire, like Garmin, then at the listener you connect the data wire to A, and connect B to the negative side of the power supply.

Installing our NMEA0183 to Wi-Fi Device

When deciding where to install the device, there are three things to consider: power, NMEA data, and  the Wi-Fi signal. Power is often taken from the same circuit as the boat's instruments, as it only draws a small current, and has an internal self-resetting fuse. NMEA data is often taken from the output of the chart polotter, as this generally outputs all NMEA data down one wire. If you want to send data out from your app to the instruments, this may go to the chart plotter for routes and waypoints, and/or to the autopilot if you want to control it directly - the input and output can go to different devices.

On most vessels the Wi-Fi signal is more than adequate wherever the device is located, as fibreglass is almost totally transparent to Wi-Fi signals. However if your boat is metal, carbon fibre or heavy wood, or particularly large, you may need to check for the best location. The easiest way to do this is to connect the device to a long wire for power. Put the device in a location, and then check the Wi-Fi signal strength round the boat using the signal strength display in your phone or laptop (or you can get the T-shirt), waiting a couple of minutes in each spot to adjust to the signal there.

When you have selected the position, secure the device with four M3 or M4 countersunk screws (or Sta-Lok or Velcro). Lead your wires to the device and bare the ends, then connect up. The wires should be 0.32 - 3.3mm2 (12 - 22AWG), bared 7mm (¼"). Switch on power and the green power LED will light up, with some others flashing briefly. Switch on the instruments so they are sending data to the device, and the Rx and Wi-Fi lights will flash as data comes in. If Rx doesn't flash, check the wiring to the instruments and their setting. If Rx is flashing but not Wi-Fi, then try swapping over the A and B wires. Finally, check data going the other way by firing up an app that sends data out over Wi-Fi, and you should see the Rx and Wi-Fi lights flashing.