Example of PGN generation from Signalk paths

[tonybentley]

There are cases where signalk plugins will not be able to provide exact NMEA 2000 PGNs to display on Raymarine, Garmin, B&G, or other proprietary chartplotters. We can programmatically generate PGNs using Node-Red and Signalk's global application context.

The use case is for using the environment.inside.temperature path, which comes from a Shelly H&T IoT wifi sensor, and send the values to a Raymarine Axiom+ chartplotter via a NMEA2000 datasource connection with Signalk.

The interval to send values will be different for each display, but in this example I will choose every 5 seconds using the Inject node that is provided by Node-Red.

Now that we have an interval to send updates from, we can use the function node to programmatically generate the PGN format.

Each function node represents a path from Signalk. For example, the first function node will be environment.inside.temperature + environment.inside.humidity and the second will be environment.outside.temperature + environment.outside.humidity. Both of these environmental parameters use the 130311 PGN for displaying on a chartplotter.

To generate the PGN object, we will use the extensive reference document that Kees Verruijt has developed for canboat. It is the library Signalk uses for NMEA 2000 parsing and creation of PGNs.

I will choose the PGN 130311 reference to define the proper keys and values for the PGN object.

For the inside temperature and humidity sources, there are lookup tables. I will choose the value that represents inside, which is 0 for humidity and 2 for temperature.

Next we will use the Signalk app global scope to access the path for each value:

const app = global.get('app')
const humidity = app.getSelfPath('environment.inside.humidity.value')
const temperature = app.getSelfPath('environment.inside.temperature.value')

And finally build the object for serializing into a NMEA 2000 PGN:

msg.payload = {
    sid: 1234,
    pgn: 130311,
    'Humidity Source': 0,
    Humidity: humidity * 100,
    'Temperature Source': 2,
    Temperature: temperature
}
node.log(JSON.stringify(msg))
return msg;

Finally, to serialize the object we will need to use the Signalk node signalk-send-nmea2000. Enter the value nmea2000JsonOut for the NMEA 2000 JSON Event and nmea2000Out for the NMEA 2000 Event in the node configuration, which will ensure the PGN is sent to the NMEA 2000 bus.

You can inspect the log entry using the Signalk Server Log page. I recommend removing all fields from the Debug input field so you can see each log entry coming from Node-Red. You should see the log coming from node.log() to log custom messages or turn on the switch under signalk-server:events:nmea2000JsonOut to see the PGN in JSON format.

Once you have all three nodes, you will connect them and deploy to see it working.

In the screenshot above, I have one node for inside, and another for outside.

Happy coding!