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How to track down NMEA hardware issues: Current.

How to track down Current problems with NMEA

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A lot of devices on the NMEA network run off POB or power over Bus which just means that they pull their 12V current directly from the NMEA backbone.

The other day I had an issue with my Rudder angle indicator disappearing from the network and when it was there giving false readings.

The rudder Angle indicator is a device that tracks rudder angle and connects directly to the NMEA network via the backbone. First thing I needed to do was replace its connector with a test harness to see if it had enough voltage available.

The Rudder angle indicator is powered directly from the NMEA network.

To achieve this the NMEA backbone has a 12V power feed, at least one, putting voltage onto the NMEA cable pair for 12V.

All NMEA cables have 5 separate wire connections:

  • RED = 12V positive feed for NMEA
  • BLACK = 12V negative (Earth) for NMEA
  • BLUE = low Data signal
  • WHITE/YELLOW = high Data signal

+ a shield cable.

Each of the connectors for NMEA around your boat will have 12V connection on it with, hopefully 12V of current or more accurately 13V minimum available.

Now by building a simple harness I could test the voltage at each NMEA backbone connection point and this turned up the issue for my Rudder indicator.

But some devices on your NMEA network also require the correct amount of current available to operate correctly. Most will easily get this as their Amp needs are very modest, but some might not be so modest. My Forward looking sonar for instance consumes a reasonable amount of current and if its not available will start to cause issues.

So how do you test for current??

The easiest way to do this is create a setup like you see here

This uses a SIMNET connector (most of my own boat uses SIMNET as its backbone) and the red and black wires from this are then connected to a multimeter. But instead of taking it back to a multimeter directly you can continue it through to a male connector making sure you leave the RED cable exposed.

Then you can use this to “Jumper” between the NMEA backbone and the device you are having issues with.

The RED or positive cable can be tested for current flow using a clamp meter such as the Fluke 325 shown in these photo’s.

Fluke have a great description of how to measure current on their website here.

Basically you close the jaws of you clamp meter around the positive lead whilst the network is running and select the correct type of current and of course Amp reading. (Amps/DC in our case).

Then you can record the current being drawn and if it is deficient compared to the manufacturers specs for the device, you have a problem.

Now the problem could be the device as well as not enough current being available on the network (which usually means a short somewhere along the way). Practically most NMEA devices draw very low current and current will not be the limiting factor, therefore if they are drawing too little, it is mostly the device that has an issue. If you can try replacing it with one you know is working.

If that doesn’t work then you need to start testing for shorts in the power lines of your NMEA backbone. A continuity tester (or testing resistance using a multimeter) will tell you if you have a short. Unfortunately at this point you need two adaptors to be able to connect into two different ends of your back bone.

fortunately, these faults are rare, most NMEA hardware faults are comms wires issues, breakages, or power (Voltage Issues). Other than voltage drops, the the most common issue is the devices themselves not the network.

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