Are you Positive about that Negative ?

A customer recently contacted us asking why he was measuring a negative voltage on his NMEA0183 Output and suggesting that his AIS had a fault. He confirmed that he had his multi-meter’s Red lead on the Output+ and the Black lead on the Output-, so how on earth could it be measuring a negative voltage unless the AIS was faulty.

Well normally when you measure the voltage of say a battery, the two terminals are clearly labelled + and – so as long as you get your multi-meter’s leads round the right way the measured voltage is always positive. However, with most NMEA0183 signals, you are dealing with a “differential” voltage where the positive and negative connections both have pulsing signals on them and it is the difference between the voltages on the two connections that you are measuring.

Looking at the diagram above, you can see the pulsing signals are going between 0v and +3.3v on both connections, but they are inverted so when one signal goes to 3.3v the other goes to 0v and vice versa. This has the interesting effect of making the measured voltages alternate between +3.3v, when the + connection is at 3.3v and the – at 0v, and then -3.3v when the + is 0v and the – is 3.3v.

Most AIS units, particularly receivers only output data when they receive an AIS target report, so if there are not many AIS vessels in the area, it is not unusual to measure -3.3v on the output. Couple this with the fact that AIS data is at a higher baud rate (38400) which means the pulses are much shorter and may not be detected by the multi-meter, it is always good practice to use an LED to test NMEA0183 outputs as detailed in our Tech Note.

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