It's is possible to very effectively test NMEA Data simply using an LED (Light Emitting Diode) you can buy from any good electronics shop. An LED is a simple quick test of the signal, and if it can barely light up your LED, there's probably not enough power to transmit the data. Furthermore on differential systems, as the diode only allows a current one way, you can try the diode in both polarities to ensure that both sides of the differential system is functioning.

NMEA 0183 Connections use what we call opto-isolators, this is a simple component that converts a signal into light pulses, with a small internal LED and then senses these pulses and converts them back into a signal. This helps prevent any ground loops between NMEA 0183 devices. Also under the NMEA standards, devices should be able to cope with over voltage and reverse polarity, but ultimately this is down to the manufacturer.

The next video lesson looks at some essential NMEA fault finding skills.