Hi,
I found this comment on the sideimagesoft forum from Humminbird Greg reguading NEMA:
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Rudiger is correct: NMEA Output only. No NMEA Input.
You should be able to take the NMEA output off of the NMEA pigtail wire. This is the shorter cable that comes out of the GPS Receiver’s cable connector. Take the black plastic cap off and you should have a Red, White and Black wire. Terminate the Red wire so it does not make an electrical connection to anything else. The White wire is the NMEA Output (NMEA +) and the Black wire is the NMEA Ground (NMEA -). Set the NMEA Output menu to “On” and you should be ready.
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Maybe this will help answer your question.
Roger