Has a simplified but not simple description of NMEA Protocol.
NMEA 0183 is a protocol for command and data exchange between devices. Based on 'sentences' it is a pretty rich palette for controlling GPS and other such nav goodies. However your particular GPS/NavDev/FishFinder has it's own set of interface requirements likely based on NMEA 0183 protocol. Check with your vendor for details and suitable upload/download tools. Designing your own is not for the faint of heart or weak of codesmithing. I have used Garmin and Trimble nav devices for many years and they seem to have a good reputation for customer assistance.
anasasi (Not NMEA compliant)