I bet there is a code sequence or unhandled buffer overflow that the HDS errors on.

In my experience hooking up the heading output from my AP to my HDS 7, the HDS didn't work with the NMEA0183 but did work after I converted NMEA0183 to NMEA2000 externally. The documentation said it should have worked.

I suspect Lowrance doesn't test their product with anything other than Simnet or NMEA2000. The NMEA0183 seems to be a weak implementation.