kdratch

Hello all. I am installing a VDO gps speedo and I want to use my Garmin chartplotter as a speed source. The speedo wiring has NMEA0183-a and NMEAO183-B. There is no Tx or Rx designation. Are those wires the Rx to connect to my Tx from the chartplotter? Or am I missing something? The speedo also has sender signal + and - wires, but I am thinking that they are for a dedicated speed sensor of some sort. Any thoughts?