You are correct: the Ethernet/NMEA 2000 will not allow you to communicate to your NMEA 0183 DSC radio - however, the NMEA 0183 output will. To get this from an internal GPS unit like the 798cXi HD Si unit that you have you will need the AS-HHGPS cable. Directions will be packaged with the cable for connection (White wire is NMEA + and Black wire is ground) and to let you know to set the NMEA Output menu to "On".