Does your Simrad RC42N have a cable coming out with a Simnet connector on it?
My memory is that a data cable runs through a water seal into the unit where it can connect via two terminal blocks, one with red and black for NMEA-2000 power and the other for yellow and blue for NMEA 2000 data, and that you can hook whatever compatible cable type you'd like, SimNet, NMEA-2000, or ST-NG to it. Alternately, I remember another terminal block or two for NMEA-0183. If this is the case, you should be able to use a NMEA-2000 cable with no adapter.
If I'm wrong, or you'd prefer to leave what is there there and you've got a cable with a SimNet connector coming out, is it male or female? Maybe one, or both, of these would help?