The Simrad IS20 and IS12 instuments are simnet, and therefor NMEA 2000 compatable. NMEA 2000 is a drop cable to backbone system, if you wire it like a NMEA 2000 network (unlike a SIMNET daisychain network) each device and share data with the other.
I am in the middle of installing a NMEA 2000 Network on our boat.
1. Simrad IS 12 Combi depth/speed.
2. Simrad TP 32 Tillerpilot
3. Garmin GMI 10 Display
4. Lowrance 7300 HD chartplotter
4a. Lowrance puck GPS that came with the Chartplotter, powered by the network
When they are all connected to the NMEA network, the Tilerpilot can use the GPS to go to waypoints, Garmin can diplay depth, speed from either paddle wheel or GPS, and Bearing to waypoiint, etc.
If I add a wind transducer, I can do so with out buying another display, save for calibration issues. If I buy a simrad IS20 transducer, I can display wind date on the Garmin display, but I may have to either borrow a Simrad diplay to caibrate the transducer, if needed, or pay a installer to calibrate it for me.
It all plays nicely together, or at least it should. THe Lowrance connectors mate with the garmin connectors, etc.
Go to Maretron : Marine Electronic Instruments
if you want to learn more about NMEA 2000 networking.
NOTE, NMEA 2000 is not the same as NMEA 0183. Compltetly different beasts.