It's in the "Words"
Well, there is NEMA 0183 and there are it's "Versions" 1.0, 1.5, 2.0 etc. That sets the standards for electrical connectivity to the communication "bus". All devices on the bus communicate and share information equally.
Then you have to see what "words" or "phrases" each device recognizes. Raymarine uses proprietary "phrases" for its "Seatalk" interface. That way it can handle all the data on the interfaces and between their instruments. The problem arises when you try and interface a non-Seatalk device that does not regonize the phrases that Raymarine sends/recieves. Your new plotter may or may not recognize all the Raymarine phrases. In the communications appendices of both the Raymarine devices and your new plotter, down around oh say page 200 (just kidding) you will see a listing of the interfaces supported and the phrases recognized. You will need to make sure the Baud Rate, and NEMA version are the same for all devices, and then see what phrases are common between the new plotter and the Seatalk format.
Have I lost you yet? It's really quite complicated, and I think that instrument vendors do that specifically to avoid the "mix and match" approach to onboard data systems and have you buy everything from "Them" and not other vendors.
I had a heck of a time getting my Garmin to talk to my Nexus network, compounded by the fact that Garmin is notorious for mixed up wiring on the NEMA connections.
Good luck! Unfortunately the manufacturers are ZERO help trying to connect to things outside of their own product range!