[SkunkBoat]

I have an Echo 94sv, probably one version older than yours.
Really nice machine. Side view is great for wreck fishing.

NMEA2k makes connecting things together very easy. Unlike old interfaces that required a pair of wires for every different signal, it uses a single pair of wires for the data and that data has a standard format that every machine understands. It also has a pair of power wires so it can power remote sensors with the same cable.

You will have to figure out how to connect to the Mercury.(edit: You might need an adapter from SmartGuage to NMEA2K) If it has a NMEA2k cable going to the smart gauge then you just connect that to a drop on your backbone and insert "T"s and drop cables to your smart gauge and Echo. It will work on both at same time. You can add "T"s to the backbone and (on a large boat) even extend the backbone with a cable.

The thing to remember is that the drop cables and backbone cables are different. The drops come off the T and go to devices and the backbones extend the network. You also need to "terminate" the network. Your kit will include a terminator.

You can't add Ts to drops. Each device must connect to the backbone thru its own drop.