Now Scott helped me get the Fusion data into the Signal K server. I have made a dashboard displaying all the status of the Fusion systems.
It would be nice to be able to hit 'mute' and change volume and sources from the 'media' tab of my Node Red Dashboard.
Clearly you can't modify the server data for volume as these are broadcast on a different pgn.
So you need to talk to the head unit directly.
Scott's
signalk-fusion-stereo server plug manages this, and I have verified by throwing alarms in and getting them out of the speakers.
Taking the example of a single volume slider.
The approach I am taking is :

Use a subscribe node to set the volume slider to the present value.
Movement events of the slider are formatted into Actisense format and sent to the N2K network through the YDEN-2 interface configured in bidirectional TCP mode.
I picked the Actisence format over the JSON format as I could not work out how to include the network address of the Fusion head unit in that format.
However .. though the volume slider tracks the rotation of the volume knob, the set volume fails.
The format function is based on my reading od Scott's implementation,
I haven't got the NMEA analyser out to see what is actually being fired, that is my next step.
A review and comments that get this working would ease along the next great chunk of integration work.
Thanks
Mub
-- The Node Red code is:
[{"id":"171ac3d3.4166fc","type":"signalk-subscribe","z":"af015e98.b4011","name":"","mode":"sendAll","flatten":true,"context":"vessels.self","path":"entertainment.device.fusion1.output.zone1.volume.master","source":"","period":"5000","x":300,"y":280,"wires":[["844810fc.ec8e7"]]},{"id":"844810fc.ec8e7","type":"ui_slider","z":"af015e98.b4011","name":"","label":"Volume","tooltip":"","group":"7067103.e483bf","order":1,"width":0,"height":0,"passthru":true,"outs":"end","topic":"topic","topicType":"msg","min":0,"max":"26","step":1,"x":480,"y":280,"wires":[["5d478867.733f78"]]},{"id":"2e72b1e5.6f160e","type":"signalk-send-nmea2000","z":"af015e98.b4011","name":"","nmea2000Event":"nmea2000out","nmea2000JsonEvent":"nmea2000JsonOut","x":850,"y":280,"wires":[]},{"id":"5d478867.733f78","type":"function","z":"af015e98.b4011","name":"","func":"n2k_msg= (new Date()).toISOString()+\",6,126720,1,28,8,a3,99,19,00,\"+msg.payload.toString(16)+\",\"+msg.payload.toString(16)+\",\"+msg.payload.toString(16)+\",\"+msg.payload.toString(16);\n\n\nmsg.payload = n2k_msg;\n\n\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","x":640,"y":280,"wires":[["2e72b1e5.6f160e"]]},{"id":"7067103.e483bf","type":"ui_group","name":"Fusion Volume","tab":"a30ce661.173488","order":1,"disp":true,"width":"6","collapse":false},{"id":"a30ce661.173488","type":"ui_tab","name":"Media","icon":"dashboard","disabled":false,"hidden":false}]
--