I'm sort of talking to the controller. Certainly it knows I'm talking and is responding.
I rewrote the code to replicate the 3 stage start up sequence (from post 21) and then connected the controller to the arduino and it seems to be responding. As far as the data being received from the controller in response to the data I'm sending, it looks identical to what's received when it's connected to the arduino.
The only difference in the data, and the main issue I seem to be having, is that on one of the 2 rs485 data pins the idle is HIGH whereas on the mixer it's LOW (see attached).
I'd like to resolve the data issue in order to eliminate it from the investigation (plus I think the arduino's signal is wrong as per the rs485 spec / requirements / guidelines / whatever).
Aside from the data issue I've found that when I connect the controller to the arduino and power everything on, the screen stays on the "00", the backlight stays lit and the buttons are unresponsive. However, if I start everything up and then move the controller connections to the mixer (which is powered on patiently waiting) the backlight then goes out which is the appropriate standby mode and the buttons are responsive. Better yet, if I then move the connections back to the arduino, the buttons remain responsive. I can perform all of the usual actions on the buttons until I attempt to put it back into standby at which point the buttons then become unresponsive once more.
Clearly the unresponsive buttons is a blocker for real use but I may continue to work on the code that handles the state, temp, pressure, etc. until I have a breakthrough or somebody points out the glaringly obvious mistake to me...