Just curious if you made any progress with this. I am now on my second hot tub that has been a royal PITA debugging and I absolutely hate the archaic control boards. It looks like they designed them in the 1980's and then never touched the design since. All the simple control stuff is easy, all the sensors and safety switches are easy. Programming is easy. The problem is I can't find any documentation on the interface used to talk between the control board and the top-side board. I had an old one (from the previous spa) that was replaced because it failed, so I ripped it apart and I see the RJ11 just goes into 4 wires in the control box. Assuming one is power and one is ground, that leaves 2 wires. The communication is obviously bidirectional as the control board can send data like error codes and sensor information (spa temp) to the top-side control and the topside control can send information back to the control board like button presses for jets or temp adjustment, etc.
Since these boards looks to be as old as dirt, my guess would be either a UART or something like I2C where the control board master polls the top-side slave.
Before I dig into this, anybody figure it out already?
