Here are pictures of the twin controller for the two units and the master controller for the whole system.
The controllers have several functions all operated by sending serial commands to the individual controller. The controllers also report back their status when any event occurs (such as temperature change or going into defrost) and this is then displayed on the master controller.
The functions allow for the unit to be switched on and off, set to operate in cool or heat, force a defrost cycle and interrogate the unit status. Each one also has an override switch that will force the unit into heating mode (whilst retaining the defrost cycle if required).
All the commands are sent from the master controller. The temperature of the water tank is displayed on the top line (after the time), followed by the unit that is being sent commands (the number in brackets).
Each unit (1,2,3) displays its status (on/off) mode C(ool)/H(eat) and the temperature of the evaporator.
Next job - install the controllers in the external unit, fit a decent keyboard and build the master controller into its box.
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