[BradC]

Well, that schematic corresponds pretty closely with the configuration on that outdoor board.

So I'll eat my words. It uses a one wire serial protocol on terminal 1 of the unit-to-unit link.

Having said that, you can quite easily remove the CPU chip and drive all your solenoids from the inputs of IC8 using logic levels, so you don't need to interface to any high voltages. In addition, you have a thermistor connected to the main cpu also which you could connect directly to an arduino or equivalent.