Did you ever make this long range extension cable? I moved into a new home with MUCH more property, and as such, currently has two separate irrigation timers (8 channels on each), that are located about 75′ from each other. I want to replace them with my OSpi and an Expansion board, if the SPI is slow enough to not have noise issues. Otherwise, I’ll need to buy an additional opensprinkler unit, which would be quite a bit more expensive.
The alternative is, of course, just extend the 24VAC lines for the valves to the OSpi’s location, and install the expandion board there.