Spud, outside the US, minisplit systems dominate, and because of their efficiency, they are starting to be very popular here as well. However, all the normal thermostats from Nest etc... aren't designed to interface with these types of systems. That's because these systems have continuously variable cooling or heating power, not just on or on high or on low. So if you use an adapter to a conventional stat, you lose efficiency.

The most popular brand of these systems in the US is from Mitsubishi. It turns out the control mechanism from their thermostats to the various indoor units is simple serial port communications, and all their indoor units have a common plug, the CN105 interface.

Turns out folks had reverse engineered the interface specs and built arduino code to talk to it from an nodeMCU. You can see the code here: https://github.com/SwiCago/HeatPump

What would be the best way to interface this code to Homeseer? In the arduino plugin or a different approach?