Reverse engineering modbus protocol
Recently, I started an effort to integrate the heat recovery ventilation (HRV) system into my Home Assistant setup. The house is equipped with a Zehnder ComfoAir E300, but after an extensive search I couldn't find any existing integrations for this particular model. I did however found an issue hinting at an undocumented connector that was connected to a level shifter, suggesting a serial interface was available.
Schematic showing the IC connected to C3.
I disassembled the device, probed the PCB with a multimeter and followed the traces from the connector. The result can be seen in the figure above.