Yesterday, I hooked up the "central station" and recorded a bunch of data. I tried as many settings as I could, focusing on identifying how commands are sent to the air conditioner and how status are retrieved.

And after just two hours of opening PulseView files, extracting data and compiling everything in Excel here are the result.

• status inquiry

      - RAC adapter message: MT P="address" C="checksum"[CR]

      - air conditioner response: OK P="value" C="checksum"[CR]

• value setting

      - RAC adapter message: ST P="address","value" C="checksum"[CR]

      - air conditioner response: OK[CR]

That's all there is to it! The checksum calculation is straight forward, so I won't go into details here.
For a list of available addresses and possible values, check out the file "Analysis result (2020_06_14).xlsx".

As a confirmation, I connected my FTDI USB to TTL serial adapter directly to the air conditioner, grounded the H-LINK pin and sent a few commands to the air conditioner: everything worked fine.

So the next step will be to learn about Zigbee communication system and explore how I can implement a wireless remote functionality in Home Assistant.