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

And after just 2 hours of opening PulseView files, extracting data and compiling everything in Excel here is 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]

And that's all it is! The checksum calculation is quite straight forward so I won't explain it.
For a list of available addresses and possible values, you can check 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 see how I can implement a wireless remote functionnality in Home Assistant.