Thanks to you I have been able to connect to my MSZ-GC22.
I'm experiencing a strange thing like one or two others : the communication works @ 9600 bauds (even parity & 1 stop bit), not 2400. What is strange is that Rich Hardley @https://nicegear.nz/blog/hacking-a-mitsubishi-heat-pump-air-conditioner seems to have also an MSZ-GC (or MSZ-GE). The electronic board has the same reference... I'm in France, I don't know if the configuration may be country specific or if it may also depend on the external unit (an MXZ-3A64 in my case).
So far the GC22 answers to the connect command (FC 7A 01 30 01 00 54 54) and then to room temperature request (FC 62 01 30 10 03 00 00 08 00 ... 00 52 52).
I'm connected directly to my PC through an FT232 Serial-USB converter working in 5V and I have written a little C program under Windows. I will probably add 1k serial resistors on the Tx and the Rx to prevent a big mess in case something unwanted happens.
I'm now going to investigate further with an MSZ-FD25, as soon I will receive the PAP05 connectors (I've soldered directly on the connector on my MSZ-GC for the first tests as I did not want to wait). I'm also planning to use an ESP8266 and develop a full remote control with wifi.