My first step was to open the gateway and to check the pinout.
I can confirm the pinout posted by Florian (https://hackaday.io/project/168959-let-me-control-you-hitachi-air-conditioner)
Nevertheless the function of pin N°6 is still unclear for me. Let's check that later.
I mesured 5V for Rx & Tx echanges.
I'll come up soon with the first paquets...