I started to reverse engineer the UART protocol for Fujitsu Airstage units (the protocol used between the air handler and the official UTY-TFSXJ4 WiFi dongle). Since ha_airstage is working pretty great locally with the official dongle, I’m not sure I’ll continue building my own ESPHome integration, but I wanted to share what I learned in case anyone wants to pick up the trail.
The UART seems to be 9600 baud, 8 bit + 1 stop bit, no parity, inverted signal. Logic analyzer captures are in the captures dir. Everything else is a GPT5 vibe-coded interpreter for those captures (and at first glance the interpretation seems very sane!)