What was done
A two-way integration over the unit's own control panel bus, with every protocol claim checked on a real installation.
agjendem wanted to control a Flexit SL4 R air handling unit from Home Assistant without Flexit's Modbus adapter. The README says the integration was reverse engineered and written together with Claude Code, and it states plainly how far to trust that.
How it was done
An M5Stack ATOM Lite with an RS485 module sits in parallel with the control panel in a spare socket, powered from the bus. The proprietary protocol is documented in a protocol file with marks for what is verified, inferred or unknown. The README lists mistakes found along the way, such as a checksum window off by one byte and a status byte misread three times.
Results
Reading covers supply air temperature, fan levels and duty, heat demand, boost, filter alarm and filter timer. Writing covers fan level, setpoint, boost, afterheater and filter reset, verified against the control board's own broadcasts. A climate entity combines them. The unit does not need to be opened.