Zehnder ComfoSpot 50 Communication Protocol

This is an attempt at reverse engineering the communication protocol used by the Zehnder ComfoSpot 50 ventilation unit with heat recovery.

It has the following control board:

The control board of Zehnder Comfospot 50. Connector “SENSOR X8” is used for the temperature and humidity sensor while “BUS X7” is for the external control panel.

The “BUS X7” has four pins labeled +, A, B and - which are used for connecting to an external ComfoLED control panel which replicates the functionality of the built-in control panel. The 8-pin chip below the “BUS X7” is a MAX487 RS-485/RS-422 transceiver.

It appears to be using the UART protocol at 9600 baud rate, 9 data bits, 1 stop bit and 0 parity bits. The one extra bit is used to indicate an address byte and a data byte in a “single master, multiple slaves” or “single controller, multiple responders” setup. With the most significant bit (MSB) indicating the the byte type (multi-drop bus or MDB) we get two address bytes and the rest as data bytes.

The device appears to be sending two sequences of bytes on pin A at 76ms intervals:

Data packets sent by the master/controller device at 76ms intervals.
Data packets sent by the master/controller device at 76ms intervals.

First packet:

Data sent to address 0x01 0x04 in hex notation.
Data packet sent to device address 0x01 0x03 in hex notation.

and the second packet:

Data sent to address 0x01 0x04 in hex notation.
Data sent to address 0x01 0x04 in hex notation.

I’ve haven’t been able to figure out which devices these could be and what data is being sent to them.

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