“illegal function” ist the modbus reply given for a register that is read-only.
i looked into the document an there i found the following:
The Ventilation unit supports the following commands of Modbus protocol:
– Read holding registers (0x03);
– Write multiple Holding registers (0x10).
so you chose the right function
maybe what is described in this post helps:
that means you would have to write to 324 instead of 325,
because the adress 40325 starting from 40000 translates to the 324th register …
modbus documentation often is like that…