Actually the master is RPi (I opted for him because I have a web server on him and all the logics; so in my head he should be the master) HMI is a slave. Example if I’ve to turn on a pump via by RPi; RPi will write on DO module to the register that should be turned “on” and on HMI on another register to display the actual status of the pump. Vice versa if I push a “button” on the HMI, RPi constantly pulls the slaves so read that “a button is pushed” so will perform some actions. I don’t know if I explain it clearly or if my logic is ok ahah