I would like to establish communciation between a rasppi used as a controller and 4 other rasppi as interfaces to digital input/outputs. The communication between the interface rasppis and the digital I/Os will be done via I2C.
What do you recommend I use for the communiation between controller rasppi and the interface rasppis?
OPCUA, Modbus TCP, or MQTT?
Is it normal to use OPC UA for controllers? I heard it was used mostly for monitoring data in the plant and not for controls.