Networks carry information. Never protection.
Five layers, each with one job. When a network fails, the line either keeps running or stops safely.
MQTT, Modbus TCP, OPC UA
The heartbeat is a wire.
The Jetson's 1 Hz square wave reaches PLC input X0.5 through an opto-isolator. It does not travel over MQTT, Modbus or any other network, so a network fault cannot fake a healthy AI.
The gateway translates.
The Raspberry Pi 5 takes MQTT from the Jetson, writes Modbus TCP registers to the PLC and exposes the same data as OPC UA tag names. Changing the PLC vendor changes the gateway, not the AI node or the dashboard.
If the gateway's counter stops for 5 s, the PLC raises an alarm and discards stale requests. The line keeps running.
Remote is the weakest link, by design.
The dashboard views and requests. Remote start works only with the key switch at REMOTE. Losing the dashboard or the internet loses the view, nothing else.