Gateway with Node-RED (Raspberry Pi)

What you need

• Raspberry Pi 4 (2GB+ RAM) or any Linux machine with Node.js
• Ethernet cable to connect to the PLC network
• Internet access (WiFi or a second ethernet cable)
• PLC details: IP address, OPC UA port, Node IDs to monitor

Step 1: Install Node-RED

If you are using Raspberry Pi OS, Node-RED installs with a single command:

bash <(curl -sL https://raw.githubusercontent.com/node-red/linux-installers/master/deb/update-nodejs-and-nodered)

Answer "y" to all prompts. When done:

# Start Node-RED as a service (starts automatically on boot)
sudo systemctl enable nodered
sudo systemctl start nodered

Open Node-RED in your browser: http://<raspberry-IP>:1880

Step 2: Install the OPC UA plugin

In the Node-RED interface, go to Menu (hamburger) → Manage Palette → Install and search for:

node-red-contrib-opcua

Click "Install". This adds the OPC UA nodes to the left panel.

Step 3: Create the OPC UA → MQTT flow

Drag these nodes onto the canvas and connect them in order:

3.1 "OPC-UA Client" node

• Endpoint: opc.tcp://<PLC-IP>:4840 (the address of the PLC's OPC UA server)
• Action: Subscribe
• Node IDs: the nodes you want to monitor (e.g. ns=2;s=Temperature)

3.2 "Function" node (transform to OPC UA Pub/Sub JSON)

Function node code:

// Transforma data change de OPC UA a formato Pub/Sub JSON
var nodeName = msg.topic.split(";").pop() || "Unknown";
var value = msg.payload;

msg.payload = JSON.stringify({
    "MessageType": "ua-data",
    "PublisherId": "Gateway-" + env.get("HOSTNAME"),
    "Messages": [{
        "DataSetWriterId": 1,
        "Payload": {}
    }]
});

// Parsear para agregar el campo dinamico
var obj = JSON.parse(msg.payload);
obj.Messages[0].Payload[nodeName] = {
    "Value": value,
    "SourceTimestamp": new Date().toISOString(),
    "StatusCode": 0
};
msg.payload = JSON.stringify(obj);

return msg;

3.3 "MQTT Out" node

• Server: MQTT broker URL (e.g. mqtt://broker.hivemq.com:1883)
• Topic: the topic configured in Rela-ai (e.g. plant-north/gateway-line1)
• QoS: 0

3.4 Connect

[OPC-UA Client] → [Function] → [MQTT Out]

Click Deploy (red button at the top right).

Step 4: Verify

Open a terminal and subscribe to the topic to check if messages are arriving:

mosquitto_sub -h broker.hivemq.com -t "plant-north/gateway-line1" -v

You should see JSON messages every time a value changes in the PLC.

Step 5: Configure Rela-ai

1. In the dashboard, create a source with protocol OPC UA → Via Gateway MQTT
2. Set the broker URL and topic you used in the MQTT Out node
3. Rela-ai starts receiving data automatically

Monitoring multiple nodes

To monitor several PLC nodes at once, add multiple "OPC-UA Client" nodes (one per variable) and connect them all to the same Function node. The Function node will accumulate the fields in the Payload.

Alternative: use a single OPC-UA Client node with Action "Subscribe" and configure multiple Node IDs separated by commas.

Tips

• Automatic reconnection: Node-RED reconnects automatically if the PLC or broker disconnects.
• Persistence: If the Raspberry Pi restarts, Node-RED starts automatically (if you enabled the service).
• Node-RED Dashboard: You can add a local dashboard in Node-RED to view data in real time from the plant floor.
• Security: If the PLC requires OPC UA authentication, configure the username/password in the OPC-UA Client node.