IIoT Gateway with Raspberry Pi – Bridging Protocols

In this course, you’ll learn how to turn a Raspberry Pi into an IIoT gateway capable of translating Modbus data to MQTT and vice versa.

Requirements

• Raspberry Pi with Raspbian Lite (SSH enabled), Docker, and Node-RED.
• Python 3.7+ with pymodbus and paho-mqtt.

Installing Node-RED

docker run -d --name nodered -p 1880:1880 nodered/node-red

Access http://<RPi-IP>:1880 to design flows.

Creating Modbus → MQTT Flows

1. Add Modbus (read/write) and MQTT nodes in Node-RED.
2. Configure the Modbus host (IP/serial) and the MQTT broker.
3. Design a flow that:
   • Reads registers from a Modbus simulator and publishes every X seconds to an MQTT topic.
   • Receives MQTT commands and writes them to the Modbus device.

Python Gateway (gateway.py)

from pymodbus.client.sync import ModbusTcpClient
from paho.mqtt.client import Client

# Configuration
tcp = ModbusTcpClient('192.168.0.20')
mqtt = Client()
mqtt.connect('broker.local')

# MQTT callback
def on_message(client, userdata, msg):
    # Parse payload and write to Modbus
    tcp.write_register(10, int(msg.payload))

mqtt.on_message = on_message
mqtt.subscribe('plant/area1/command')

# Main loop
while True:
    rr = tcp.read_holding_registers(0, 5)
    mqtt.publish('plant/area1/data', rr.registers)
    mqtt.loop()

Deployment with Systemd

[Unit]
Description=IIoT Gateway Service
After=network.target

[Service]
ExecStart=/usr/bin/python3 /home/pi/gateway.py
Restart=always
User=pi

[Install]
WantedBy=multi-user.target

Enable and start:

sudo systemctl enable gateway.service
sudo systemctl start gateway.service

Security

• Configure UFW to restrict ports (1880, 502, 1883).
• Use TLS on the MQTT broker and in Python (mqtt.tls_set()).
• Limit access via ACLs in Node-RED.

In the next post, we’ll connect Siemens S7 PLCs to the cloud using this gateway.