MQTT_to_ModBUS_bridge

MQTT to ModBUS bridge

[TOC]

1.Introduction

This guide shows a NodeRED flow for a Modbus RTU (serial) to MQTT bidirectional bridge.

2. Preparation

2.1. Hardware

2.1.1. Master device

• RAK7391 WisGate Developer Connect + WisBlock IO RAK5802

In this example, we use a RAK7391 board to interface RAK5802. there are two WisBlock IO Connecter on the RAK7391 already, you can connect RAK5802 with any one of them.

2.1.2. Slave Device

In this example we will first create a Modbus sensor (a.k.a. master or sender) using a WisBlock Starting Kit with a RAK1901 Temperature and Humidity Sensor and a RAK5802 IO Module.

• WisBlock Starter Kit (WisBlock Base RAK5005-O + WisBlock Core RAK4631) * 1

• RAK5802 * 1

  

• WisBlock Sensor RAK1901 * 1

2.1.3. Connection diagram

We are going to connect the RAK5812 modules on both sides (WisBlock and RAK7391) this way: A to A, B to B and share the ground.

WisBlock	RAK7391
A	A
B	B
GND	GND

2.3. Software

2.3.1. Slave device

The code can be found under the rak5802_modbus_device folder. The example is

You can open it directly with the Arduino IDE but you will first have to have it installed as well as the RAK6430 BSP. Check the RAK4631 Quick Start Guide to know more.

• ArduinoIDE
• RAK4630 BSP

At the top of the example sketch you have links to install the required libraries using the Arduino IDE Library Manager.

• Arduino RS485 library
• ArduinoModbus library
• Sparkfun SHTC3 Humidity and Temperature Sensor Library

Then compile and upload the sketch.

2.3.2. Master device

Create a docker compose file with the following content.

version: '3.7'

services:

  nodered:
    image: nodered/node-red:latest
    container_name: NodeRed
    user: node-red
    restart: unless-stopped
    group_add:
      - dialout
    devices:
      - "/dev/ttyUSB0:/dev/ttyUSB0"
      - "/dev/ttyUSB1:/dev/ttyUSB1"
    volumes:
      - 'node-red-data:/data'
    ports:
      - "1880:1880"

volumes:
  node-red-data:

Run docker compose up at the same directory with docker compose file to start NodeRED container. You can then browse to http://{host-ip}:1880 to get the familiar Node-RED web interface.

2.3.3. Flow dependencies

The NodeRED flow we are going to use has two dependecies:

• @mrcamilletti/node-red-contrib-serial-modbus-api
• node-red-contrib-aedes

You can install them from the NodeRED web UI on the Manage Palette option of the main menu or using the command line:

docker exec -it NodeRed @mrcamilletti/node-red-contrib-serial-modbus-api
docker exec -it NodeRed node-red-contrib-aedes
docker restart NodeRed

Remember to reboot your container after the nodes installation is completed.

3. Run example

3.1. Import flow and deploy

After all the preparation is finished, you can import the flow now, the new flow should look like this:

There are three part in this flow:

3.1.1 MQTT broker

Aedes MQTT broker is a internal mqtt broker in this flow.

3.1.2 Polling

Polling section publish topics to get LED status, temperature data and humidity data.

3.1.3 Subscribe topic and deal with get/set commands

This section subscribe mqtt topics and deal with get/set commands. For example, set LED ON or OFF, get temperature or humidity data, get configuration.

In the connection diagram provided above, one of the RAK5802 ModBUS module is mounted to WisBlock slot#1 on RAK7391, thus in the modebus api request node, we defined the Modbus server to /dev/ttyUSB0:9600. If the RAK5802 ModBUS module is mounted to WisBlock slot#2 on RAK7391, please change it to /dev/ttyUSB1:9600.

After import and deploy the flow, we can find polling result in the debug window. The payloads are all raw data. For example, in the following figure, you will find number 51, 0, and 3005, which means the humidity is 51%, the LED is OFF, and the temperature is 30.05 ℃.

3.2. Use mqtt client to publish and subscribe topic

The topics the flow supports are as followings.

Topic	Action
bridge/<device name>/<register name>/status	Polled value for the given register
bridge/<device name>/<register name>/write XXX	Write value to the given registry
bridge/<device name>/<register name>/read	Force reading the given registry
bridge/config	Returns current configuration

The <device name> and <register name> are defined in configuration json object which is in Mqtt2ModbusRequestApi and ModbusRequestApi2Mqtt node of flow.

var config = {
  "devices": [
    {
        "id": 42, "name": "LED01",  "type": "coil", "address": 0, "quantity": 1
    },
    {
        "id": 42, "name": "sensor01", "type": "holding", 'quantity': 2,
        "registries": [
          { 'name': 'humidity', 'address': 0, 'polling': 60 },
          { 'name': 'temperature', 'address': 1, 'polling': 60 }
        ]
    }
  ]
}

We use mosquitto_sub and mosquitto_pub to subscribe and publish mqtt topic. Please install them before

sudo apt install mosquitto-dev
sudo apt install mosquitto-clients

Then we can test mqtt topics.

For example, use the command bellow to control LED on. 172.23.0.2 should be replaced with the ip of mqtt broker. In this example we deployed NodeRED in container, so the ip is container's ip.

mosquitto_pub -h 172.23.0.2 -t bridge/LED01/write -m 1

Use the command bellow to control LED off.

mosquitto_pub -h 172.23.0.2 -t bridge/LED01/write -m 0

And we can also subscribe topics. For example , use the command bellow to subscribe temperature.

mosquitto_sub -h 172.23.0.2 -t bridge/sensor01/temperature/status

Then you can receive the temperature data 60 seconds later. If you want receive it immediately, you can run mosquitto_sub -h 172.23.0.2 -t bridge/sensor01/temperature/read -m "" at another terminal.

4. License

This project is licensed under MIT license.