Modbus TCP/IP to MQTT Publisher in Python (JSON Payload)

Last Code Revision Date: 03-Sep-2025

Modbus TCP/IP to MQTT JSON Publisher

This project bridges Modbus TCP/IP devices and MQTT brokers, publishing Modbus register values as JSON payloads to main MQTT topics for each data type. It is built with the pymodbus and paho-mqtt libraries and is designed for flexible, custom use cases.

Library Version Details

Features

• Supports all Modbus data types: coils, discrete inputs, holding registers, input registers
• Per-address reading for maximum flexibility (handles consecutive and non-consecutive addresses)
• Publishes only when values change
• All values for each data type are published as a single JSON object to the main topic
• Configurable update interval
• Robust MQTT connection with auto-reconnect and status callbacks
• Easy configuration via config.py

Project Structure

modbus-mqtt-publisher
├── src
│   ├── main.py               # Main application logic
│   ├── modbus_client.py      # Modbus TCP client wrapper
│   ├── mqtt_publisher.py     # MQTT publisher with auto-reconnect
│   └── config.py             # User configuration for Modbus/MQTT/topics/interval
├── requirements.txt          # Python dependencies
├── README.md                 # Project documentation
└── .gitignore                # Version control ignore file

Installation

1. Clone the repository:

   git clone <repository-url>
   cd modbus-mqtt-publisher
   

2. Install dependencies:

   pip install -r requirements.txt
   

Configuration

Edit src/config.py to set:

• Modbus server IP and port
• MQTT broker IP and port
• Register types and addresses to read (can be any list)
• MQTT topics for each register type
• Update interval (in seconds)

Example:

MODBUS_READ_CONFIG = {
    "holding_registers": {
        "addresses": [0, 1, 2, 5, 6, 7],
        "mqtt_topic": "modbus/holding_registers"
    },
    "coils": {
        "addresses": [0, 2, 4],
        "mqtt_topic": "modbus/coils"
    }
}
UPDATE_INTERVAL = 5  # seconds

Usage

Start the publisher:

python src/main.py

• The app will read configured Modbus addresses at the set interval.
• Only changed values are published to MQTT.
• All values for each data type are sent as a single JSON object to the main topic.

Example MQTT payload for holding registers:

{
  "type": "holding_registers",
  "values": {
    "0": 123,
    "1": 456,
    "2": 789,
    "5": 101,
    "6": 202,
    "7": 303
  }
}

Testing

• Ensure your Modbus server and MQTT broker are running and accessible.
• Use any MQTT client to subscribe to main topics (e.g. modbus/holding_registers, modbus/coils) and verify published JSON payloads.
• Change Modbus register values to see updates on MQTT.

Advanced

• Supports all Modbus data types (add to MODBUS_READ_CONFIG as needed).
• MQTT connection is robust and auto-reconnects if dropped.
• Easily extend for logging, batch reads, or custom payload formats if required.