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

Modbus TCP/IP to MQTT Publisher

This project bridges Modbus TCP/IP devices and MQTT brokers, publishing Modbus register values as raw payloads to individual MQTT topics. It uses pymodbus and paho-mqtt for Modbus and MQTT communication.

Library Version Details

Features

• Reads Modbus holding registers (default configuration)
• Per-address reading for flexibility (supports non-consecutive addresses)
• Publishes only when values change
• Each register/address is published to its own MQTT topic
• Configurable update interval
• Robust MQTT connection with auto-reconnect and status callbacks
• Easy configuration via src/config.py

Project Structure

modbus-mqtt-publisher-01092025
├── 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

Installation

1. Clone the repository:

   git clone <repository-url>
   cd modbus-mqtt-publisher-01092025
   

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 (currently only holding registers enabled)
• MQTT topics for each register type
• Update interval (in seconds)

Example (src/config.py):

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

To enable other data types (coils, discrete inputs, input registers), uncomment and configure them in MODBUS_READ_CONFIG.

Usage

Start the publisher:

python src/main.py

• The app reads configured Modbus addresses at the set interval.
• Only changed values are published to MQTT.
• Each value is sent to its own topic, e.g. modbus/holding_registers/2.

Testing

• Ensure your Modbus server and MQTT broker are running and accessible.
• Use any MQTT client to subscribe to topics and verify published values.
• 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 JSON payloads, logging, or batch reads if required.