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PLC Communication using PyModbus

A Python project that communicates with a PLC using the PyModbus library over the Modbus ASCII protocol. This project demonstrates how to read from and write to PLC registers and coils using a high-level Modbus library, making industrial communication simpler and more maintainable.

Features

  • Communicate with PLCs using the Modbus ASCII protocol
  • Read Holding Registers
  • Read Coils
  • Write Single Coil
  • Write Single Register
  • Write Multiple Coils
  • Write Multiple Registers
  • Convert register values into Python data types
  • Configurable serial communication settings
  • Simple and well-documented code

Technologies Used

  • Python 3
  • PyModbus
  • Modbus ASCII Protocol

Project Structure

PLC-Communication-PyModbus/
│
├── PLC Communication PyModbus.py
├── requirements.txt
├── README.md
└── LICENSE

Requirements

Install the required packages:

pip install pymodbus pyserial

or

pip install -r requirements.txt

Hardware Requirements

  • PLC supporting the Modbus ASCII protocol
  • USB-to-Serial (RS-232 / RS-485) converter (if required)
  • Python 3

Serial Configuration

The project uses the following serial settings:

Parameter Value
Protocol Modbus ASCII
Baud Rate 9600
Data Bits 7
Parity Even
Stop Bits 1
Timeout 1 second

Adjust these settings according to your PLC configuration.

Supported Functions

Read Operations

  • Read Coils (Function Code 01)
  • Read Holding Registers (Function Code 03)

Write Operations

  • Write Single Coil (Function Code 05)
  • Write Single Register (Function Code 06)
  • Write Multiple Coils (Function Code 15)
  • Write Multiple Registers (Function Code 16)

Usage

Run the program:

python main.py

Example:

read_data(function=READ_HOLDING_REGISTERS, address=0x100A, count=2)

send_data(function=WRITE_SINGLE_REGISTER,
          address=0x1000,
          value=1)

Example Output

connect to server

get and verify data

Got int32: 12345

close connection

Project Workflow

The program performs the following steps:

  1. Connect to the PLC.
  2. Send a Modbus request.
  3. Receive and verify the response.
  4. Convert register data into Python data types (when applicable).
  5. Display the received data.
  6. Close the serial connection.

Related Project

This project uses the PyModbus library for Modbus communication.

If you are interested in learning how the Modbus ASCII protocol works internally, including manual frame construction, LRC checksum calculation, serial communication, and response parsing without using external libraries, check out my companion project:

PLC Communication – Manual Modbus ASCII

https://github.com/Matin-python/PLC-Communication-Manual-Modbus-ASCII

Future Improvements

  • Support additional Modbus function codes
  • Automatic serial port detection
  • Communication retry mechanism
  • Better exception handling
  • Logging communication to a file
  • Graphical User Interface (GUI)
  • Support for Modbus RTU and Modbus TCP

License

This project is licensed under the MIT License.

Author

Mohammad Reza Bakhshandeh

Electrical Engineering (Electronics) Graduate

Interested in Industrial Automation, Embedded Systems, PLC Programming, Python Development, Computer Vision, Machine Learning, and Artificial Intelligence.