modbus-slave/README.md

Modbus Slave Simulator

A configurable Modbus slave simulator for testing Modbus TCP, RTU, and TLS connections.

Features

• Multiple Protocols: TCP, RTU (serial), and TLS support
• Configurable Data Store: Customisable sizes for coils, discrete inputs, holding registers, and input registers
• Initial Values: Set default values for all data types
• Device Identity: Configurable vendor name, product code, model, and version
• Docker Support: Ready-to-use Docker image

Installation

From Source

git clone <repository-url>
cd modbus-slave
pip install -r requirements.txt

With Docker

docker pull <username>/modbus-slave

Usage

Command Line

# TCP server (default: 127.0.0.1:5020)
python main.py

# Custom port
python main.py --port 5020

# RTU mode
python main.py --type rtu --serial-port /dev/ttyUSB0 --slave-id 2

# Custom data store sizes
python main.py --holding-registers-size 1000 --input-registers-size 500

# Set initial values
python main.py --holding-registers-default 100 --coils-default true

Docker

# Run with defaults
docker run -p 5020:5020 <username>/modbus-slave

# Run with custom arguments
docker run -p 5020:5020 <username>/modbus-slave --holding-registers-size 1000

Command Line Options

Option	Description	Default
-t, --type	Server type (tcp, rtu, tls)	tcp
-h, --host	TCP bind host	127.0.0.1
-p, --port	TCP/TLS port	5020
-s, --slave-id	Modbus slave ID	1
--serial-port	Serial port (RTU)	/dev/ttyUSB0
--baudrate	Serial baudrate	9600
--coils-size	Number of coils	100
--holding-registers-size	Number of holding registers	100
--input-registers-size	Number of input registers	100
--holding-registers-default	Default value for holding registers	0
-v, --verbose	Increase verbosity	-

Modbus Data Model

Function Code	Data Type	Access
0x01/0x05/0x0F	Coil	Read/Write
0x02	Discrete Input	Read Only
0x03/0x06/0x10	Holding Register	Read/Write
0x04	Input Register	Read Only

Testing

Test with pymodbus client:

from pymodbus.client import ModbusTcpClient

client = ModbusTcpClient('127.0.0.1', port=5020)
result = client.read_holding_registers(0, 10)
print(result.registers)
client.close()

Licence

See LICENCE file.