Simple Modbus Simulator

A simple Modbus TCP server simulator that reads configuration from a YAML file and serves defined coils, discrete inputs, holding registers, and input registers.

Installation

go build -o modbus-sim .

Usage

./modbus-sim [config.yaml]

If no config file is specified, it defaults to config.yaml.

Configuration

Example Config

server:
  url: "tcp://localhost:5502" # Server listen address
  timeout: 30 # Connection timeout in seconds
  max_clients: 5 # Max concurrent connections
  allow_undefined_registers: true # Return 0 for undefined addresses
devices:
  - name: "device1"
    modbus_id: 1
    data:
      coils:
        "0": true
        "1": false
        "10": true
      discrete_inputs:
        "0": true
        "1": false
        "100": true
      holding_registers:
        "0": 100
        "1": 200
        "100": 1234
      input_registers:
        "0": 1000
        "1": 2000
        "100": 5000
  - name: "device2"
    modbus_id: 2
    data:
      coils:
        "0": false
        "1": true
      holding_registers:
        "0": 1000
        "1": 2000
      input_registers:
        "0": 5000
        "1": 6000

Server Options

Option	Type	Default	Description
url	string	tcp://localhost:5502	Listen address
timeout	int	30	Connection timeout (seconds)
max_clients	int	5	Max concurrent connections
allow_undefined_registers	bool	false	Return 0 for undefined addresses

Device Options

Option	Type	Description
name	string	Device name (for display)
modbus_id	uint8	Modbus unit ID
data.coils	map	Coil addresses (bool)
data.discrete_inputs	map	Discrete input addresses (bool)
data.holding_registers	map	Holding register addresses (int)
data.input_registers	map	Input register addresses (int)

Testing

You can test the server using the modbus-cli tool:

# Build the CLI
go build -o modbus-cli github.com/simonvetter/modbus/cmd/modbus-cli

# Read coils
./modbus-cli --target tcp://localhost:5502 rc:0+10

# Read holding registers
./modbus-cli --target tcp://localhost:5502 rhr:0+10

# Read input registers
./modbus-cli --target tcp://localhost:5502 rir:0+10

# Read discrete inputs
./modbus-cli --target tcp://localhost:5502 rdi:0+10

# Write a coil
./modbus-cli --target tcp://localhost:5502 wc:0:true

# Write a holding register
./modbus-cli --target tcp://localhost:5502 wh:0:1234

Notes

• Address keys in the YAML are strings (e.g., "0", "100") to avoid issues with leading zeros
• Modbus addresses are 0-indexed in this simulator
• Multiple devices are supported with different unit IDs

TODOs

[ ] add logging