Modbus TCP Server

A TypeScript implementation of a Modbus TCP server for industrial automation and IoT applications.

Features

• Full Modbus TCP Protocol Support: Implements all standard Modbus TCP function codes
• TypeScript: Fully typed for better development experience
• Memory Management: Efficient data store for holding registers, input registers, coils, and discrete inputs
• Configurable: Flexible configuration for ports, address ranges, and limits
• Error Handling: Comprehensive error handling with proper Modbus exception codes
• Testing: Complete test suite with Jest
• Cross-Platform: Works on Windows, Linux, and macOS

Supported Function Codes

• 0x01 - Read Coils
• 0x02 - Read Discrete Inputs
• 0x03 - Read Holding Registers
• 0x04 - Read Input Registers
• 0x05 - Write Single Coil
• 0x06 - Write Single Register
• 0x0F - Write Multiple Coils
• 0x10 - Write Multiple Registers

Installation

npm install

Usage

Basic Server

import { ModbusTCPServer } from 'modbus-tcp-server';

// Simple usage with defaults
const server = new ModbusTCPServer();

// Or customize specific settings
const server = new ModbusTCPServer({
  port: 5020,                    // Custom port
  minRegisterAddress: 40001,     // Start at standard holding register range
  maxRegisterAddress: 40100      // Limit to 100 registers
});

// Start the server
await server.start();

// Access the data store
const dataStore = server.getDataStore();

// Set some data
dataStore.setHoldingRegister(100, 1234);
dataStore.setCoil(200, true);

// Read data directly from server
console.log('Holding Register 100:', server.getHoldingRegisterValue(100));
console.log('Coil 200:', server.getCoilValue(200));
console.log('All data:', server.getAllData());

// Stop the server
await server.stop();

Reading Data

The server provides convenient methods to read data directly:

// Read individual values
const holdingValue = server.getHoldingRegisterValue(100);
const inputValue = server.getInputRegisterValue(200);
const coilValue = server.getCoilValue(300);
const discreteValue = server.getDiscreteInputValue(400);

// Read ranges
const holdingRange = server.getHoldingRegisterValues(100, 5); // Read 5 registers starting at 100
const inputRange = server.getInputRegisterValues(200, 3); // Read 3 registers starting at 200

// Get all data
const allData = server.getAllData();

// Get data summary
const summary = server.getDataSummary();
console.log(`Holding: ${summary.holdingRegisters}, Input: ${summary.inputRegisters}`);

Command Line Usage

# Start with default settings (port 502)
npm start

# Start with custom port
npm start -- --port=5020

# Start with custom address range
npm start -- --min-address=50000 --max-address=54000

# Show help
npm start -- --help

Default Configuration

The server comes with sensible defaults:

const DEFAULT_CONFIG = {
  port: 502,                    // Standard Modbus TCP port
  host: '0.0.0.0',             // Listen on all interfaces
  unitId: 1,                   // Default unit ID
  maxHoldingRegisters: 10000,  // Memory limits
  maxInputRegisters: 10000,
  maxCoils: 10000,
  maxDiscreteInputs: 10000,
  minRegisterAddress: 1,       // Address range
  maxRegisterAddress: 65535
};

Configuration Options

| Option | Environment Variable | Default | Description | |--------|---------------------|---------|-------------| | --port | MODBUS_PORT | 502 | Server port | | --host | MODBUS_HOST | 0.0.0.0 | Server host | | --unit | MODBUS_UNIT_ID | 1 | Unit ID | | --min-address | MIN_REGISTER_ADDRESS | 50000 | Minimum register address | | --max-address | MAX_REGISTER_ADDRESS | 54000 | Maximum register address | | --max-holding | MAX_HOLDING_REGISTERS | 10000 | Max holding registers | | --max-input | MAX_INPUT_REGISTERS | 10000 | Max input registers | | --max-coils | MAX_COILS | 10000 | Max coils | | --max-discrete | MAX_DISCRETE_INPUTS | 10000 | Max discrete inputs |

Development

Build

npm run build

Development Mode

npm run dev

Testing

# Run tests
npm test

# Run tests in watch mode
npm run test:watch

# Run tests with coverage
npm run test:coverage

Linting

# Check for linting errors
npm run lint

# Fix linting errors
npm run lint:fix

# Format code
npm run format

API Reference

ModbusTCPServer

Main server class for handling Modbus TCP connections.

Constructor

constructor(config: ModbusServerConfig)

Methods

• start(): Promise<void> - Start the server
• stop(): Promise<void> - Stop the server
• getDataStore(): ModbusDataStore - Get the data store instance

ModbusDataStore

Data store for managing Modbus registers and coils.

Holding Registers (Read/Write)

• setHoldingRegister(address: number, value: number): void
• getHoldingRegister(address: number): number
• setHoldingRegisters(startAddress: number, values: number[]): void
• getHoldingRegisters(startAddress: number, quantity: number): number[]

Input Registers (Read Only)

• setInputRegister(address: number, value: number): void
• getInputRegister(address: number): number
• getInputRegisters(startAddress: number, quantity: number): number[]

Coils (Read/Write)

• setCoil(address: number, value: boolean): void
• getCoil(address: number): boolean
• setCoils(startAddress: number, values: boolean[]): void
• getCoils(startAddress: number, quantity: number): boolean[]

Discrete Inputs (Read Only)

• setDiscreteInput(address: number, value: boolean): void
• getDiscreteInput(address: number): boolean
• getDiscreteInputs(startAddress: number, quantity: number): boolean[]

Utility Methods

• getAllRegisters() - Get all register data
• clearAllData() - Clear all data

Examples

Multiple Server Instances

For applications requiring multiple Modbus servers (e.g., different inverters):

# Terminal 1: First inverter
npm start -- --port=5020 --min-address=50000 --max-address=52000

# Terminal 2: Second inverter  
npm start -- --port=5021 --min-address=52001 --max-address=54000

Environment Variables

export MODBUS_PORT=5020
export MODBUS_HOST=127.0.0.1
export MIN_REGISTER_ADDRESS=50000
export MAX_REGISTER_ADDRESS=54000
npm start

License

MIT

Contributing

1. Fork the repository
2. Create a feature branch
3. Make your changes
4. Add tests
5. Submit a pull request

Support

For issues and questions, please use the GitHub issue tracker.