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0.2.1 • Public • Published

node-modbus-rtu Build Status

Pure NodeJS implementation of ModbusRTU protocol using node-serialport and Bluebird promises

Implementation notes

This library implement ONLY ModbusRTU Master and only most important features:

  • 03 Read Holding Registers
  • 06 Write Single Register
  • 16 Write Multiple Registers

Coil functions (readCoils, writeCoils) is not implemented yet. But you can fork and add this.

Minimal requirements

NodeJS >=5

if you have older NodeJS version, you should install modbus-rtu@0.1.* version or update NodeJS (the 8.0 version is out, how long you will be use legacy builds? :) )


The simplest way, install via npm, type to console:

npm i modbus-rtu serialport --save


  1. Queue. This is a killer-feature of this library. Behind the scene it use a simple queue. All request what you do stack to this queue and execute only if previous was finished. It means that using this library you can write to modbus without waiting a response of previously command. That make you code much cleaner and decoupled. See examples below.

  2. Promises Promises is a great pattern for the last time. Promises make async code more clean and readable. All communication functions return promises, so you can easily process data or catch exceptions.


The basic example

const SerialPort = require('serialport');
const ModbusMaster = require('modbus-rtu').ModbusMaster;
//create serail port with params. Refer to node-serialport for documentation
const serialPort = new SerialPort("/dev/ttyUSB0", {
   baudRate: 2400
//create ModbusMaster instance and pass the serial port object
const master = new ModbusMaster(serialPort);
//Read from slave with address 1 four holding registers starting from 0.
master.readHoldingRegisters(1, 0, 4).then((data) => {
    //promise will be fulfilled with parsed data
    console.log(data); //output will be [10, 100, 110, 50] (numbers just for example)
}, (err) => {
    //or will be rejected with error
//Write to first slave into second register value 150.
//slave, register, value
master.writeSingleRegister(1, 2, 150).then(success, error);


Queue turn this:

//  requests
master.readHoldingRegisters(1, 0, 4).then((data) => {
    master.readHoldingRegisters(2, 0, 4).then((data) => {
        master.readHoldingRegisters(2, 0, 4).then((data) => {
            master.readHoldingRegisters(2, 0, 4).then((data) => {

Into this:

master.readHoldingRegisters(1, 0, 4).then((data) => {
master.readHoldingRegisters(2, 0, 4).then((data) => {
master.readHoldingRegisters(3, 0, 4).then((data) => {
master.readHoldingRegisters(4, 0, 4).then((data) => {

This makes possible to write code in synchronous style.

Check more examples in /examples folder in repository.

The main problem

Communicating via serial port is sequential. It means you can't write few requests and then read few responses.

You have to write request then wait response and then writing another request, one by one.

The first problem is, if we call functions in script in synchronous style (one by one without callbacks), they will write to port immediately. As result response from slaves will returns unordered and we receive trash.

To deal with this problem all request instead of directly writing to port are put to the queue, and promise is returned.

API Documentation

new ModbusMaster(serialPort, [options])

Constructor of modbus class.

  • serialPort - instance of serialPort object
  • options - object with Modbus options

List of options:

  • responseTimeout: default 500
  • debug: default false; enable logging to console.


new ModbusMaster(new SerialPort("/dev/ttyUSB0", {
    baudRate: 9600


readHoldingRegisters<T>(slaveint, startint, lengthint, [dataType = DATA_TYPES.INT])Promise<T[]>;

Modbus function read holding registers.

Modbus holding register can store only 16-bit data types, but specification does'nt define exactly what data type can be stored.

Registers could be combined together to form any of these 32-bit data types:

  • A 32-bit unsigned integer (a number between 0 and 4,294,967,295)
  • A 32-bit signed integer (a number between -2,147,483,648 and 2,147,483,647)
  • A 32-bit single precision IEEE floating point number.
  • A four character ASCII string (4 typed letters)

More registers can be combined to form longer ASCII strings.
Each register being used to store two ASCII characters (two bytes).

To parse this combined data types, you can get raw buffer in callback and parse it on your own.

By default bytes treated as signed integer.

Supported Data Types

  • DATA_TYPES.UINT A 16-bit unsigned integer (a whole number between 0 and 65535)
  • DATA_TYPES.INT A 16-bit signed integer (a whole number between -32768 and 32767)
  • DATA_TYPES.ASCII A two character ASCII string (2 typed letters)

List of function arguments:

  • slave - slave address (1..247)
  • start - start register for reading
  • length - how many registers to read
  • dataType - dataType or function. If function is provided, this will be used for parsing raw buffer. dataType is one of DATA_TYPES

Returns Promise<T[]> which will be fulfilled with array of data


const {ModbusMaster, DATA_TYPES} = require('modbus-rtu');
const master = new ModbusMaster(serialPort);
master.readHoldingRegisters(1, 0, 4).then((data) => {
    //promise will be fulfilled with parsed data
    console.log(data); //output will be [-10, 100, 110, 50] (numbers just for example)
}, (err) => {
    //or will be rejected with error
    //for example timeout error or crc.
master.readHoldingRegisters(1, 0, 4, DATA_TYPES.UINT).then((data) => {
    // data will be treat as unsigned integer
    console.log(data); //output will be [20, 100, 110, 50] (numbers just for example)
master.readHoldingRegisters(1, 0, 2, (rawBuffer) => {
    //buffer here contains only data without pdu header and crc
    return rawBuffer.readUInt32BE(0);
}).then((bigNumber) => {
    //promise will be fullfilled with result of callback
    console.log(bigNumber); //2923517522


writeSingleRegister(slaveint, registerint, valueint, [retryCount=10]) -> Promise<void>

Modbus function write single register. If fails will be repeated retryCount times.

  • slave - slave address (1..247)
  • register - register number for write
  • value - int value
  • retryCount - int count of attempts. Set 1, if you don't want to retry request on fail.

Returns Promise


const master = new ModbusMaster(serialPort);
master.writeSingleRegister(1, 2, 150);


writeMultipleRegisters(slaveint, startint, array[int]) -> Promise<void>

Modbus function write multiple registers.

You can set starting register and data array. Register from start to array.length will be filled with array data

  • slave - slave address (1..247)
  • start - starting register number for write
  • array - array of values

Returns promise


new ModbusMaster(serialPort, (master) => {
  master.writeMultipleRegisters(1, 2, [150, 100, 20]);


To run test, type to console:

npm test

Or run manually entire test (by executing test file via node).

Please feel free to create PR with you tests.


  1. Add rest modbus functions


npm i modbus-rtu

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