Newtonian Physics Mockumentary


    0.2.4 • Public • Published


    This is a fork of thekip/node-modbus-rtu to handle Modbus RTU like protocol over TCP Sockets

    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-tcp --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 or timeouted. 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

    import net from 'net';
    import Reconnect from 'node-net-reconnect';
    import { ModbusMaster } from './../src/';
    const socket = net.Socket();
    const options = {
      host: '',
      port: 5008,
      retryTime: 1000, // 1s for every retry
      retryAlways: true, // retry even if the connection was closed on purpose
    const recon = new Reconnect(socket, options);
    const modbusMaster = new ModbusMaster(socket, {
      debug: true,
    setInterval(() => {
      modbusMaster.readHoldingRegisters(1, 107, 2);
    }, 1000);
    // Modbus starts automatically on "connect" event


    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(socket, [options])

    Constructor of modbus class.

    • socket - instance of net socket object
    • options - object with Modbus options

    List of options:

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


    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


    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


    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


    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.


    npm i modbus-rtu-tcp

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