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Implementation of the Modbus TCP/ASCII/RTU master for node.js. This module is based on h5.modbus, and contains extensions and improvements for use with Control Solutions MODBUS products.

It can be used by itself - the utilities implement basic MODBUS interactions and test features - or as a library module to develop product-specific extensions.


Install nodejs for your platform ( This will make the node and npm executables available.

Using the Utility Programs

Copy or clone the repository to a new folder (e.g via Github .zip file download or cloning).

Navigate to the folder that contains the package.json file. The instructions/paths below are relative to this folder. Forward-slash (Unix/Mac) file paths are used; Windows users should adjust to backslash\file\paths as necessary.

Use npm install to install dependent packages.

Configuration for Utility Programs

Configuration (for example, how the programs communicate with the target devices) can be controlled in multiple ways. These are listed in order of priority (command line options have precedence over environment variables have precedence over config.json)

  • Editing the config.json file
  • Setting operating system environment variables(s). For example, if MODBUS_PORT=COM1 is set as an environment variable, it will be used instead of a port specified in config.json.
  • Using command line options: running the utility programs with the -h option will list the available command line options for that utility.


The config.json must be a properly-formatted JSON file:

  "port" : {
    "name" : "COM1",
    "options" : {
      "baudrate": 19200,
      "rts": false,
      "dtr" : false
  "master" : {
      "transport": {
        "type": "RTU",
        "slaveId" : 127,
        "connection": {
            "type": "serial",
            "serialPort": null
      "suppressTransactionErrors": true,
      "retryOnException": false,
      "maxConcurrentRequests": 1,
      "defaultUnit": 1,
      "defaultMaxRetries": 1,
      "defaultTimeout": 1000


The "port" object is used to configure a serial port connection. It has the following settings:

  • name the operating system name for the hardware serial port. To find the available ports, check your operating system device manager, or use the node examples/ports.js example program to list available ports.
  • options control how the port operates, and are taken from the options offered by the nodejs serialport module:
    • baudrate: defaults to 9600. Should be one of: 115200, 57600, 38400, 19200, 9600, 4800, 2400, 1800, 1200, 600, 300, 200, 150, 134, 110, 75, or 50. Custom rates as allowed by hardware is supported.
  • dataBits: Data Bits, defaults to 8. Must be one of: 8, 7, 6, or 5.
  • stopBits: Stop Bits, defaults to 1. Must be one of: 1 or 2.
  • parity: Parity, defaults to 'none'. Must be one of: 'none', 'even', 'mark', 'odd', 'space'

The "websocket" object controls the behavior of a "websocket" type connection. It has the following settings:

  • url the websocket URL to connect
  • reconnection whether to reconnect automatically (true)
  • reconnectionAttempts (Infinity) before giving up
  • reconnectionDelay how long to initially wait before attempting a new reconnection (1000). Affected by +/- randomizationFactor, for example the default initial delay will be between 500 to 1500ms.
  • reconnectionDelayMax maximum amount of time to wait between reconnections (5000). Each attempt increases the reconnection delay by 2x along with a randomization as above randomizationFactor (0.5), 0 <= randomizationFactor <= 1
  • timeout connection timeout before a connect_error and connect_timeout events are emitted (20000)

The "master" object controls the behavior of the MODBUS master. It has the following options:

  • transport: Determines how messages will be framed and encoded for transport over the connection. rtu for (standard) MODBUS RTU, ascii for (standard) MODBUS ASCII, ip for (standard) MODBUS TCP/IP, and tunnel for (Control Solutions proprietary) tunneling of MODBUS commands over an RTU-based network. Using tunnel allows cs-modbus to act like a master, even as it participates on the physical network as a slave. This setting requires the 'real' MODBUS master to implement SLAVE_COMMAND polling, as defined in CS document DOC0003824A-SRS-A ** slaveId is only used for the tunnel transport; it defines the slave address that cs-modbus will monitor for SLAVE_COMMAND messages. ** eofTimeout: the timeout in milliseconds used to detect the end-of-frame in RTU and tunnel connections. These transports do not have an explicit end of message indicator; it is provided by measuring idle time on the bus. This value should be at least 3.5 character times, at the chosen baud rate. For example, at 19200 this value should be about 20. ** connection defines the physical connection that will be used to communicate with the MODBUS network. *** type: The types of supported connections are: tcp, udp, serial. *** serialPort: (required for serial connections). Set to an instance of the node-serialport module. *** TCP and UDP connections require additional parameters that are not detailed here; refer to lib/connnections/TcpConnection.js and lib/connections/UdpConnection.js for additional details.

  • suppressTransactionErrors: (boolean) determines whether errors detected at the transaction level will throw exceptions (which must be caught by the application code) or not.

  • retryOnException: (boolean) determines whether the master will retry the message if the slave returns an exception code, or simply fail the message.

  • maxConcurrentRequests: (integer) determines how many transactions may be attempted simultaneously. This should be '1' for serial connections using RTU or ASCII transport. A value of '2' provides an efficiency boost for TUNNEL transport over serial. TCP and UDP connections can support a higher number of simultaneous transactions. Note: the application may submit multiple requests to the master without concern for this maximum; additional requests will simply be queued until the connection is able to accept them.

  • defaultUnit: (integer): the default MODBUS unit identifier to transmit messages to. Can be overridden on a message-by-message basis.

  • defaultMaxRetries: (integer) the number of times to retry an unsuccessful transaction before failing it. Can be overridden on a message-by-message basis

  • defaultTimeout: (integer) the number of milliseconds to wait for a response from the slave. This can be tweaked to maximize performance of a given system depending on the connection speed, etc. Can be overridden on a message-by-message basis.

mb Utility

The mb utility allows simple MODBUS interactions with an attached slave. After configuring the connection, run the mb utility. node mb -h shows the available command line options node mb read holding 0 3 reads three registers starting at address 0 node mb read holding 0 3 -v reads the same three registers and outputs verbose diagnostic information, including the exact bytes transmitted and received over the link.

If the cs-modbus package is installed 'globally': npm install csllc/cs-modbus.git -g the mb utility will be available at any terminal prompt, regardless of folder. In this case, it is simplest to use the environment variable configurations rather than edit the config.json file. For example: set MODBUS_PORT=COM1 set MODBUS_SLAVE=10 set MODBUS_BAUD=19200 (notice 'set' is used on Windows platforms, 'export' is the Mac equivalent)

Using cs-modbus to build an application

Create a new folder and navigate there in a command prompt.

Add the MODBUS module to your nodejs project npm install csllc/cs-modbus.git

If you intend to use a serial-port based MODBUS connection, you need npm install serialport

Basic Use

Create a new file (demo.js) in your project folder and insert the following into it:

// Include the module
var modbus = require('cs-modbus');

// Include the serial port handler, and open /dev/ttyAMA0
// (replace the port name with an appropriate one for your
// system)
var SerialPort = require('serialport').SerialPort;
var serialPort = new SerialPort('/dev/ttyAMA0', {
  baudRate: 9600

// Configure the master
// In this case we set up for MODBUS-RTU over the serial port
// we just declared.
var master = modbus.createMaster({
  transport: {
    type: 'rtu',
    connection: {
      type: 'serial',
      serialPort: serialPort

// When the master is initialized..
master.once('connected', function()
  // Read a set of discrete inputs from the slave device with address 1
  // (the parameters of this command will depend on what
  // kind of slave you are connected to)
  var t1 = master.readDiscreteInputs(0x0000, 8, {
    unit: 1

    // The following hooks the transaction complete event, and prints out the result
  t1.on('complete', function(err, response)
    if (err)
      console.error('[Error] %s', err.message);

The examples or utility programs may be helpful in understanding how to interface to the library.

In order to run the examples, refer to the configuration instructions above (eg config.json)

Ports lists all of the serial ports present on the system - which may help identify the correct port to use for the connection.

Inspect USB is a straightforward approach to opening a serial port connection and querying the device's ID information (ReportSlaveId message). The various events are hooked to show the progression of a typical message.


Clone or fork the repository, and edit the files to make the necessary changes.

Units tests are provided and should be updated for any code changes: npm test

The tests include linting the code with JSHINT, running all unit tests, and producing test coverage reports.


The module does not at this point use a consistent code style; please follow the convention in the file you are editing, and/or the style as enforced by JSHINT.

Test Coverage

npm test includes generation of code coverage reports. To view them, review the build/coverage/lcov-report/index.html file.


This project is released under the MIT License.


Implementation of the Modbus TCP/ASCII/RTU master for node.js.







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