jsmodbus Documentation

You can install jsmodbus as a local dependency for your project or globally to use its Command Line Interface (CLI).

# Install as a local dependency
npm install jsmodbus

# Install globally to use the CLI
npm install -g jsmodbus

The ModbusAbstractRequest is an abstract base class used to define the structure of Modbus requests. While you typically use concrete implementations provided by the library, understanding this interface is useful when implementing custom request types.

Key properties available on a request include:

• body: The Modbus function code and its specific parameters (of type ModbusRequestBody).
• unitId, slaveId, and address: These are aliases for the same value representing the target device ID (type number).
• byteCount: The calculated number of bytes in the request payload.
• createPayload(): An abstract method that returns a Buffer representing the serialized Modbus request.

The library uses the debug module for internal logging. To view debugging information, set the DEBUG environment variable. To see all debug output, use *.

export DEBUG=*

To implement a Modbus TCP server, wrap a standard Node.js net.Server instance with modbus.server.TCP.

const modbus = require('jsmodbus')
const net = require('net')
const netServer = new net.Server()
const server = new modbus.server.TCP(netServer)

netServer.listen(502)

To create a TCP client, instantiate Modbus.client.TCP by passing a Node.js net.Socket instance and a unitId.

const Modbus = require('jsmodbus')
const net = require('net')
const socket = new net.Socket()
const unitId = 1 // Replace with your unit ID
const client = new Modbus.client.TCP(socket, unitId)

const options = {
  'host' : '127.0.0.1',
  'port' : 502
}

socket.connect(options)

To create an RTU client, instantiate Modbus.client.RTU by passing a serialport instance and an address.

const Modbus = require('jsmodbus')
const SerialPort = require('serialport')

const options = {
  baudRate: 57600
}

const address = 1 // Replace with your unit address
const socket = new SerialPort("/dev/tty-usbserial1", options)
const client = new Modbus.client.RTU(socket, address)

The client API uses Promises for Modbus operations. When calling readCoils(offset, count), the response object contains the data in resp.response.body.coils (an Array) and resp.response.body.payload (a Buffer).

// Assuming socket and client are already initialized
// For serialport, use socket.on('open', ...) instead of 'connect'
socket.on('connect', function () {
  client.readCoils(0, 13).then(function (resp) {
    // resp structure: { response : [TCP|RTU]Response, request: [TCP|RTU]Request }
    // Data location: resp.response.body.coils (Array) or resp.response.body.payload (Buffer)
    console.log(resp);
  }, console.error);
});

socket.connect(options);

After installing the module globally, you can access the Command Line Interface. Use the --help flag to view available commands and information.

jsmodbus --help

When working with a ModbusAbstractResponse, you can retrieve the target device identifier using any of the following three property aliases. They all return the same number value representing the Modbus unit/slave ID:

• unitId
• slaveId
• address

These properties are read-only.

The library exports several namespaces to help with protocol handling and debugging:

• codes: Modbus exception and function codes.
• errors: Error objects thrown by the library.
• requests: Available Modbus request types, including UserRequest.
• responses: Modbus response types.
• limits: Protocol-specific constants and limits.