ModbusClient

A wrapper for TCP sockets built for the Modbus protocol.

Installation

To install the package simply run the following command

npm install modbusclient

Usage

The package provides a central utility ModbusClient class which implements various methods for handling communications with a Modbus server.

Creating the client

import { ModbusClient } from 'modbusclient'

const client = new ModbusClient(502, '127.0.0.1', () => console.log('Connected'))

As seen above, the constructor takes 3 arguments: the port where the Modbus Server is listening on, the IP address of the machine on which the server runs on and a callback function which is executed once a connection has been established.

Sending a request

To send a request, you need to use the send method defined on the ModbusClient class, which takes as its only argument a ModbusRequest object.

The ModbusRequest object contains the following properties:

• functionCode - the Modbus function code to be requested;
• noOfRegisters - the total number of register values to be returned;
• slaveId - the slave from which to request data;
• startAddress - the register from which noOfRegisters register values will be returned.

let request = {
  // Read Input Registers Function
  functionCode: 4,
  // Requesting 3 register values
  noOfRegisters: 3,
  // The Modbus Slave from which data is requested
  slaveId: 0,
  // The first register to read values from
  startAddress: 10
}

// Sending the request
client.send(request)

The Response object

The response object contains the following properties:

• transactionId - the order number of the current response;
• functionCode - the function code which was previously requested;
• noOfRegisters - the total number of registers returned;
• registerValues - array of integers which contain the requested register values.

Receiving the response

The client implements a Promise based API, therefore the send method returns a Promise which resolves once a response is received from the server. As such, the response is passed as an argument to the Promises' then method. If the request fails, the Promise rejects with an appropriate error message.

client
  .send(request)
  // Processing the response
  .then(response => log(response))
  // Handling possible errors
  .catch(err => console.log(err))

function log(response) {
  console.log(`Transaction ID: ${response.transactionId}`)
  console.log(`Function code: ${response.functionCode}`)
  console.log(`Total number of registers: ${response.noOfRegisters}`)
  console.log(`Values: ${response.registerValues.join(', ')}`)
}

If the above snippet encounters no errors, the displayed result will be:

Transaction ID: 1
Function code: 4
Total number of registers: 3
Values: 2, 3, 10

ES6 Async / Await

You can also use the ES6 async / await syntactic sugar to make the code look synchronous. The following example should produce the same result as the previous one:

function log(response) {
  console.log(`Transaction ID: ${response.transactionId}`)
  console.log(`Function code: ${response.functionCode}`)
  console.log(`Total number of registers: ${response.noOfRegisters}`)
  console.log(`Values: ${response.registerValues.join(', ')}`)
}

async function example() {
  try {
    const response = await client.send(request)
    log(response)
  } catch (error) {
    console.log(`Error: ${error}`)
  }
}

example()

TypeScript

Additional type annotations for the ModbusRequest and ModbusResponse objects can be imported with:

import { ModbusRequest, ModbusResponse } from 'modbusclient/types'
import { ModbusClient } from 'modbusclient'

const client: ModbusClient = new ModbusClient(502, '127.0.0.1', () => {})
const request: ModbusRequest = {
  functionCode: 4,
  noOfRegisters: 3,
  slaveId: 0,
  startAddress: 10
}
client.send(request).then((response: ModbusResponse) => console.log(response))