Modbus TCP/IP

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Modbus TCP/IP Stream

Build Status
This is a very simple module that uses NodeJS Streams2 to read Modbus TCP data and convert it to JSON and vice-versa.


If you're just starting or just trying to use this library, I advise you to try modbus-stream. It's more complete (it supports all standard function codes) and it was designed to work with TCP, RTU and ASCII modes.
npm install modbus-tcp


var modbus = require("modbus-tcp");
var client = new modbus.Client();
var server = new modbus.Server();

// link client and server streams together
// if you have a socket (stream) you can just
// call client.pipe(socket) or server.pipe(socket)

server.on("read-coils", function (from, to, reply) {
    return reply(null, [ 1, 0, 1, 1 ]);

// read coils from unit id = 0, from address 10 to 13
client.readCoils(0, 10, 13, function (err, coils) {
    // coils = [ 1, 0, 1, 1 ]

Client Methods

All of the following read methods have the form method(unitId, from, to, next) and write methods have the form method(unitId, addr, val, next), where next is an
optional function called if the server replies (with the same transactionId) to the sent message.
  • readCoils
  • readDiscreteInputs
  • readHoldingRegisters
  • readInputRegisters
  • writeSingleCoil
  • writeSingleRegister
  • writeMultipleCoils
  • writeMultipleRegisters

Addresses are exactly as is in protocol, so if you see a paper talking about address 40001 this usually means first record address of that function so it means address 0.

Server Events

  • read-coils
  • read-discrete-inputs
  • read-holding-registers
  • read-input-registers
  • write-single-coil
  • write-single-register
  • write-multiple-coils
  • write-multiple-registers
  • data

This last event is triggered when an unknown function code is received.