modbus-tcp

Modbus TCP/IP

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

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 
client.pipe(server.pipe());
server.pipe(client.pipe());
 
server.on("read-coils", function (fromtoreply) {
    return reply(null, [ 1, 0, 1, 1 ]);
});
 
// read coils from unit id = 0, from address 10 to 13 
client.readCoils(0, 10, 13, function (errcoils) {
    // coils = [ 1, 0, 1, 1 ] 
});

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.

  • 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.