// 'RIR' contains the "Function Code" that we are going to invoke on the remote device
var RIR = require('modbus-stack').FUNCTION_CODES.READ_INPUT_REGISTERS;

// IP and port of the MODBUS slave, default port is 502
var client = require('modbus-stack/client').createClient(502, '10.0.1.50');

// 'req' is an instance of the low-level `ModbusRequestStack` class
var req = client.request(RIR, // Function Code: 4
                         0,    // Start at address 0
                         50);  // Read 50 contiguous registers from 0

// 'response' is emitted after the entire contents of the response has been received.
req.on('response', function(registers) {
  // An Array of length 50 filled with Numbers of the current registers.
  console.log(registers);
  client.end();
});

var FC = require('modbus-stack').FUNCTION_CODES;

// 'handlers' is an Object with keys containing the "Function Codes" that your MODBUS
// server will handle. Anything function code requested without a handler defined here
// will have the Server transparently respond with Exception Code 1 ("Illegal Function")
var handlers = {};

// Define a handler for "Read Input Registers". We'll just respond with the register
// number requested. In a real-world situation, you'd probably look up these values from
// a database, etc.
handlers[FC.READ_INPUT_REGISTERS] = function(request, response) {
  var start = request.startAddress;
  var length = request.quantity;
  
  var resp = new Array(length);
  for (var i=0; i<length; i++) {
    resp[i] = start + i;
  }
  response.writeResponse(resp);
}

require('modbus-stack/server').createServer(handlers).listen(502);