Modbus RTU Tutorial: How to Read Input Registers with M-Duino PLC

/*
   Copyright (c) 2018 Boot&Work Corp., S.L. All rights reserved
   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU Lesser General Public License as 
published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Lesser General Public License for more details.
   You should have received a copy of the GNU Lesser General Public License
 along with this program. If not, see <http://www.gnu.org/licenses/>.
 */ #include <ModbusRTUMaster.h> // Define the ModbusRTUMaster object, 
using the RS-485 or RS-232 port (depending on availability)

#if defined HAVE_RS485_HARD
#include <RS485.h>
ModbusRTUMaster master(RS485); #elif defined HAVE_RS232_HARD
#include <RS232.h>
ModbusRTUMaster master(RS232); #else
ModbusRTUMaster master(Serial1);
#endif uint32_t lastSentTime = 0UL;
const uint32_t baudrate = 38400UL; 
//////////////////////////////////////////////////////////////////
void setup() {
 Serial.begin(9600UL); // Start the serial port
#if defined HAVE_RS485_HARD
 RS485.begin(baudrate, HALFDUPLEX, SERIAL_8E1);
#elif defined HAVE_RS232_HARD
 RS232.begin(baudrate, SERIAL_8E1);
#else
 Serial1.begin(baudrate, SERIAL_8E1);
#endif // Start the modbus master object.
 // It is possible to define the port rate (default: 19200)
 master.begin(baudrate);
}
////////////////////////////////////////////////////////////////////////////////////////////////////
void loop() {
  // Send a request every 1000ms
  if (millis() - lastSentTime > 1000) {
    // Send a Read Input Register request to the slave with address 31
    // It requests for 1 register starting at address 0
    // IMPORTANT: all read and write functions start a Modbus transmission, but they are not
    // blocking, so you can continue the program while the Modbus functions work. To check for
    // available responses, call master.available() function often.
    if (!master.readInputRegisters(31, 0, 1)) {
      // Failure treatment
    }
    lastSentTime = millis();
  }

  // Check available responses often
  if (master.isWaitingResponse()) {
    ModbusResponse response = master.available();
    if (response) {
      if (response.hasError()) {
        // Response failure treatment. You can use response.getErrorCode()
        // to get the error code.
      } else {
        // Get the register value from the response
        uint32_t reg = response.getRegister(0);

        Serial.print("Reg: ");
        Serial.println(reg);
      }
    }
  }
}