Electrical – Modbus RTU using SoftwareSerial

arduinocommunicationrs232

Im trying to control a temperature controller using Arduino MEGA over SoftwareSerial.

Im using Modbus-Master-Slave-for-Arduino library and I tried to test using example code below.
It gives only 0 for each item of the received data array au16data[].

Im using RS-485 serial transceiver module for interface to temperature controller and SoftwareSerial pins. But, There is no any pin definition for DE and RE pins. How can I connect A and B terminal of the controller to Arduino?

#include <ModbusRtu.h>
#include <SoftwareSerial.h>

// data array for modbus network sharing
uint16_t au16data[16];
uint8_t u8state;

/**
 *  Modbus object declaration
 *  u8id : node id = 0 for master, = 1..247 for slave
 *  u8serno : serial port (use 0 for Serial)
 *  u8txenpin : 0 for RS-232 and USB-FTDI 
 *               or any pin number > 1 for RS-485
 */
Modbus master(0); // this is master and RS-232 or USB-FTDI via software serial

/**
 * This is an structe which contains a query to an slave device
 */
modbus_t telegram;

unsigned long u32wait;

SoftwareSerial mySerial(12, 13);//Create a SoftwareSerial object so that we can use software serial. Search "software serial" on Arduino.cc to find out more details.

void setup() {
  Serial.begin(9600);//use the hardware serial if you want to connect to your computer via usb cable, etc.
  master.begin( &mySerial, 9600 ); // begin the ModBus object. The first parameter is the address of your SoftwareSerial address. Do not forget the "&". 9600 means baud-rate at 9600
  master.setTimeOut( 2000 ); // if there is no answer in 2000 ms, roll over
  u32wait = millis() + 1000;
  u8state = 0; 
}

void loop() {
  switch( u8state ) {
  case 0: 
    if (millis() > u32wait) u8state++; // wait state
    break;
  case 1: 
    telegram.u8id = 0x01; // slave address
    telegram.u8fct = 0x04; // function code (this one is registers read)
    telegram.u16RegAdd = 0x03E8; // start address in slave
    telegram.u16CoilsNo = 0x01; // number of elements (coils or registers) to read
    telegram.au16reg = au16data; // pointer to a memory array in the Arduino

    master.query( telegram ); // send query (only once)
    u8state++;
    break;
  case 2:
    master.poll(); // check incoming messages
    if (master.getState() == COM_IDLE) {
      u8state = 0;
      u32wait = millis() + 2000; 
        Serial.println(au16data[0]);//Or do something else!
        Serial.println(au16data[1]);//Or do something else!
        Serial.println(au16data[2]);//Or do something else!
        Serial.println(au16data[3]);//Or do something else!
        Serial.println(au16data[4]);//Or do something else!
        Serial.println(au16data[5]);//Or do something else!
        Serial.println(au16data[6]);//Or do something else!
        Serial.println(au16data[7]);//Or do something else!
        Serial.println(au16data[8]);//Or do something else!
        Serial.println(au16data[9]);//Or do something else!
        Serial.println(au16data[10]);//Or do something else!
        Serial.println(au16data[11]);//Or do something else!
        Serial.println(au16data[12]);//Or do something else!
        Serial.println(au16data[13]);//Or do something else!
        Serial.println(au16data[14]);//Or do something else!
        Serial.println(au16data[15]);//Or do something else!
    }
    break;
  }
}

Best Answer

These appear to be Driver Output and Receiver Output enable pins.

Your website link shows that you should do the following to these pins

RE..........Receiver Output Enable (Low to enable)

DE..........Driver Output Enable (high to enable)

Enables are not part of RS-485 and so won't be handled by the library, you'll have to control these pins yourself by using a DigitalWrite() before you transmit/receive.

Related Topic