Modbus RTU slave library

Question

I want to use an Arduino Genuino Zero as a Modbus slave. I tried to search on google, and found a library. When I try using the library, I get a timeout error.

Can anyone tell me how to solve that error? Did anyone manage to get the Arduino Modbus RTU slave library working?

Answer 1

so i edited code your as below #define mySerial Serial1

byte test_data[8] = {0x01,0x03,0x00,0x64,0x00,0x02,0x85,0xD4};

int data_count = 5;


void setup() {

mySerial.begin(9600);
Serial.begin(9600);
delay(1000);

}
int count = 0; 
void loop() {

  if(count%10 == 0) {
    Serial.println();
    send_data();
  }

  count++;

  while(mySerial.available()>0)
  {
    byte b=mySerial.read();
    delay(1);
    Serial.print(b,HEX);
    Serial.print(" ");
  }
  //Serial.println(count);
  while(Serial.available())
  {

    byte a=Serial.read();
    delay(1);
    mySerial.print(a);

  }


  delay(1000);

}

void send_data() {
  Serial.println("send data");


  delay(1);
  for(char i=0;i<8;i++){
    mySerial.write(test_data[i]);
    delay(1);
    Serial.print(test_data[i],HEX);
    Serial.print(" ");
  }

  Serial.println();

  delay(1);

Answer 2

I am unable to simulate and see the result right now; I hope if the library functions properly this might be a communication issue. I can add a one step debug for you to check and see what is happening.

Your device is acting as a Modbus RTU slave, So take another Arduino, make it act it like a master device, and see what response is coming from the slave device. Here is a glimpse of code I used to make arduino as a master and see the response coming in Serial terminal.

    #include <SoftwareSerial.h>
    #define rxPin 2
    #define txPin 3

    #define RS485_pin 10
    //SoftwareSerial mySerial(rxPin, txPin);
    #define mySerial Serial1


    byte test_data[8] = {0x01,0x03,0x00,0x64,0x00,0x02,0x85,0xD4};

    int data_count = 5;
    byte rcvdata[15];
    int param_value_int[7];

    void setup() {
      // put your setup code here, to run once:
    //mySerial.begin(19200,SERIAL_8E1);
    mySerial.begin(9600);
    Serial.begin(9600);
    delay(1000);
    pinMode(RS485_pin, OUTPUT);
    digitalWrite(RS485_pin,LOW);
    delay(100);
    }
    int count = 0; 
    void loop() {

      if(count%10 == 0) {
        Serial.println();
        send_data();
      }

      count++;

      while(mySerial.available()>0)
      {
        byte b=mySerial.read();
        delay(1);
        Serial.print(b,HEX);
        Serial.print(" ");
      }
      //Serial.println(count);
      while(Serial.available())
      {
        digitalWrite(RS485_pin,HIGH);
        byte a=Serial.read();
        delay(1);
        mySerial.print(a);
        digitalWrite(RS485_pin,LOW);
       // Serial.print(a);
      }


      delay(1000);
      //while(1);
    }

    void send_data() {
      Serial.println("send data");

      digitalWrite(RS485_pin,HIGH);
      delay(1);
      for(char i=0;i<8;i++){
        mySerial.write(test_data[i]);
        delay(1);
        Serial.print(test_data[i],HEX);
        Serial.print(" ");
      }

      Serial.println();
      digitalWrite(RS485_pin,LOW);
      delay(1);
    }

Here you can use Software Serial or normal Serial1 or Serial2 on devices like Mega. Make sure if it is RS485 based you have to toggle the RS485 read-write pin each time to enable and disable one feature at a time as RS 485 uses half-duplex communication.

In this case test_data[] uses Function code FC03 Read holding register. You can make your custom data. I am assuming you are familiar with all the types available.Still there is a great site [ link ] to look up to if you don't know the basics. The last two bytes are checksum. And there is an online checksum calculator available if you need also https://www.lammertbies.nl/comm/info/crc-calculation.html

In this way you can at least debug and share the result, so that we can help more.