Sample codes to write modbus via MAX485

Projects Networking, Protocols, and Devices
1

i am looking for sample codes to write modbus to another device (PLC) via MAX485

2

Hi @chickbuildljtelebrico

Try to Google arduino rs485 example, or arduino rs485 modbus

There are plenty of examples online available, and even Gemini, for instance, provides a code snippet right out of the box.

3

There are also many examples right here! A simple search for RS485 or modbus should provide examples.

You may be lucky and find the exact code you need but if not, be prepared to experiment yourself.

4

if this means, that you are searching for a Modbus Server (Slave) implementation, you can check my Library which comes with several examples.

But I suggest you define precisely what your device should really do.

5

thank you so much. i am able to read RFID cards via Pin 2&3. I want to write the read data to modbus device thru MAX485 (connected to Pin 0/Rx & Pin 1/Tx to RO and DI of MAX485. However, response of Modbus Poll (thru USB-485 converter) is checksum error/insufficient bytes.

6

just to clarify something

your device+RFID will read tags. That device is a Modbus Slave.
The PLC is the Modbus Master. The PLC will ask your device regarding the last readed tag.
Your Device should answer the PLC request.

Ist that correct?

7

yes it is. my Allen Bradley micro850 PLC is configured as Master.

8

Please show your code using code tags.

9 
#include <ArduinoModbus.h>
#include <ArduinoRS485.h>
#include<ModbusRTUServer.h>
#include<HardwareSerial.h>
#include<SoftwareSerial.h>

#define DE_RE_PIN 10

const byte slaveAddress = 1; // Modbus slave address
const int holdingRegisterAddress = 0; // Address of the holding register to write
int valueToWrite = 1234; // Value to write to the holding register


#define MAX_BITS 100                 // max number of bits 
#define WEIGAND_WAIT_TIME  3000      // time to wait for another weigand pulse.  

unsigned char databits[MAX_BITS];    // stores all of the data bits
unsigned char bitCount;              // number of bits currently captured
unsigned char flagDone;              // goes low when data is currently being captured
unsigned int weigand_counter;        // countdown until we assume there are no more bits
 
unsigned long facilityCode=0;        // decoded facility code
unsigned long cardCode=0;            // decoded card code
const int numHoldingRegisters = 10;


void ISR_INT0()
{
  //Serial.print("0");   // uncomment this line to display raw binary
  bitCount++;
  flagDone = 0;
  weigand_counter = WEIGAND_WAIT_TIME;  
 
}
 
// interrupt that happens when INT1 goes low (1 bit)
void ISR_INT1()
{
  //Serial.print("1");   // uncomment this line to display raw binary
  databits[bitCount] = 1;
  bitCount++;
  flagDone = 0;
  weigand_counter = WEIGAND_WAIT_TIME;  
}

void setup()
{
    ModbusRTUServer.begin(1, 9600);
  
  int address;
  int nb;
  address=1;
  nb = 1;
      // ... start serial/network ... 
    
     // Configure 10 holding registers starting at address 0x00 (40001) 
     ModbusRTUServer.configureHoldingRegisters(0x00, numHoldingRegisters);
  //ModbusRTUServer.begin(id, baudrate, config);
  
  //Serial.println("Modbus RTU Client Example - Write Holding Register");
  if (!ModbusRTUClient.begin(9600, SERIAL_8E1)){
    Serial.println("Failed to start Modbus RTU Client");
    while (true);
  }
  pinMode(13, OUTPUT);  // LED
  pinMode(2, INPUT);     // DATA0 (INT0)
  pinMode(3, INPUT);     // DATA1 (INT1)
 
  Serial.begin(9600);
  
  Serial.println("RFID Readers");
 
  // binds the ISR functions to the falling edge of INTO and INT1
  attachInterrupt(0, ISR_INT0, FALLING);  
  attachInterrupt(1, ISR_INT1, FALLING);
  weigand_counter = WEIGAND_WAIT_TIME;
    
}
 
void loop()
{
  ModbusRTUServer.poll();
  // This waits to make sure that there have been no more data pulses before processing data
  if (!flagDone) {
    if (--weigand_counter == 0)
      flagDone = 1;	
  }
 
  // if we have bits and we the weigand counter went out
  if (bitCount > 0 && flagDone) {
    unsigned char i;
 
    Serial.print("Read ");
    Serial.print(bitCount);
    Serial.print(" bits. ");
 
    // we will decode the bits differently depending on how many bits we have
    // see www.pagemac.com/azure/data_formats.php for mor info
    if (bitCount == 32)
    {
      // 35 bit HID Corporate 1000 format
      // facility code = bits 2 to 14
      for (i=2; i<16; i++)
      {
         facilityCode <<=1;
         facilityCode |= databits[i];
      }
 
      // card code = bits 15 to 34
      for (i=16; i<32; i++)
      {
         cardCode <<=1;
         cardCode |= databits[i];
      }
 
      printBits();
    }
    else if (bitCount == 26)
    {
      // standard 26 bit format
      // facility code = bits 2 to 9
      for (i=1; i<9; i++)
      {
         facilityCode <<=1;
         facilityCode |= databits[i];
      }
 
      // card code = bits 10 to 23
      for (i=9; i<25; i++)
      {
         cardCode <<=1;
         cardCode |= databits[i];
      }
 
      printBits();  
    }
    else {
      // you can add other formats if you want!
     Serial.println("Unable to decode."); 
    }
 
     // cleanup and get ready for the next card
     bitCount = 0;
     facilityCode = 0;
     cardCode = 0;
     for (i=0; i<MAX_BITS; i++) 
     {
       databits[i] = 0;
     }
  }
 
}
 
void printBits()
{
      // this function is for reading the access card tags. it is working now.
      Serial.print("FC = ");
      Serial.print(facilityCode);
      Serial.print(", CCJT = ");
      Serial.println(cardCode); 
      uint8_t serverAddress = 1;
      uint16_t registerAddress = 1;
      uint16_t valueToWrite = cardCode;
      Serial.print("Attempting to write value ");
      Serial.print(valueToWrite);

    // this function aims to write to modbus register. 
    //but checksum error at modbus poll
      uint16_t sensorvalue = cardCode;
      int id1;
      id1 = 1;
      int regadd;
      regadd=400001;
      ModbusRTUServer.holdingRegisterWrite(id1,sensorvalue);
      Serial.print(", sensorva = ");
      Serial.println(sensorvalue);
     
}