I have two mega arduino and want to them communicate with modbus rtu together, i use below link circuit for this : http://www.microcontroller-project.com/rs485-communication-between-arduino-mega-and-arduino-pro-mini.html

but dont work!! while they work in simple rs485 and without modbus with same above link code !?

my code in modbus mode is based on this forum : https://forum.arduino.cc/index.php?topic=176142.0


    //#include <LiquidCrystal.h>

#include <SimpleModbusMaster.h>

   The example will use packet1 to read a register from address 0 (the adc ch0 value)
   from the arduino slave (id=1). It will then use this value to adjust the brightness
   of an led on pin 9 using PWM.
   It will then use packet2 to write a register (its own adc ch0 value) to address 1
   on the arduino slave (id=1) adjusting the brightness of an led on pin 9 using PWM.

//////////////////// Port information ///////////////////
#define baud 9600
#define timeout 1000
#define polling 200 // the scan rate
#define retry_count 50

// used to toggle the receive/transmit pin on the driver
#define TxEnablePin 2

#define LED 9

// The total amount of available memory on the master to store data

// This is the easiest way to create new packets
// Add as many as you want. TOTAL_NO_OF_PACKETS
// is automatically updated.
  TOTAL_NO_OF_PACKETS // leave this last entry

// Create an array of Packets to be configured
Packet packets[TOTAL_NO_OF_PACKETS];

// Masters register array
unsigned int regs[TOTAL_NO_OF_REGISTERS];
int In22=22;

// LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

void setup()
//  Serial1.begin(9600);
// lcd.begin(16, 2);
  // Initialize each packet
  modbus_construct(&packets[PACKET1], 1, READ_HOLDING_REGISTERS, 0, 5, 0);
//  modbus_construct(&packets[PACKET2], 1, PRESET_MULTIPLE_REGISTERS, 1, 1, 0);

  // Initialize the Modbus Finite State Machine
  modbus_configure(&Serial1, baud, SERIAL_8N2, timeout, polling, retry_count, TxEnablePin, packets, TOTAL_NO_OF_PACKETS, regs);

  pinMode(LED, OUTPUT);

void loop()

//  regs[0] = analogRead(0); // update data to be written to arduino slave
//regs[1] =In22;
//  analogWrite(LED, regs[0]>>2); // constrain adc value from the arduino slave to 255
//  regs[1];
 if (Serial1.available()){
//  lcd.setCursor(6, 1);
//    lcd.print(regs[0]);



#include <SimpleModbusSlave.h>

/* This example code will receive the adc ch0 value from the arduino master.
   It will then use this value to adjust the brightness of the led on pin 9.
   The value received from the master will be stored in address 1 in its own
   address space namely holdingRegs[].

   In addition to this the slaves own adc ch0 value will be stored in
   address 0 in its own address space holdingRegs[] for the master to
   be read. The master will use this value to alter the brightness of its
   own led connected to pin 9.

   The modbus_update() method updates the holdingRegs register array and checks

   The Arduino serial ring buffer is 64 bytes or 32 registers.
   Most of the time you will connect the arduino to a master via serial
   using a MAX485 or similar.

   In a function 3 request the master will attempt to read from your
   slave and since 5 bytes is already used for ID, FUNCTION, NO OF BYTES
   and two BYTES CRC the master can only request 58 bytes or 29 registers.

   In a function 16 request the master will attempt to write to your
   slave and since a 9 bytes is already used for ID, FUNCTION, ADDRESS,
   NO OF REGISTERS, NO OF BYTES and two BYTES CRC the master can only write
   54 bytes or 27 registers.

   Using a USB to Serial converter the maximum bytes you can send is
   limited to its internal buffer which differs between manufactures.

#define  LED 9 

// Using the enum instruction allows for an easy method for adding and
// removing registers. Doing it this way saves you #defining the size
// of your slaves register array each time you want to add more registers
// and at a glimpse informs you of your slaves register layout.

//////////////// registers of your slave ///////////////////
  // just add or remove registers and your good to go...
  // The first register starts at address 0
  HOLDING_REGS_SIZE // leave this one
  // total number of registers for function 3 and 16 share the same register array
  // i.e. the same address space

unsigned int holdingRegs[HOLDING_REGS_SIZE]; // function 3 and 16 register array

void setup()
  /* parameters(HardwareSerial* SerialPort,
                long baudrate,
        unsigned char byteFormat,
                unsigned char ID,
                unsigned char transmit enable pin,
                unsigned int holding registers size,
                unsigned int* holding register array)

  /* Valid modbus byte formats are:
     SERIAL_8N2: 1 start bit, 8 data bits, 2 stop bits
     SERIAL_8E1: 1 start bit, 8 data bits, 1 Even parity bit, 1 stop bit
     SERIAL_8O1: 1 start bit, 8 data bits, 1 Odd parity bit, 1 stop bit

     You can obviously use SERIAL_8N1 but this does not adhere to the
     Modbus specifications. That said, I have tested the SERIAL_8N1 option
     on various commercial masters and slaves that were suppose to adhere
     to this specification and was always able to communicate... Go figure.

     These byte formats are already defined in the Arduino global name space.

  modbus_configure(&Serial1, 9600, SERIAL_8N2, 1, 2, HOLDING_REGS_SIZE, holdingRegs);

  // modbus_update_comms(baud, byteFormat, id) is not needed but allows for easy update of the
  // port variables and slave id dynamically in any function.
  modbus_update_comms(9600, SERIAL_8N2, 1);

  pinMode(LED, OUTPUT);

void loop()
  // modbus_update() is the only method used in loop(). It returns the total error
  // count since the slave started. You don't have to use it but it's useful
  // for fault finding by the modbus master.

//  holdingRegs[0] = 123; // update data to be read by the master to adjust the PWM
  holdingRegs[1] = 123; // update data to be read by the master to adjust the PWM
    holdingRegs[2] = 123;
//    holdingRegs[3] = 123;
//    holdingRegs[4] = 123;
  //  analogWrite(LED, holdingRegs[PWM_VAL]>>2); // constrain adc value from the arduino master to 255
  /* Note:
     The use of the enum instruction is not needed. You could set a maximum allowable
     size for holdinRegs[] by defining HOLDING_REGS_SIZE using a constant and then access
     holdingRegs[] by "Index" addressing.
     holdingRegs[0] = analogRead(A0);
     analogWrite(LED, holdingRegs[1]/4);


1 Answer 1


I even didn't check the Modbus part of your code but the first think what I found: You are using the D2 pin for LCD and at the same time as TXEnablePin...

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