Modbus RS485 Communication using Arduino as Master

[engr_hussain_sheikh]

I am trying to read holding registers from a Schneider PowerLogic PM5650 using Arduino. Communication I am using is RS485. Moreover, I am using TTL to 485 module with following pinout
RS485 module----------------- Schneider
A ---------------------------------- A
B ---------------------------------- B

RS485 module ----------------Arduino UNO
DI -------------------------- TX (Pin 1)
DE -------------------------- Pin 3
RE -------------------------- Pin 2
RO -------------------------- RX (Pin 0)
Vcc to Arduino 5 v and GND to Arduino GND
So when I use simple RS485 to USB and use software (Modscan) to Read the Holding Register, it shows me the data when I put in the Register Address. But when I try to do same with the Arduino uno I can’t get any data. This is the code I am using

#include <ModbusMaster.h>

#define MAX485_DE 3

#define MAX485_RE_NEG 2

ModbusMaster node;

void preTransmission()

{

digitalWrite (MAX485_RE_NEG, 1);

digitalWrite (MAX485_DE, 1);

}

void postTransmission()

{

digitalWrite (MAX485_RE_NEG, 0);

digitalWrite (MAX485_DE, 0);

}

void setup()

{

pinMode(MAX485_RE_NEG, OUTPUT);

pinMode(MAX485_DE, OUTPUT);

// Init in receive mode

digitalWrite (MAX485_RE_NEG, 0);

digitalWrite (MAX485_DE, 0);

// Modbus communication runs at 9600 baud

Serial.begin(9600);

// Modbus slave ID 1

node.begin(1, Serial);

// Callbacks allow us to configure the RS485 transceiver correctly

node.preTransmission(preTransmission);

node.postTransmission(postTransmission);

}

void loop()

{

uint8_t resultMain;

resultMain = node.readHoldingRegisters(0x3035, 6);

Serial.println("Reading Register: ");

if (resultMain == node.ku8MBSuccess)

{

Serial.println("--------");

Serial.print ("Voltage Avg: ");

Serial.println(node.getResponseBuffer(0x00));

Serial.print ("V avg 2: ");

Serial.println(node.getResponseBuffer (0x01));

Serial.print ("Data_3: ");

Serial.println(node.getResponseBuffer(0x04));

Serial.print ("Data_4: ");

Serial.println(node.getResponseBuffer(0x05));

}

delay(1000);

}

This is my Output

This the the register Address

I am stuck in it. Please guide what should I do

[bobcousins]

@engr_hussain_sheikh

resultMain = node.readHoldingRegisters(0x3035, 6);

How did you decide on 0x3035 ? The addresses in the table appear to be decimal, so it should be 3035, assuming you need to subtract 1.

[engr_hussain_sheikh]

I tried decimal as well result remain the same.
HEX value of address is BDB, tried it as well (0x0BDB) but it does not work either

[bobcousins]

@engr_hussain_sheikh

DI -------------------------- TX (Pin 1)
DE -------------------------- Pin 3
RE -------------------------- Pin 2
RO -------------------------- RX (Pin 0)

You are using the same serial port as you are you using for serial monitor, that is not a good idea.

@engr_hussain_sheikh

// Modbus communication runs at 9600 baud

Serial.begin(9600);

How did you decide 9600 is the correct baud rate? Are you sure that 8,N,1 framing is correct?

Please post a link to the data sheet for the device you are using.

[engr_hussain_sheikh]

i have tried different serial pin as well but result is the same

Data Sheet

files

Schneider Device

[engr_hussain_sheikh]

Any change i need to do in my code or circuit ?

[bobcousins]

Uno only has one serial port, so if you are using that for the serial monitor, you will need to use SoftwareSerial for communication.

[engr_hussain_sheikh]

can you suggest me code changes and pin changes ?

[bobcousins]

Here is an example sketch I used to control a relay module. You should be able to adapt it to your needs.

/*!
 * @file
 * @brief Example ModbusMaster on Arduino Uno.
 * @author Bob Cousins
 *
 * Example showing use of ModbusMaster to control a relay module.
 * SoftwareSerial is used because Serial is used for serial monitor.
 */
 
#include <ModbusMaster.h>
#include <SoftwareSerial.h>

#define TX 3
#define RX 2

#define TX_ENABLE   4
#define RX_ENABLE_N 5

#define BAUD_RATE 9600

#define SLAVE_ADDRESS 1

ModbusMaster master;

SoftwareSerial modbus_serial (RX, TX);

void pre_transmit (void)
{
  digitalWrite (TX_ENABLE, 1);
  digitalWrite (RX_ENABLE_N, 1);
}

void post_transmit (void)
{
  digitalWrite (TX_ENABLE, 0);
  digitalWrite (RX_ENABLE_N, 0);
}

void setup() 
{
  pinMode (TX, OUTPUT);
  pinMode (RX, INPUT);

  pinMode (TX_ENABLE, OUTPUT);
  pinMode (RX_ENABLE_N, OUTPUT);

  Serial.begin (115200);

  modbus_serial.begin (BAUD_RATE);
  master.begin (SLAVE_ADDRESS, modbus_serial);

  master.preTransmission(pre_transmit);
  master.postTransmission(post_transmit);
}

void loop() 
{
  uint8_t result;

  // turn on relay 0
  result = master.writeSingleCoil (0, 1);
  if (result == ModbusMaster::ku8MBSuccess)
    Serial.println("It worked!");
  else
  {
    Serial.print("I can't do that: ");
    Serial.println (result, HEX);
  }

  delay (1000);

  // turn off relay 0
  result = master.writeSingleCoil (0, 0);
  delay (1000);
}

[engr_hussain_sheikh]

I used this code

#include<ModbusMaster.h>


#define MAX485_DE 3
#define MAX485_RE_NEG 2


ModbusMaster node;

void preTransmission()
{
  digitalWrite(MAX485_RE_NEG, 1);
  digitalWrite(MAX485_DE, 1);
}

void postTransmission()
{
  digitalWrite(MAX485_RE_NEG, 0);
  digitalWrite(MAX485_DE, 0);
}

void setup() {
  
  Serial.begin(9600);
 // Serial2.begin(9600);

  pinMode(MAX485_RE_NEG, OUTPUT);
  pinMode(MAX485_DE, OUTPUT);

  // Init in receive mode
  digitalWrite(MAX485_RE_NEG, 0);
  digitalWrite(MAX485_DE, 0);

  //My slave uses 9600 baud
  delay(1000);
  Serial.println("starting arduino: ");
  Serial.println("setting up Serial ");
  Serial.println("setting up RS485 port ");
//  slave id
 node.begin(1, Serial);
  node.preTransmission(preTransmission);
  node.postTransmission(postTransmission);
}

void loop() {
  uint16_t result;
  
//slave address and length
  result = node.readHoldingRegisters(3035 ,2);

  if (result == node.ku8MBSuccess)
  {
      Serial.print("V avg : ");
      Serial.println(node.getResponseBuffer(1));
      Serial.print("Flow : ");
      Serial.println(node.getResponseBuffer(2));
  }

  Serial.print("\n");
  
  delay(500);
}

and its giving me this output

When I change the register address the output changes
Thanks for your time, plz look into this and guide

[engr_hussain_sheikh]

V avg on power analyzer is 245.5

[engr_hussain_sheikh]

This Code is giving me the values as when I use CT with the Schneider meter and check I avg register it start giving me values and as I remove load from CT it gives me 0. it means it is giving me register values now how can I interpret those values ?

This is the code in which i am using voltage register (V avg) value on meter was around 245-250

#include<ModbusMaster.h>


#define MAX485_DE 3
#define MAX485_RE_NEG 2


ModbusMaster node;

void preTransmission()
{
  digitalWrite(MAX485_RE_NEG, 1);
  digitalWrite(MAX485_DE, 1);
}

void postTransmission()
{
  digitalWrite(MAX485_RE_NEG, 0);
  digitalWrite(MAX485_DE, 0);
}

void setup() {
  
  Serial.begin(9600);
 // Serial2.begin(9600);

  pinMode(MAX485_RE_NEG, OUTPUT);
  pinMode(MAX485_DE, OUTPUT);

  // Init in receive mode
  digitalWrite(MAX485_RE_NEG, 0);
  digitalWrite(MAX485_DE, 0);

  //My slave uses 9600 baud
  delay(1000);
  Serial.println("starting arduino: ");
  Serial.println("setting up Serial ");
  Serial.println("setting up RS485 port ");
//  slave id
 node.begin(1, Serial);
  node.preTransmission(preTransmission);
  node.postTransmission(postTransmission);
}

void loop() {
  uint16_t result;
  
//slave address and length
  result = node.readHoldingRegisters(3035 ,2);

  if (result == node.ku8MBSuccess)
  {
      Serial.print("V avg : ");
      Serial.println(node.getResponseBuffer(0));
      Serial.print("Flow : ");
      Serial.println(node.getResponseBuffer(1));
      
  }

  Serial.print("\n");
  
  delay(500);
}

Output:

[markd833]

I think the problem may be in the way you are interpreting the modbus response from the meter.

That table you posted in your post #1 says that the "Voltage L-N Avg" is a 32 bit value (2x INT16s) but should be interpreted as a FLOAT32.

I think that to get the whole value you would need to request 2 registers - as your code in post #12 does. However, the 2 values returned are the 2 halves of the 32-bit floating point number. You need to combine the 2 16-bit values received into one 32-bit value which you would then interpret as a floating point number.

With a bit of massaging, I get the following:

17273 = 0x4379
56781 = 0xDDCD

If I combine the 2 into one 32- bit value, I get:

0x4379DDCD

When that is interpreted as an IEE754 32-bit floating point number, it works out as 249.8664V.

[engr_hussain_sheikh]

Thanks @markd833 , that really helps can you plz help me in providing code lines that i could use in my code to convert it in the required formate.

[markd833]

Have a read of this:

Reform float32 from two unsigned int16 [SOLVED]

Hi I'm stuck trying to reform a 32 bit float from 2 unsigned 16 bit ints. I'm using an arduino as a modbus master to read a flow meter. the registers in the meter are all 16 bit but the device uses 2 registers to represent a 32 bit float. I can read the 16 bit ints but I' having difficulty reforming them to a 32 bit float (using big endian) I came across this as a suggestion but I cant make it work: #include uint16_t regs[2]; //reg[0] & reg[1] contain the 16 bit ints long int num = ((reg[0…

[engr_hussain_sheikh]

Thanks, i will use it and post the working code here