Modbus_Arduino, How to use arduino as slave to read and write coils

Projects Programming
1

As the title mentioned, I am looking for a way to read and write a coils in arduino with Modbus

#include <modbus.h>
#include <modbusDevice.h>
#include <modbusRegBank.h>
#include <modbusSlave.h>  
#define RS485TxEnablePin 8
#define RS485Baud 9600
#define RS485Format SERIAL_8N2
modbusDevice regBank;
modbusSlave slave;

int LEDpin = 3;
int buttonPin = 2;

int brightness = 0;
int buttonValue = 0;
int dutyCycle = 0;

void setup() {
    pinMode(LEDpin,OUTPUT);
    pinMode(buttonPin,INPUT);

    regBank.setId(1);
    regBank.add(40001);
    
    slave._device = &regBank;  
    slave.setBaud(&Serial,RS485Baud,RS485Format,RS485TxEnablePin); 
    Serial.flush(); 
}

void loop() {
  dutyCycle = regBank.get(40001);
  buttonValue = digitalRead(buttonPin);
  if(buttonValue == HIGH){        
    analogWrite(LEDpin,255);  
    delay(10000);    
  }
  analogWrite(LEDpin,dutyCycle);
  slave.run();  
}

I have try to use another code to read coil but the result show as below :

from pymodbus.client.sync import ModbusSerialClient as ModbusClient
import time

scale = ModbusClient(method='rtu', port='/dev/ttyUSB0', parity='N', baudrate=9600, bytesize=8, stopbits=2, timeout=1)
connection = scale.connect()

scale.write_coil(0, True, unit=1)
time.sleep(1)
rr = scale.read_coils(0, 8, unit=1)
print(rr.bits)

Output:
[False, False, False, False, False, False, False, False]

Anyone have any suggestion? Thank you~

2

Before moving to modbus protocol communication you have to check whther your rs485 is working properly or not. USE brays terminal to check this. Many a times we forgot that max485 is half duplex chip.

check this first and then revert back .

Regards,
P.R.B

3

Hi PRB_tech,thanks for your help,

I have tried to use Bray Terminal to check my RS485 is connect or not, and the attachment on below is the result after I connect to the port, there is nothing happened when I click connect

I also have tried like the link as below:

I also have a weird experience before, when I try to write a value to the Modbus Register through Arduino code by itself and read the value of register from computer(python code as I posted above) and I can read the value correctly.

But if I try to write value to Modbus register through computer(python code) and read it from computer , its output is still zero (that mean the write fucntion didn't work properly),but my arduino code will let a LED light up if the value of register is not zero, the fact is that LED light! but I cant read the value out ...

Thanks for help