Unexpected behaviour in ModbusRTU_Server_LED example

[benjaminSchilling33]

When I run the ModbusRTU_Server_LED example on an Arduino board (R4 Mini + SPE Shield - which includes RS485),
modbusRTUServer.poll() always return the same value, as if the internal buffer or something has not been cleared after polling.

Is this expected? I can't send a new value because it always contains the first received value.

[vishnumaiea]

The ModbusRTU_Server_LED is for testing the Modbus RTU coil data exchange. Is the example code working without any modifications (except for pin and port modifications)? What is the behaviour? How are you sending the data to the Server device?

[benjaminSchilling33]

I'm using this arduino code on Arduino Uno R4 Mini with SPE Shield.
According to SPE shield schematic, RX and TX are connected to pins 1 and 2. DE is connected to pin 8 and RE is connected to pin 7.

//===================================================================================//
/*
  Filename: ModbusRTU_Server_LED.ino [RP2040]
  Description: This example demonstrates the use of a Modbus RTU server to listen for Modbus
  requests and turn an LED on and off. The request can come from any Modbus RTU client
  including the ModbusRTU_Client_LED example. We define 4 coils but only use the `0x00`
  one to turn the LED on and off. We are using hardware serial port for the RS-485
  interface.

  This code was tested with the Raspberry Pi Pico RP2040 board. For ModbusRTU emulation,
  you can use the Modbus Mechanic (https://github.com/SciFiDryer/ModbusMechanic) software.
  
  Framework: Arduino, PlatformIO
  Author: Vishnu Mohanan (@vishnumaiea, @vizmohanan)
  Maintainer: CIRCUITSTATE Electronics (@circuitstate)
  Library Version: 0.0.9
  License: MIT
  Source: https://github.com/CIRCUITSTATE/CSE_ModbusRTU
  Last Modified: +05:30 19:11:37 PM 28-05-2025, Wednesday
 */
//===================================================================================//

#include <CSE_ArduinoRS485.h>
#include <CSE_ModbusRTU.h>
//===================================================================================//
// Macros and constants

// You can define the serial port pins here.
#define   PIN_RS485_DE        8
#define   PIN_RS485_RE        7
#define   PIN_RS485_RX        1
#define   PIN_RS485_TX        0

#define   PORT_RS485          Serial1 // The hardware serial port for the RS-485 interface

//===================================================================================//
// Globals

// Declare the RS485 interface here with a hardware serial port.
RS485Class RS485 (PORT_RS485, PIN_RS485_DE, PIN_RS485_RE, PIN_RS485_TX); // Port, DE, RE, TX

// Create a Modbus RTU node instance with the RS485 interface.
CSE_ModbusRTU modbusRTU (&RS485, 0x01, "modbusRTU-0x01"); // (Port, Device Address, Device Name)

// Create a Modbus RTU server instance with the Modbus RTU node.
CSE_ModbusRTU_Server modbusRTUServer (modbusRTU, "modbusRTUServer"); // (CSE_ModbusRTU, Server Name)

const int ledPin = LED_BUILTIN;

//===================================================================================//

void setup() {
  // Initialize the default serial port for debug messages.
  Serial.begin (9600);
  delay (1000);
  Serial.println ("CSE_ModbusRTU - Modbus RTU Server LED");

  // // Optional for RP2040. Set the serial port pins here.
  // PORT_RS485.setRX (PIN_RS485_RX);
  // PORT_RS485.setTX (PIN_RS485_TX);

  // Initialize the RS485 port manually.
  // This particualr begin() call is specific to RP2040-Arduino.
  // If you are using a different controller, change the begin() call accordingly.
  PORT_RS485.begin (9600, SERIAL_8N1);

  // Initialize the RS485 interface. If you are initializing the RS485 interface
  // manually, then the parameter can be empty.
  RS485.begin();

  // Configure the LED.
  pinMode (ledPin, OUTPUT);
  digitalWrite (ledPin, LOW);
  
  // Initialize the Modbus RTU server.
  modbusRTUServer.begin();

  // Configure four coils starting at address 0x00.
  // The first coil will control the LED.
  modbusRTUServer.configureCoils (0x00, 4);

  // Enable/Disable the debug messages here.
  CSE_ModbusRTU_Debug:: enableDebugMessages();
  // CSE_ModbusRTU_Debug:: disableDebugMessages();
}

//===================================================================================//

void loop() {
  delay(5000);
  // Poll for Modbus RTU requests.
  int requestReceived = modbusRTUServer.poll();

  if ((requestReceived != -1) && (requestReceived < 0x80)) {
    // Read the current value of the coil.
    int coilValue = modbusRTUServer.readCoil (0x00);
  
    if (coilValue == 1) {
      Serial.println("Enable LED");
      // Coil value set, turn on the LED.
      digitalWrite (ledPin, HIGH);
    }
    else if (coilValue == 0) {
      Serial.println("Disable LED");
      // Coil value clear, turn off the LED.
      digitalWrite (ledPin, LOW);
    }
    else {
      Serial.println ("Error reading coil");
    }
  }
}

//===================================================================================//

I have my logic analyzer attached to the pins of the Arduino UNO R4 mini and get the following capture:
My client only sends the modbus data once. If i disconnect the client board (like disable power connection, I still see the RX line receiving stuff (as can be seen on the right).

On the arduino serial terminal I get:

receive(): Received ADU: 0x01 0x05 0x00 0x00 0xFF 0x00 0x8C 0x3A
receive(): ADU CRC passed
poll(): Received request to write single coil 0x0
poll(): Writing value 0x01
send(): Sending ADU: 0x01 0x05 0x00 0x00 0xFF 0x00 0x8C 0x3A
Enable LED
receive(): Received ADU: 0x01 0x05 0x00 0x00 0xFF 0x00 0x8C 0x3A
receive(): ADU CRC passed
poll(): Received request to write single coil 0x0
poll(): Writing value 0x01
send(): Sending ADU: 0x01 0x05 0x00 0x00 0xFF 0x00 0x8C 0x3A
Enable LED
receive(): Received ADU: 0x01 0x05 0x00 0x00 0xFF 0x00 0x8C 0x3A

I dont get why I'm constantly receiving the data again, while the client is not sending anything again...

[vishnumaiea]

The server is repeatedly printing the message probably because the previous data is still in the buffer. I need to test this and I will update you after.