Server does not work when using different protocols

[HWHardsoft]

I encountered a problem when I mixed setting coil registers and querying holding registers in a program. It works 95% of the time, but occasionally the holding register cannot be set. When I only query holding registers or only set coil registers, everything works great.
It doesn't seem to be the client's fault either. When I analyze the data sent by the client, it seems that the correct protocols are always being sent. It appears that the server occasionally does not respond because it detects a CRC error.

#include <CSE_ArduinoRS485.h>
#include <CSE_ModbusRTU.h>


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

const int ledPin = 2;
int counter = 0;

// Declare the RS485 interface here with a hardware serial port.
// auto send/receive: Dip Switch  S1: OFF - ON - ON - OFF
RS485Class RS485 (Serial2, -1, -1, -1); // (Serial Port, DE, RE, TX) 
//RS485Class RS485 (Serial1, 6, 6, 1); // (Serial Port, DE, RE, TX) 

// Create a Modbus RTU node instance with the RS485 interface.
CSE_ModbusRTU modbusRTU (&RS485, 0x01, "modbusRTU-0x01"); // (RS-485 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)

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

void setup() {
  // Initialize the default serial port for debug messages
  Serial.begin (115200);
  delay (1000);
  Serial.println ("Modbus RTU Server with Arduino UNO R4");

  // Initialize the RS485 interface. If you are initializing the RS485 interface
  // manually, then the parameter can be empty.
  Serial2.setPins(16,17); // pins of U5 
  RS485.begin (9600,SERIAL_8N1);

  // Initialize the Modbus RTU server
  modbusRTUServer.begin();

  // Configure the LED
  pinMode (ledPin, OUTPUT);
  digitalWrite (ledPin, LOW);


  // Configure four coils starting at address 0x00
  // build-in LED is coil 0x00
  modbusRTUServer.configureCoils (0x00, 4);

  // Configure four holding registers
  // status of build-in LED is holding register  0x00 
  // status of counter is holding register  0x01
  modbusRTUServer.configureHoldingRegisters (0x00, 4);

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

}

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

void loop() {
  // increment counter
  counter++;

  // holding registers
  // save adc values 
  modbusRTUServer.writeHoldingRegister(0x00,analogRead(2));
  modbusRTUServer.writeHoldingRegister(0x01,analogRead(4));
  modbusRTUServer.writeHoldingRegister(0x02,analogRead(35));

  // save counter value
  modbusRTUServer.writeHoldingRegister(0x03,counter);
  
  // Poll for Modbus RTU requests
  int requestReceived = modbusRTUServer.poll(); 

  // proceed Modbus
  if ((requestReceived != -1) && (requestReceived < 0x80)) {
    Serial.println ("Request received");

    // Read the current value of the coil
    int coilValue = modbusRTUServer.readCoil (0x00);

    // set led according coilValue  
    if (coilValue != 0) {
      // Coil value set, turn LED on
      digitalWrite (ledPin, HIGH);
      
    } else if (coilValue == 0) {
      // Coil value clear, turn LED off
      digitalWrite (ledPin, LOW);
    
    }
  }
}

[HWHardsoft]

Addendum: It doesn't matter whether I mix the protocols. The problem always occurs, even if I only read holding registers in the client. Apparently, this is a timing problem or an overflow of a variable for the protocol length.

receive(): Received ADU: 0x01 0x03 0x00 0x00 0x00 0x04 0x44 0x09
receive(): ADU CRC passed
poll(): Received request to read holding registers 0x0 to 0x3
Address: 0x0, Value: 0x132
Address: 0x1, Value: 0xD2
Address: 0x2, Value: 0x107
Address: 0x3, Value: 0xB432
setCRC(): CRC is 0x22B9
send(): Sending ADU: 0x01 0x03 0x08 0x01 0x32 0x00 0xD2 0x01 0x07 0xB4 0x32 0xB9 0x22
Request received
**receive(): Received ADU: 0x01
checkCRC(): Error - ADU length is less than 3.
receive(): ADU CRC failed
receive(): Received ADU: 0x03 0x00 0x00 0x00 0x04 0x44 0x09
checkCRC(): Error - CRCs do not match. Found: 0x449, Calculated: 0xC361
receive(): ADU CRC failed**
receive(): Received ADU: 0x01 0x03 0x00 0x00 0x00 0x04 0x44 0x09
receive(): ADU CRC passed
poll(): Received request to read holding registers 0x0 to 0x3

[vishnumaiea]

If it happens occasionally and can not be reproduced consistently, then it might be a data loss or corruption issue in UART or serial lines. You can try reducing the serial baud and check if the issue persists. You can also try increasing the baud and see if the issue happens more frequently.

Meanwhile I will check if there is a possibility in the code for something like this to happen.