MODBUS RTU CRC errors

Projects Networking, Protocols, and Devices
1

Hello,

I'm using the modbus master library direct from the IDE (ModbusMaster.h)

I'm reading registers of a device which works pretty fine.
But sometimes the slave shows an CRC error. So i sniffed the communication with "device monitoring studio" and see that the arduino sometimes send a request beginning with a lot of zeroes:

 01 03 10 01 00 06 90 C8  <--- Good request
 01 03 10 01 00 06 90 C8  <--- Good request
 01 03 10 01 00 06 90 C8  <--- Good request
....
00 00 00 00 00 00 00 00 01 03 10 01 00 06 90 C8   <--- Bad request with CRC error

But all request are fired from the same function.
I don't understand why it sends the request sometimes with 8 nulled words.
This are the functions:

void readModbusRegister()
{

  ecu.readHoldingRegisters(4097, 6); //read 6 registers beginning from offest 4097

  if (result == ecu.ku8MBSuccess)  //if transmission is good, decode Data
  {
    decodeModbusRegister();

  }

}


void decodeModbusRegister()  //read single word registers and write into data array
{

  for (j = 0; j < 6; j++)
  {
    data[j] = ecu.getResponseBuffer(j);
  }

  printValues(); //after decoding, print data
}

Can it be old data in the serialbuffer?

regards
Bastian

2

So i sniffed the communication with "device monitoring studio"

What kind of tool is that?

What hardware do you use for the transmission of the ModBus protocol. ModBus RTU is standardized over RS-485. Do you use such hardware? I guess you have noise on the line, that's why the ModBus standard uses always a parity bit (even or odd) but I guess you don't use that, are you?

Can it be old data in the serialbuffer?

No.