Interfacing Modbus RTU with PIC

Replies

• 

  shahrul

  Now, I see the pattern.
  
  Example read holding register.
  Request protocol:
  

  Slave ID: 1 byte
  Function code: 1 byte
  Starting Address: 2 bytes
  Quantity of address: 2 bytes
  CRC-16: 2 bytes

  Response protocol:
  

  Slave ID: 1 byte
  Function code: 1 byte
  Byte count: 1 byte
  Register value: (2*Quantity of address) bytes
  CRC-16: 2 bytes

  This is the example Tx and Rx data
  

  Tx: 01 03 00 00 00 03 05 CB
  Rx: 01 03 06 00 01 00 01 00 04 4C B6
  

  Last 2 bytes for Tx and Rx is CRC-16 checking. My further question is, how to calculate CRC-16?
• 

  shahrul

  Now, I get the CRC-16 function.
  Example
  

  unsigned int data[]={0x01,0x03,0x00,0x00,0x00,0x03};
  
  int CRC16(int DataLength){
  unsigned int i,j,CheckSum;
  CheckSum=0xFFFF;
  for (j=0;j0;i--){
          if((CheckSum)&0x0001==1) CheckSum=(CheckSum>>1)^0xA001;
          else CheckSum=CheckSum>>1;}}
  CRC_high=(CheckSum>>8)&0x00FF;    
  CRC_low=CheckSum&0x00FF;
  CheckSum=((CheckSum>>8)&0x00FF)|((CheckSum<<8)&0xFF00);
  return CheckSum;
  }
  

  will return value CRC=0x05CB
• 

  shahrul

  Now, I have problem on parity bit. I have done 8bit UART, that is NONE PARITY.
  The parity bit is setup with 9th bit. I try send this, but it fail at certain data.
  

  response[0]=0x01;
  response[1]=0x03;
  response[2]=0x06;
  response[3]=0x00;
  response[4]=0x00;
  response[5]=0x00;
  response[6]=0x01;
  response[7]=0x00;
  response[8]=0x02;
  CRC16(9);
  response[9]=CRC_low;
  response[10]=CRC_high;}
  
  for(i=0;i<=10;i++){
  TX9D=even_parity(response);
  uart_transmit(response);}

• 

  Ashraf HZ

  This is quite informative stuff. Keep us updated on your progress 😀