Calculate checksum on received and transmitted ASCII string
Example C code program
//INT8U is an 8 bit unsigned integer.
INT8U itAscRecBuf[82];
//ASCII receive character buffer
INT8U AscHexToBin(INT8U, INT8U *);
//ASCII hex to binary conversion
INT8U AsciiToHex( INT8U);
//Ascii to Hex conversion
//Calculate checksum on a received ASCII string, return checksum value.
//It should equal 0 if good.
INT8U CalcCheckSum(void)
{
 INT8U i,length, cc;
 length = AscHexToBin(2, &itAscRecBuf[0]);
 //get length value,
 //first two characters
 cc = AscHexToBin(2, &itAscRecBuf[length]);
 //get checksum value
 //at end of string.
 for (i=0;i<length ;i++ )
 {
   cc += itAscRecBuf[i];
     //get string value and add it to
     //checksum
 }
 return(cc);
 //good checksum should equal 0
}

//ascii hex to binary, width 1 or 2
INT8U AscHexToBin(INT8U Width, INT8U * DataPtr)
{ INT8U aVal;
// accumulated value
aVal = AsciiToHex(*DataPtr);
 DataPtr++;
 if (Width == 2)//two digits wide, else 1 digit wide
 {
   aVal = aVal << 4;
     aVal += AsciiToHex(*DataPtr);
 }
 return(aVal);
}

//Ascii to Hex conversion
INT8U AsciiToHex( INT8U Value )
{
 switch ( Value )
 {
     case 'A':
        return( 10 );
     case 'B':
        return( 11 );
     case 'C':
        return( 12 );
     case 'D':
        return( 13 );
     case 'E':
        return( 14 );
     case 'F':
        return( 15 );
     default:
        return( Value - 0x30 );
  }
}

/* **************************************************************
To generate the checksum for an ASCII string to be transmitted, clear the checksum value (CC = 0;)
Add each byte of the string to be transmitted to the checksum value
(CC += ASCII byte).
Do a two’s compliment of the checksum (CC = (CC ^ 0xff) + 1;).
Convert the checksum’s upper and lower nibble’s to ASCII hex.
Send a carriage return (0x0D) and line feed (0x0A).
The following is an example C code for sending the checksum after building the initial checksum value.
*/

//send checksum + carriage return, line feed
//Comm2_Put_Ch() sends the byte out the serial data port.
INT8U NibToAsc(INT8U);
void SendChecksum(INT8U CC)
//enter with checksum value added up
{
  CC = (CC ^ 0xFF) + 1;
    //calculate two's compliment
    Comm2_Put_Ch(NibToAsc(CC >> 4));
    Comm2_Put_Ch(NibToAsc(CC & 0x0F));
    Comm2_Put_Ch(0x0D);
    Comm2_Put_Ch(0x0A);
}

INT8U NibToAsc(INT8U Nib)
{
  if (Nib < 0x0A)
      return( (INT8U)(Nib + 0x30));
    else if (Nib <= 0x0F)
      return(Nib + 0x37);
        //converts to 0x0A -> 0x0F hex value

    return(0x20); //error
}