Example 
An example of a C language function performing CRC generation is shown on the 
following pages. All of the possible CRC values are preloaded into two arrays, _
  which are 
simply indexed as the function increments through the message buffer. One array 
contains all of the 256 possible CRC values for the high byte of the 16-bit CRC _
  field, and 
the other array contains all of the values for the low byte. 
Indexing the CRC in this way provides faster execution than would be achieved by 
calculating a new CRC value with each new character from the message buffer. 
Note: This function performs the swapping of the high/low CRC bytes internally. _
  The 
bytes are already swapped in the CRC value that is returned from the function. _
Therefore 
the CRC value returned from the function can be directly placed into the _
message for 
transmission.

unsigned char *puchMsg ; 	
 A pointer to the message buff. containing binary data to be used for _
   generating the CRC 
unsigned short usDataLen ; The quantity of bytes in the message buffer. 
 
The function returns the CRC as a type unsigned short. CRC Generation Function
 
unsigned short CRC16(puchMsg, usDataLen)
unsigned char *puchMsg ;		/* message to calculate CRC upon */
unsigned short usDataLen ;		/* quantity of bytes in message  */
{
	unsigned char uchCRCHi = 0xFF ;	/* high CRC byte  initialized */
	unsigned char uchCRCLo = 0xFF ;	/* low CRC byte initialized  */
	unsigned uIndex ;		/* will index into CRC lookup*/
				/* table                     */
	while (usDataLen--)		/* pass through message buffer    */
		{
		uIndex = uchCRCHi ^ *puchMsg++ ;  /* calculate the CRC   */
		uchCRCHi = uchCRCLo ^ auchCRCHi[uIndex] ;
		uchCRCLo = auchCRCLo[uIndex] ;
		}
	return (uchCRCHi << 8 | uchCRCLo) ;
}

High Order Byte Table 
/* Table of CRC values for high-order byte */
 
static unsigned char auchCRCHi[] = {
0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 0x01, 0xC0, 
0x80, 0x41, 0x00, 0xC1, 0x81, 0x40, 0x01, 0xC0, 0x80, 0x41, 
0x00, .......
} ; 
 
Low Order Byte Table 
/* Table of CRC values for low-order byte */
 
static char auchCRCLo[] = {
0x00, 0xC0, 0xC1, 0x01, 0xC3, 0x03, 0x02, 0xC2, 0xC6, 0x06, 
0x07, 0xC7, 0x05, 0xC5, 0xC4, 0x04, 0xCC, 0x0C, 0x0D, 0xCD, 
0x0F, 0xCF, 0xCE, 0x0E, 0x0A, 0xCA, 0xCB, 0x0B, 0xC9, 0x09, 
0x08, ...........
} ;

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