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6739389EN/FB - Page 17
/***************************************************************************************************** */
/* FUNCTION CALL: crc = crc16 (message, length); */
/* with char *message; message = address of message */
/* */
/* int length; length of received message (including CRC) */
/* expressed in number of bytes */
/* int crc; = CRC16 calculated from the "address", "code" and */
/* "information" fields. */
/* */
/* RETURN VALUE: calculated crc 16 (int crc) */
/******************************************************************************************************/
int crc16 (message, length)
unsigned char message [ ]; /* buffer containing message */
/* for which the crc16 is to be calculated. */
int length; /* length of message to be checked */
/* (including crc16) */
{ /* beginning of the function */
int i ; /* loop variable */
int j ; /* calculation and displacement variable */
union { /* calculated crc16: */
int ival ; /* - whole */
unsigned char cval [ 2 ]; /* - table of 2 characters */
/* unsigned is important since otherwise the sign */
/* extension causes negative displacements with */
/* respect to the beginning of the table */
} crcal;
/*$ initialization of calculated crc */
crcal.ival = 0XFFFF;
i = 0;
/*$ correction of the length to be checked: remove the */
/*$ received crc16 from the length to be checked */
length = length - 2;
/*$ WHILE there are bytes to be checked DO */
while ( i < length )
{
/*$ calculate the table index */
j = (int) ( message [ i ] ^ crcal.cval [ CPH ] );
/*$ most significant byte */
crcal.cval [ CPH ] = tbcrch [ j ] ^ crcal.cval [ CPL ];
/*$ least significant byte */
crcal.cval [ CPL ] = tbcrcl [ j ];
/*$ next byte */
i++;
}; /*$ END WHILE there are bytes to be checked */
/*$ return the calculated crc */
return ( crcal.ival );
} /* end of function */
Note: if the CRC16 is computed
using table lookup, the most
significant byte is transmitted first.
JBUS protocol (continued)