YASKAWA Varispeed G7, Modbus Communication Manual

Page: 31 / 92

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CRC-16 Calculation Example in C
void getMBCRC(char *, int, char *) // function prototype
void getMBCRC(char *buf, int bufLen, char *crc) { // Function name and parameter list returning a void
// *buf pointer to character array used to calculate CRC
// bufLen number of characters to calculate CRC for
// *crc pointer to the array that contains the calculated CRC
unsigned long crc_0 = 0xffff; // Declare and initialize variables
unsigned long crc_1 = 0x0000; // Declare and initialize variables
int i,j; // Declare and initialize variables
for (i=0; i<bufLen; i++) { // Loop through characters of input array
crc_0 ^= ((unsigned long)buf[i] & 0x00ff); // XOR current character with 0x00ff
for (j=0;j<8;j++) { // Loop through characters bits
crc_1 = (crc_0 >> 1) & 0x7fff; // shift result right one place and store
if (crc_0 & 0x0001) // if pre-shifted value bit 0 is set
crc_0 = (crc_1 ^ 0xa001); // XOR the shifted value with 0xa001
else // if pre-shifted value bit 0 is not set
crc_0 = crc_1; // set the pre-shifted value equal to the shifted value
} // End for loop - Loop through characters bits
} // End for loop - Loop through characters of input array
crc[0] = (unsigned char)((crc_0/256) & 0x00ff); // Hi byte
crc[1] = (unsigned char)(crc_0 & 0x00ff); // Lo byte
return; // Return to calling function
} // End of CRC calculation function