word CalcCRC (word crc, byte ser_data) {
// Update the CRC for transmitted and received data using
// the CCITT 16bit algorithm (X^16 + X^12 + X^5 + 1).

    crc  = (unsigned char)(crc >> 8) | (crc << 8);
    crc ^= ser_data;
    crc ^= (unsigned char)(crc & 0xff) >> 4;
    crc ^= (crc << 8) << 4;
    crc ^= ((crc & 0xff) << 4) << 1;
	return crc;
}