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50 CRC16 Type Function returning UnsignedInteger Invocation CRC16(data, count, crcPoly, crcInit, crcFlags) Parameter Method Type Description data ByRef anyType The data bytes to add to the CRC value. count ByVal integral The number of bytes to process. crcPoly ByVal UnsignedInteger The CRC polynomial to use. crcInit ByVal UnsignedInteger The initial value of the CRC. crcFlags ByVal integral Flag bits that control the CRC computation. Discussion This function computes the CRC value over a number of data bytes using a specified polynomial and initial value. The values to use for the polynomial and the initial value depend on the style of CRC that you need to generate. See the discussion below for further details. The flags parameter contains bits that control aspects of the CRC computation as described in the table below. Flag Values for the CRC Compuation Constant Hex Binary Description zxCRCRefIn &H01 xxxx xxx1 Each input data bytes will be “reflected”. zxCRCRefOut &H02 xxxx xx1x The final CRC value will be “reflected”. The remaining bits are reserved for future use and should always be zero. In this context, the term “reflection” refers to reversing the order of the bits in a data item so that the most significant becomes the least significant and vice versa. For a multi-byte data item, the bits in each byte are reversed and the order of the bytes is reversed as well. Although this function will typically be used to compute the CRC value for an entire block of data at once, it may also be used in a byte-by-byte or data burst mode. To do so, you would pass the computed CRC value from the previous iteration as the initial value. Note, however, that you shouldn’t use the zxRefOut flag bit in this case. Rather, if you need reflected output you would perform the bit reversal on the final CRC value when you reach the end of the data stream. You can reverse the bit order of a 16-bit value by using the following code fragment. crc = MakeWord(FlipBits(HiByte(crc)), FlipBits(LoByte(crc))) CRC algorithms can be described by a parametric model known as the RockSoft model (see http://www.repairfaq.org/filipg/LINK/F_crc_v34.html#CRCV_005). This CRC implementation supports the POLY, INIT, REFIN and REFOUT parameters of the model with WIDTH=16 and XOROUT=0. If necessary, you can easily implement a non-zero XOROUT parameter by using the following code fragment. crc = crc Xor XorOutValue The Rocksoft model parameters for commonly used CRC computations are given in the table below.