A simple parity value calculated on columns of data rather than rows, were each successive set of data bit is XOR'd with the prior set to form a new set of parity values.
LRC = LRC ^ dataByte;
ormovf dataByte,w xorwf LRC
LRC's by themselves, can only detect errors, but have a slight advantage over standard parity bits in that they are less sensitive to burst errors in serially transmitted data. For example with a drop-out burst error of 3 bits starting after the first bit of the first byte:
10110001 even parity = 0 10000001 even parity = 0 (still) 11011011 even parity = 1 11011011 even parity = 1 -------- -------- 01101010 = LRC 01011010 = LRC
Note that if the data were being transmitted in parallel, the advantage reverts to simple parity, as the LRC becomes sensitive to burst errors.
An 8-bit LRC such as this is equivalent to a CRC using the polynomial x8+1
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<A HREF="http://techref.massmind.org/techref/method/error/lrc.htm"> Longitudinal Redundancy Check Data Link Error Detection / Correction Method </A>
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