My apologies Michael (and other readers whom I may have mislead). I was indeed thinking of Hamming Codes (my grey matter is beginning to decompose). So a CRC is a more sophisticated checksum that can't simply be defeated by TWO complimentary error conditions.
I actually use a similar, but 2-byte CRC routine, that follows the ModBus CRC generation philosophy - but I've never taken the time to analyse its method - just assume that Modicon know their comms transmission stuff. I sometimes wonder WHY this should be any more robust than a simple addition or XOR - but assume it is. Still a 1 in 65k chance of an error being missed though...
Dave