Thanks for the reply. It gave me the hints I needed to solve my problem. I could not change how the checksum works, I'm trying to modify an existing system. What I ended up noticing is that the word following the checksum is its complement. If I replace the checksum and its complement with any two words that sum to FFFF (IE. FFFF 0000, or 7F7F 8080) and then calculate the checksum, the resulting value and its complement wouldn't change the calculated checksum when substituted back in.