I've written a complete module for the 990 its posted in crestron forum on yahoo groups. To answer your question, if I remember correctly the checksum is only one byte, which means you drop the overflow. In other words if your calc produces a 123 sum, the checksum value is 23. Please note there is an error in the documentation on the checksum the 990 sends back to you, so if your checking it, you'll need to look at the data it sends and verify it. I can't remember if it included one more byte or it containted one less byte. Just look at both ways and you should be able to determine it quickly, or you can look at my module at groups.yahoo.com and search for crestron.