Refnum, in simplist terms, is the 'address/reference' to a particular file. If you have a 'Null Refnum' message, you probably have lost the reference number to a file. The refnum is only valid while the file is open, once it closes, the refnum goes away too. Make sure your file is where your VI thinks it should be; you may need to backtrack through the block diagram(s)to confirm such, including using the Highlight Execution function.
Good Luck, Doug
