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According to the increasing number of questions about this communication protocol, it would be time to rewrite the MODBUS library. I also suggest to add it to the NI device drivers installer.
This could be the place to list the expected modifications. Some comments and bugs are already listed in above linked page.
make sure you are logged into the NI community which is different from the ni forums and the ni.com page. There should be a gray bar near the top that says you are not logged in.
To clarify, neither option is currently officially supported. I try to help out where I can on the new library, but the old one is just a developer zone document.