I have a similar issue. In my case, the customer is using VISA for a completely unrelated product that barely works. I can't risk making any changes to the VISA configuration, or I will probably prevent the other program from working.
I downloaded NI-Serial, and started the install. But it turned out, it was already installed. Not that it helps any (so far). I can't find any functions (using Search: serial) that relate to serial without being VISA.
I hate to download code from unknown sources, but I guess that's my only option. Thanks for the work, whoever wrote it! (and for posting the link)
___________________
CLD, CPI; User since rev 8.6.