The RTOS used by LVRT can call DLLs. I'm not sure if there are any special requirements, but I am fairly sure that they will destroy the real-time part of your app, because you take the ability to manage this out of the OS's control. Then again, I could be wrong. Maybe if your code is threadsafe and you call it properly it will work fine. I haven't done this myself, anyway.
You can find a document about calling external code in LV by clicking Help>>Search the LabVIEW Bookshelf. You can probably also find some reference by searching the LVRT bookshelf. You can also search the DevZone area of this site for many documents.
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