Be aware that there are multiple libraries that are called"Rotary". In the SDT.h header file in the T41 project, you'll see:
#include <Rotary.h> ? ? ? ? ? ? // https://github.com/brianlow/Rotary
We always place the download URL as a comment for any library that is "non-standard" to the IDE install. That is the case here. Once installed, you can look at the library's examples to see how to use it, or use the code I posted earlier.
Jack, W8TEE
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jjpurdum