Lets assume that your LabVIEW loop is running slower than your Arduino Loop. In that case LabVIEW will read the entire serial output you posted above. If you pass this into the case structure it will not patch 'p' because you are passing in many many characters, not just the 'P'.
First try creating an indicator on the output of the VISA read to see exactly what you are getting from the Arduino. Once you know you are getting the correct output from the Arduino in LabVIEW you can work on using LabVIEW string fucntions to parse the data.
Let me know if that makes sense or if you have more questions.
Thanks!
-Sam K
LabVIEW Hacker
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