If a PID VI receives an invalid input, such as NaN (Not a Number), the VI outputs NaN until you reinitialize the VI. However, you can use the Not A Number/Path/Refnum? function to check for invalid inputs and respond in a way that is appropriate to the application. The following block diagram uses the Not A Number/Path/Refnum? function with a Select function and a Feedback Node to reuse the last valid input when the VI receives an invalid input.

Block diagram showing fault protection using Not A Number/Path/Refnum? and Select functions.

Refer to the General PID with Fault Protection VI in the labview\examples\control\PID directory for an example of a single-channel PID implementation that includes fault protection.