In Sum.vi you set the Execution option Run when opened which is not suitable for a subvi. Remove it!
Second, in the Window Appearance properties, customize and set Show front panel when called and Close afterwards if originally closed, otherwise the front panel will not show up when Password calls Sum.
Third, as you already know how to use event structures, put your sum in an event triggered when either x or y are changed. If you want to stick polling values, don't do it at the maximum speed: you don't need to compute the sum every microsecond! Put a wait into the loop instead: a 100 ms wait should be ok, but even a 0 ms wait would help. Interactive while loops without a wait tend to suck too much processor time.
Paolo
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