Please see the attached file
Starting from a blank VI, I:
1. Created a while loop
2. Inserted a signal generator and accepted the default values
3. Copy and pasted that signal generator in the while loop
4. Inserted a Compound Arithmetic express VI
5. Wired the first signal generator to the Compound Arithmetic VI (wire was fine)
6. Wired the 2nd signal generator to the Compound Arithmetic VI and got the error that the source and the sink are two different types
I don't get any errors when using the ADD express VI. Using a pyramid of ADD VI's isn't a good solution, especially when I'll have 8 or more inputs that need to be aded.