When the controller doesn't communicate an error is displayed. Unless you told the controller driver to not display any errors. By default, it will display an error such as, "2: Comm err Std 1" or something similar.
The AnalogNumeric box will not display "??" or anything like that if it is not associated with a tag. You can use a script to set the value in an AnalogNumeric box and setting its value to "??" would be very awkward when the user wished to manually set the box's value.
Beijer Electronics, Inc. | Applications Engineer