It is possible to debug a library in the TIA Portal but there are some restrictions.
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FCs cannot be debugged, although it is possible to perform a certain amount of debugging indirectly by writing the values you want to see to an instance DB. You can do this by, for example, passing a parameter instance to the FC and then writing the values to the parameter instance. You can then monitor the values as shown in the following.
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FBs cannot be debugged, but it is possible to monitor the values of instance DBs of these FBs. Simply go online, double-click on the instance DB and then click on the icon to start monitoring.
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Modifying operands is possible as long as the instance in which the value is present is not representing a class in AX. If it is, you can only watch the values and not change them.