From OpenSCADAWiki
The tab "Diagnostic" (Fig.4.5j) contains form of the diagnostic messages of the data source. Many data sources are external devices with accessing to the data by a network connection or a system bus then there possible different emergency situations on access to the data. Display all their to the field "State" of the controller object impossible due it displays only current access data state. Due the diagnostic you can trace all the emergency situations at view of messages, which forming by the controller object for the specified range of time and by the selected messages level. Besides to certain diagnostic information some modules of the data sources can provide here debug exchange dumps on the messages level "Debug (0)".