OPC Servers that lose communication with their data source (such as a PLC, DCS, Analyzer, etc), should indicate that the process values now have "Bad Quality". In addition, they should also indicate the reason for this "Bad Quality". For example, an OPC Server can indicate the Quality value is "Bad" because it lost communication with the data source, or the item in question is "out of range" or there was a "Sensor Failure", etc.
These quality values can clearly indicate that the loss of communication was due to a failure between the OPC Server and its data source, rather than a communication problem between the OPC Client and OPC Server. Vendors can choose to implement this type of diagnosis in their OPC Server or not. Therefore, some OPC Servers provide no diagnosis at all.
The ability to diagnose problems and pass the results in a Quality parameter is an important feature that will help you to differentiate between various vendors. OPC specifications provide a list of possible error values.