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The System digital state set is intended to provide system wide 'error states'. Digital tags are usually configured to reference their own state sets which provide the 'normal' values for these tags. As you know, many digital tags can reference the same state set, but a digital tag can only reference one digital set. It's not standard practice to map a digital tag for any interface to the System state set. Some interfaces however, may require specific states to be added to the System state set to provide a specific set of error states that may be received from the data source. Other interfaces (like the RDBMS interface for example) have configuration options that allow you to specify a range within the System digital state set for error codes that can be written to the interface tags. System digital states are also written to analogue tags as error states, such as 'I/O Timeout' or 'Intf Shut' etc.
In answer to your other questions, there is no easy way to tell which interfaces (or more specifically, their tags) are mapped to which digital sets. You could do an advanced tag search where you specify the point type as digital, and the digitalset attribute equal to a specific state set name. Alternatively, just search for all digital tags in PI Builder, and then sort the returned list by the digitalset attribute. Otherwise, there is really nothing that exposes a mapping between digital tags and digital state sets. As for documentation, the System Management Guide is probably the best source of information, as well as the online documentation here.
Hope this helps.
That definitely makes a bit more sense.
Thanks for the link to that documentation - I havent found my way to the Live Library before.
As always much appreciated for your time to respond
Any time Calman!