Historize String and Time Points

[tfranklin]

I was at a customer site a week or two back and the customer asked if it was possible to historize string and time points yet.  They've been long time users so me breaking the news that it wasn't possible wasn't really the end of the world.  That said, it would be nice if this was possible.  The use case would be for storing enumerations for values that go beyond what a 3 bit discrete can provide today.  We commonly run into situations where an analog value has to be used to bring back a value of 1-x where the value represents a first out.  We'd use ST or a calc point to push the string enumeration for what it means to a string point, but the string point is never historized.  If we ever had to go back and figure out what it was, we'd have to pair up the history with an enumeration table in order to make life easier.

[BevanWeiss]

I do like the idea, and I think for Time Points it should be relatively 'trivial'.  They should compress/delta really quite well, since they can just be considered as a 64bit integer 'tick' from epoch.

The string point history sounds quite 'dangerous' though.  I can imagine it would be quite easy for this to end up misconfigured, and for someone to be trying to store thousands of strings every few hundred milliseconds (as I often see happen with OPC analog points).

[du5tin]

This would require a pretty substantial schema change to the historian database tables since the value field is a number and has been for as long as I have been working with the product. There are also lots of ValueAsInt and ValueAsReal derived columns that would break behind the scenes.

Maybe better to log string changes to events instead? 

[sbeadle]

Thank you.

We have seen this request before, but it raises questions about string length, for example. As Dustin writes you can store and read string changes in the event log.

Time data is a problem for trends which are designed to use numbers or states.

You could use an analogue to represent a time offset (e.g. a Unix Epoch of seconds since 1/1/70, or Excel days since 1/1/1900).

[BevanWeiss]

The OSIsoft PI historian allows for 'digital state sets' to be defined, which really just maps an integer to an enumeration set (1:1 mapping between integer value and string)

That might be a good first compromise within Geo SCADA Expert also 😉

The extra items I could see would be if an analog point is configured to have an enumeration set associated with it, then in the historical trends the enumeration state text would be shown alongside each value.. so instead of it just showing '1'.. it would show '1 (Low Air Fuel Ratio Trip)' if the enumeration set had the entry

Value        State Description

1               Low Air Fuel Ratio Trip

And there would probably also want to be a new schema table which extends from the CDBHistoric, and is like CDBHistoricWithEnums which just adds an additional column which is the Enumeration State Description (for the integer value associated with that historic record)  [this could obviously be done with a JOIN.. but it seems nicer to have it builtin]