Understanding virtual fields

RealTime virtual fields are field specifications for data that does not reside directly in the RealTime database (RTDB).

In PI Tag configuration, there is a field called Location2. This field indicates the collection methods that the PI-OASyS interface uses to retrieve data from the AVEVA Enterprise SCADA data source. One of these collection methods is called (Location2 = 1), and should be used if a PI tag stores data from an RTDB point’s virtual field. The virtual fields are supported through table virtual access layer functions such as v_fldget and v_fldput. Without using the virtual layer function, the PI Tag may get empty or unexpected values.

Other Location2 configurations may work, but are highly dependent on the virtual field in question. Methods that use PubSub (Location2 = 13,14,15) may work, but the value update may be delayed compared to (Location2 = 1) since, for performance reasons, not all fields are published as soon as the value changes. Since some fields may never be published, PubSub Location2 methods may work once but never update afterwards. Methods that read directly from the RTDB [(Location2 = 11,12)] may work, but can return unexpected values. You can use the DALMultiSubscriber tool to troubleshoot if a field can be used with a PubSub Location2 method. When troubleshooting, (Location2 = 1) is the preferred method.

For information on Location2 values, and how to set them, refer to the following sections:

• Methods of retrieving data from AVEVA Enterprise SCADA

• Change the PI tag configuration

AVEVA Enterprise SCADA does not have a tool that allows you to see if a field is virtual or not. The easiest way to determine this is by changing the PI Tag configuration if you are getting empty or unexpected values for a field.