Hi all,
I've tried to find the answer of my question in many places (this forum, vCampus, techsupport website, etc.) but couldn't find it.
I would like to know exactly what's going on when the PI System decided to replace the snapshot value when applying the compression algorithm.
What I understand is for a PI point on which compression is enabled, the System consider an archived value, the current snapshot value and is waiting for the next value (sent by an Interface).
When the system received this new value, it applies the compression algorithm.If this new value is within the acceptable range, then the current snapshot value is simply discarded and the new value becomes the snapshot value.
What I want to know exactly is : when is the "old" snapshot value discarded/deleted ?
I think that this value is discarded before the new value is considered as the new snapshot value.
I'm asking because I've configured a notification with PI Notifications that check the last archived value for a point and if the last event is 10 minutes old or more, it means that there is a problem with my interface. It happened so many times that my OPC Server wasn't communicating with the controller, but continued to respond to the PI OPC Interface requests. The only way I found to detect this problem is with the notification described above, using the controller's minute as the source tag. I've enabled compression to reduce archived value, so I'm expecting only 0's and 59's will be archived.
However, sometimes, I receive emails from this notification even I know that there are no problems with the OPC server or the Interface. I'm guessing it's only a question of timing (the time the old snapshot value is discarded, PI notifications check for the last archived value then the new snapshot value is updated.
What do you think about that ?