The " swinging door compression" method used by the archive allows PI to keep orders of magnitude more data on-line than conventional scanned systems. Swinging door compression discards values that fall within a threshold of a line connecting a recent time series of values that are recorded in the archive. When a new value is received by the Snapshot Subsystem, the previous value is recorded only if any of the values since the last recorded value do not fall within the compression deviation blanket (the threshold mentioned earlier). The deviation blanket is a parallelogram extending between the last recorded value and the new value with a width equal to twice the compression deviation specification. Data stored using this method is much more detailed (and therefore more useful) than in an archiving system based on averages or periodic samples.
Point Database Subsystem
For each process value to be tracked, a PI System point is defined. Each point has approximately 50 attributes. These attributes define how data is to be collected for the point and also how the data are to be stored.
In the PI System, the most recent value for each point is known as the snapshot. The Snapshot Subsystem evaluates the snapshot value to determine if it should be sent to the Archive Subsystem. The Snapshot Subsystem makes the snapshot values available to users and applications as needed.
Update Manager Subsystem
The Update Manager Subsystem sends notifications of changes in values to other applications.
PI Network Manager
The PI Network Manager provides the connection among PI subsystems residing on the PI Home node. PI Network Manager also manages network connections between the PI System and client applications. The clients may be standard products such as PI-ProcessBook, or they may be user-written PI Application Programming Interface (PI-API) programs. The PI Network Manager manages PI System security by validating clients when connections are made.
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