Real-time data collection

During real-time data collection, the interface reads events from the source Data Archive and sends them to the target server. There are two options for determining where the source data comes from:

• Archive data

  This data has passed exception testing by whatever interface accepted the data, and has passed compression testing on the source server. However, reading data from the Data Archive increases the workload on the source server, because the data is read from disk.

• Exception data

  This data has passed exception testing by the interface that accepted it, but has not passed compression testing by the source server. Reading exception data imposes less workload on the source server, because the data resides in memory. To ensure the closest correspondence between values on the source and target servers, you should configure identical compression settings on the source and target servers. To configure a PI point for exception data collection, you should assign it to scan class 1. Exception data collection cannot be used for future tags.

The source Data Archive provides a time stamp for each data event. The interface might adjust time stamps to account for time zone differences and clock drift between Data Archives. Each Data Archive must be configured with the correct time for its time zone. For individual points, you can configure time stamp adjustment by setting the Location2 point attribute.

Target point definitions can be modified while the interface is running. The interface checks with the target Data Archive for point configuration updates, every two minutes by default. These updates include adding, editing, and removing points. Processing point updates is a low priority for the interface. To prevent update processing from delaying data collection, the interface processes a maximum of 25 point updates at a time. If there are more than 25 point updates, the interface checks every 30 seconds until all updates have been processed. If a large number of point edits are made, consider stopping and restarting the interface, to force it to process all edits by rebuilding its point list.