Normal data collection

The Wonderware connector uses the Wonderware Historian SDK to collect history data and then transfer that data to the PI and AF servers in the form of PI points and an AF topology, respectively. To collect the data from the Wonderware Historian the connector queries the data by setting a required start time and end time for the query period. The start time for an initial query is defaulted to a value of Historian Server DateTime.Now minus 4 hours and the end time is always DateTime.Now. See Configure history recovery start time for new tags.

After the data for a Wonderware tag is queried initially, the start time is set to the latest valid update time plus one millisecond. Therefore, after the initial query, the start time depends upon the last successful query of the tag.

Data collection is also optimized so that tags with start times within two seconds of each other are grouped together. Grouping tags in this way ensures that single RPC's are not executed on single tags; rather a single RPC is used for a group of tags. Using a single RPC slightly alters the data collection so that the start time is defined to be the minimum last valid update time of each of the tags that are grouped into one query.

Additionally, to lessen the load on the Wonderware Historian, the maximum number of queries is limited to the number of logical processors on the connector’s computer minus 2. Each query is started in a separate thread from the computer's thread pool.

In summary, normal data collection uses the following query start and end times:

• The start time is the minimum last good update time of all tags grouped in the same query,

• The end time is DateTime.Now.