Scan class attribute (location4)

The location4 attribute configures the scan class for the PI point. The scan class determines the frequency at which input tags are scanned for new values.

For trigger-based points, set location4 to zero. For all other points, the location4 attribute must be a positive number, representing the scan class number (scan class #) to which the PI Point should be assigned. The configuration of the scan classes is done on the interface itself.

For output tags, location4 configures the output class. When necessary for load balancing, the interface distributes tags in scan class 1 across multiple OPC groups. Scan classes other than scan class 1 are assigned to separate groups for load balancing.

You can assign multiple groups to the same scan classes for PI Interface for OPC DA. For each scan class (with the exception of scan class 1), you can assign at most one polled group, one advise group, and one output group. The limits are as follows:

1 Refers to the actual maximum number of groups and not the maximum number of tags per group.

Note: We do not recommend mixing two groups within a single scan class. For scan class and grouping guidelines, see Knowledge Article 000001178 (https://customers.osisoft.com/s/knowledgearticle?knowledgeArticleUrl=2700OSI8)

The OPC standard does not guarantee that it can scan data at the rate that you specify for a scan class. If the OPC server does not support the requested scan frequency, the frequency assigned to the class is logged in the pipc.log file. If the interface workload is heavy, scans can occur late or be skipped. For more information on skipped scans, see the PI Universal Interface (UniInt) User Guide.

For more information about setting the scan class, scan offset, and update rate, see Scan class settings and update rates.