OPC server data collection

Data is read from OPC servers in groups, not as individual items. PI Interface for OPC DA creates OPC groups for PI System scan classes. (For advise PI points in scan class 1, multiple groups might be created.) The OPC server caches the most recent data.

By default, PI Interface for OPC DA reads from the cache of an OPC server. When the interface creates a group, it specifies how often the cache values for the points in that group are to be updated. The requested update rate is usually the same as the scan rate for the points. The OPC server might decline the requested rate and return the update rate that it supports for that group. The update rate to which the OPC server agrees is recorded in the local PI Message Log file.