I have an very fast OPC-DA Item and the documentation tells me:
A very fast transfer rate is required for the measuring data. The "Data" item is notified when a modification occurs. Therefore, the "Update Rate" zero is required for the group that contains this item.
If i set the updaterate in the subscripeItem() function to zero I receive an exception. Currently I work with updaterate 1. It works but sometimes i miss data.