Configure Confidence Polling
You can use Confidence Polling to configure additional polling to avoid data becoming stale by forcing points to be updated periodically. You can do this by enabling and configuring the Confidence Polling properties on the configuration Forms of either OPC UA Subscription(s) or individual OPC UA Points. If you configure confidence polling on an OPC UA Subscription, that polling will apply to all active points that are associated with that OPC UA Subscription database item.
If confidence polling is configured for both an OPC UA Subscription and some or all of its points, it is possible that two polls may be scheduled to occur simultaneously. We suggest that you configure the settings to try to avoid duplication of data polls in this situation (see below).
On any of the above configuration Forms, you can configure Confidence Polling using these properties:
-
Enabled—Use to specify whether Confidence Polling is enabled (check box selected) or disabled (check box clear). When enabled, other properties in the Confidence Polling section become available for use. Confidence Polling is disabled by default.
-
Interval—Use to define the confidence polling frequency. Enter the required interval in the OPC Time Format. You can enter the value directly in the field, or use the Interval window (accessed via the field's browse button) to specify the required value.
-
Offset—Use to specify the actual time at which polling takes place. Enter the required offset in the OPC Relative Time Format. You can enter the value directly in the field, or use the Offset window (accessed via the field's browse button) to specify the required value.
-
Source—Use to specify the location from which the data is to be obtained. Choose from:
-
Cache—Use to read the data from the OPC UA server's cache. Access to the cache data on the OPC UA server is expected to be 'fast' and should contain the latest value of the data (this depends on the point's sampling interval (update rate) and deadband optimizations) as well as the quality and timestamp. The timestamp indicates the time that the server updated or validated the value and quality in its cache.
-
Device—Use to read the data from the device (rather than from the OPC UA server itself). Accessing data from the device data is usually slower than from the cache and is expected to be used primarily for diagnostics or for particularly critical operations.
-
Either Cache or Device—Select this option for the OPC UA server to determine whether the data is obtained from the cache or the device. If you select this option, you also need to configure the Maximum Age (see below).
-
-
Maximum Age—This property only applies when the Source is set to 'Either Cache or Device'. Use to specify the maximum acceptable age of the data. Enter the required interval in the OPC Time Format. You can enter the value directly in the field, or use the Interval window (accessed via the field's browse button) to specify the required value.
The OPC UA server checks the timestamp on each item of data. Where the data for an item has been updated within the Maximum Age interval, the data is obtained from the cache. If the timestamp is not within the Maximum Age interval, the data is obtained from the device.
The confidence polling Interval for an OPC UA Subscription is set at 5 minutes, and the confidence polling Interval for one of the points that is associated with that Subscription is set at 1 minute. This setup is intended to result in confidence polls occurring at 1-minute intervals for the point, and at 5-minute intervals for the Subscription.
The polling activity that occurs every 5th minute (when 2 simultaneous polls are potentially set to occur; one for the Subscription and another for the Point) depends on the configuration of the Source and Maximum Age properties:
-
If the Subscription and Point have the same Source and Maximum Age settings, only one poll will be performed every 5th minute; the poll for the Subscription. (The OPC UA Server ignores the request to separately poll the Point at the same time, as that poll coincides with the one for the Subscription that will also include the data for that Point.)
-
If the Source and Maximum Age settings differ between the Subscription and Point, two polls will be performed every 5th minute; one for the Subscription and another for the Point. This duplication of polling for data is less efficient than the single poll performed above.