The poll rate determines how often the OPC Client polls for data from the OPC server’s data cache for the specified group’s items. All items in the group are polled at the same time. The update rate sets how often the OPC server polls the process hardware to update the server’s internal data cache for all the items in the specified group.
Setting the Poll Rate
Selecting a poll rate to use depends on whether you are using asynchronous or synchronous I/O.
Poll Rate and Asynchronous I/O
If you are using asynchronous I/O, we recommend setting the poll rate to 5 minutes (5:00). Also select the Enable Asynchronous Watchdog check box so that the client can periodically request data from the server. If you do not select this check box, the client must rely on asynchronous callbacks from the OPC server in order to refresh its data cache.
Poll Rate and Synchronous I/O
If you are using synchronous I/O, set the item’s poll rate shorter than your scan time. For example, if you have a database block with a scan time of 10 seconds, set the poll rate for the item it accesses to 5 seconds. Likewise, set the update rate to be shorter than the scan time and the poll rate.
Keep in mind that if you set poll and update rates for groups to values that are shorter than needed, you impose unnecessary stress on your system.
Example
- Database block scan time: 10 seconds
- Poll Rate: 5 seconds
- Update Rate: 2 seconds
Using a Zero Poll or Update Rate
Setting the poll rate to zero forces the driver to run at its maximum rate.
Setting the update rate to zero requests the OPC server to update its group at its maximum rate. The OPC server will return the actual, or revised, rate that it can run at. This revised rate is displayed in the Actual Rate field on the group’s statistics display in the OPC Client’s Power Tool.
See Also
Group Statistics
