How does the interface handle ExcMax for advise points?

When the OPC server sends a new value to the interface for an advise point, the interface passes this value along to the Data Archive and stores it in memory along with its timestamp. If no new value arrives by the time ExcMax has elapsed, it sends the value again using a timestamp equal to the previous timestamp plus ExcMax.

Note that this exception value does not represent an actual interaction with the data source. This is important because in some circumstances, the value can be incorrect. For example, consider a case where scan class 1, reserved for advise points, has a scan time of 10 seconds. By default, the interface configures the update rate on the OPC server to be equal to this scan time. In this case, the OPC server would poll the PLC it reads this value from every 10 seconds.

Suppose an advise point that reads this value has ExcMax set to 5 seconds. The interface receives a value from the OPC server and then, five seconds later, sends the same value again even though it might have changed on the PLC. Because the OPC server only polls the PLC every 10 seconds, it would not have advised the interface of this change. Without ExcMax set, there would be no record of this value at this time, but with ExcMax set this way, there is a record with an inaccurate value. For this reason, it is extremely important to consider the update rate when setting ExcMax. In particular, the user should never configure ExcMax to be less than the update rate. If possible, the user should set it to at least twice the update rate. See What happens if I set ExcMax equal to the update rate? for a description of problems that can arise when the two quantities are equal.