Why not just use the /TA parameter?

In cases where the user only wants to create steps in the data and does not necessarily need frequent updates, AVEVA strongly recommends using the /TA interface parameter instead of configuring ExcMax on each advise point. With this parameter set, when a new value arrives for an advise point, the interface sends the previous value with a timestamp of one update interval prior to the new value's timestamp. For example, if the update rate is set to 1 second and a new value arrives 10 seconds after the previous one, the interface sends the previous value with a timestamp of one second less than the new value's timestamp (i.e., approximately 9 seconds after the previous timestamp) and then sends the new value with its current timestamp.

This approach is preferable to using ExcMax to accomplish the same goal because the exception values that are sent this way are guaranteed to be timestamped at or near an actual update time. The only assumption in this case is that the OPC server would have advised the interface at that time had the value been different. In contrast, it is possible to (mis)configure ExcMax so that exception values are sent with timestamps that are not at update times. As a result, one cannot necessarily assume that these values are actually accurate, since the true value might have changed without the OPC server noticing the change.

In cases where the user needs to see frequent updates to an advise point that changes infrequently, the /TA parameter does not suffice. Instead, AVEVA recommends two solutions: either configure the point to poll instead of advise (this is the preferred method), or if the resulting network traffic is not acceptable, set ExcMax equal to an integer multiple of the update interval. In the latter case, ExcMax should be at least twice-and ideally many times-the update interval.