Retrieval styles, application settings, and tag settings

There are various ways to set retrieval options in the Trend application: using a retrieval style vs. using custom retrieval options, using the application-wide options vs. using tag-level options. Also, there are some differences depending on which Historian version you are using. The following guidelines help you understand which retrieval settings are actually used in a given situation.

1. Settings at the tag level override settings at the application level. For the aaHistClientTrend control, this means that the properties starting with CurrentTag... override the properties starting with RetrievalOptions...

2. Because a retrieval style definition can contain multiple sets of retrieval settings with different retrieval modes, some of the settings available for editing at the application or tag level may turn out to be irrelevant for the retrieval mode that actually gets used for a given query. However, because there is no way to know in advance which retrieval mode will be used, the settings are still available for editing. The same applies to properties in the aaHistClientTrend control.

3. At the application level, you can specify additional options for retrieving data from Historians with a version earlier than 9.0. For more information, see Configuring Other Options. These settings override any style that you may have selected at the application level. For example, if you have set these options to enable delta retrieval for periods below 15 minutes, but you have selected a style at the application level that specifies cyclic retrieval for all time periods, the Trend application enforces delta retrieval for all time periods below 15 minutes regardless of the settings in the style.

   However, if you select the style at the tag level, then the style settings override the application options. In the above example, cyclic retrieval would be used for all time periods regardless of the application settings specifying delta retrieval.

4. If there is a conflict between a setting specified in a style and a setting specified using one of the aaHistClientTrend control’s properties (for example, retrieval resolution), the style setting overrides the property setting.