Group Templates enable much of the configuration in a modular system to be performed just once—in the Group Template—with ClearSCADA automatically replicating that configuration in each of the Instances that reference the Template. (Similarly, if any of the inherited configuration needs modifying at a future date, the changes need only to be performed once, in the Group Template, with the changes automatically being replicated in each Group Instance.)
You use the Property Overrides feature to define which property values in a Group Instance are derived from a Group Template, and which property values can be overridden locally on a per Instance basis (see Understanding the Select Properties Window). You can also use expressions to override particular properties of items within the Group Template (see Use Template Expressions to Configure Property Values). There are two types of Property Overrides:
- Instance Overrides—that allow the group instance property values to be modified directly.
- Expression Overrides—that allow you to modify the group instance property values using expressions which are calculated from a set of parameters that can be modified in each group instance.
Some configuration properties are optional—they can be selected from a list, or be enabled or disabled. You need to ensure that the required optional features are specified on the configuration Forms of each item in a Group Template before specifying which properties are to be derived from that Template (see Configure the Items in a Group Template).
Certain configuration properties—such as an outstation address—need to be unique. By default, those properties that are deemed to be unique are specified as locally overridden—once you change the value of that property in an Instance, any changes that are then made to that property in the Template will not be inherited by the Instance. Properties that must be unique are fixed as locally overridden and cannot be changed from this setting.
If you remove a property override (so that it is no longer locally configurable on a per Instance basis), all Instances of that property revert to the value that is specified in the Group Template.
For examples demonstrating how Property Overrides might be used on a system, see Use Property Overrides to Optionally Assign PSTN Fallback to a Direct Outstation, and see Define which Connection Settings to use in the Instances of a Channel.