The DeltaV system provides class-based design of common elements that can be used in both batch and continuous projects, consistent with the standards defined for batch processing in ISA S88.01. With class-based modules, users can easily design reusable elements (such as control modules for valves and motors). The class is used to define the properties that are the same or similar for most of the items (such as the control logic, alarm parameters, and default settings). Then individual instances are created from the class. For example, a control module instance for a particular valve can be quickly created from a pre-configured control module class, inheriting all the common parameters, logic, etc., from the class. The control module instance specifies the I/O information that ties it to the physical piece of equipment it controls. If other parameters have been marked as being configurable at the instance level, changes can be made to those parameters as well. However, for the most part, the control module instance will be almost identical to the class from which it was created.
Changes made to a class (such as a control module class) after instances have been created from it are automatically propagated to the instances (the control modules). This is the main difference between creating a module from a predefined library template and creating a module from a class. If you create a module from a library template, there are no links between the module and the template; any changes made to the template have no effect on modules already created from that template.
In some cases, one or more parameters are different on a small number of the instances created from a class. The class designer can mark these parameters at the class level as being configurable at the instance level. There is a check box named Use default value from library object on the Properties dialog of each configurable parameter at the instance level. By default, this check box is selected so that a change in the parameter value at the class level (in the library) will propagate to the instance. The user must clear this check box to break the link to the class. The user can re-select this box to re-establish the link.Only when this box is cleared can the user modify the value of the configurable parameter.
Key points for creating classes are that few of the instances created from a class should require changes beyond the necessary I/O assignments, and the changes should involve very few parameters. If a subset of a class of valves is different enough from the original class, but the instances in the subset are similar to each other, it may be better to create a new class for that smaller group.