The ISA-95 Equipment Model is an abstract model that describes classes of logical equipment. The physical equipment is defined by physical assets. In the ISA-95 model, a logical device (equipment) usually does not change, but a physical device may change over time. For example a meter is being used, but it requires service, thus it is replaced by an identical meter. The Physical device gets changed, but in the logic of the system the equipment has not changed. The ISA-95 model further defines classes of equipment that define general functionality or features of a logical piece of equipment. These classes of equipment (EquipmentClassType) would be defined by an EquipmentClassType. Instances of these classes of Equipment are referenced by an EquipmentType, which is a definition of a logical piece of equipment. In a running system there would be multiple instances of the EquipmentType. There would also be multiple EquipmentClassTypes. Figure 24 provides an illustration of this concept. It is also acceptable to define subtypes of EquipmentType, where the associated EquipmentClassType never change and multiple instances of the subtype of EquipmentType exist.
Figure 23 illustrates the Equipment Information model which consists of Equipment Types and EquipmentClassTypes. The illustration includes the definition of subtypes of EquipmentClassType, an instance of Equipment, and a subtype of EquipmentType. Any subtype of EquipmentType includes additional restrictions on Instance of the subtype such as the instance can never remove an associated EquipmentClassType.
The role based equipment model contains information about specific equipment, equipment classes, equipment capability tests, and maintenance information associated with equipment. It can also contain information about related Physical Assets. The information about the related Physical Asset is represented as asset assignment information
The asset assignment information would be part of the Equipment model as well as part of the Physical Asset model. The asset assignment information could be time series data and is accessed by standard Attribute Services as defined in OPC 10000-4.
The asset assignment information is represented as DataVariable that is part of the Equipment or Physical Asset. The DataType of the asset assignment VariableType is a structure that represents elements of the asset assignment information. This standard only defines basic information regarding the asset assignment. Third parties may create a subtype of this VariableType to represent third party specific information of an asset assignment. (See Figure 26)