An InstanceDeclaration marked with the ModellingRule Mandatory means that for each existing BrowsePath on the instance a similar Node shall exist, but it is not defined whether a new Node is created or an existing Node is referenced.
For example, the TypeDefinitionNode of a functional block “AI_BLK_TYPE” will have a setpoint “SP1”. An instance of this type “AI_BLK_1” will have a newly-created setpoint “SP1” as a similar Node to the InstanceDeclaration SP1. Figure 26 illustrates the example.
An InstanceDeclaration marked with the ModellingRule Optional means that for each existing BrowsePath on the instance a similar Node may exist, but it is not defined whether a new Node is created or an existing Node is referenced.
In Figure 27 an example using the ModellingRules Optional and Mandatory is shown. The example contains an ObjectType Type_A and all valid combinations of instances named A1 to A13. Note that if the optional B is provided, the mandatory E has to be provided as well, otherwise not. F is referenced by C and D. On the instance, this can be the same Node or two different Nodes with the same BrowseName (similar Node to InstanceDeclaration F). Not considered in the example is if the instances have ModellingRules or not. It is assumed that each F is similar to the InstanceDeclaration F, etc.
If there would be a NonHierarchical Reference between E and F in the InstanceDeclarationHierarchy, it is not specified if it occurs in the instance hierarchy or not. In the case of A10, there could be a reference from E to one F but not to the other F, or to both or none of them.
The ExposesItsArray ModellingRule exposes a special semantic on VariableTypes having a single- or multidimensional array as the data type. It indicates that each value of the array will also be exposed as a Variable in the AddressSpace.
The Variable A having this ModellingRule shall be referenced by a forward hierarchical Reference from a VariableType B. B shall have a ValueRank value that is equal to or larger than zero. A should have a data type that reflects at least parts of the data that is managed in the array of B. Each instance of B shall reference one instance of A for each of its array elements. The used Reference shall be of the same type as the hierarchical Reference that connects B with A or a subtype of it. If there are more than one forward hierarchical References between A and B, then all instances based on B shall be referenced with all those References.
Figure 28 gives an example. A is an instance of Type_A having two entries in its value array. Therefore it references two instances of the same type as the InstanceDeclaration ArrayExpose. The BrowseNames of those instances are not defined by the ModellingRule. In general, it is not possible to get a Variable representing a specific entry in the array (e.g. the second). Clients will typically either get the array or access the Variables directly, so there is no need to provide that information.
For Object and Variable the intention of the ModellingRule OptionalPlaceholder is to expose the information that a complex TypeDefinition expects from instances of the TypeDefinition to add instances with specific References without defining BrowseNames for the instances. For example, a Device might have a Folder for DeviceParameters, and the DeviceParameters should be connected with a HasComponent Reference. However, the names of the DeviceParameters are specific to the instances. The example is shown in Figure 30, where an instance Device A adds two DeviceParameters in the Folder.
The ModellingRule OptionalPlaceholder adds no additional constraints on instances of the TypeDefinition. It just provides useful information when exposing a TypeDefinition. When the InstanceDeclaration is complex, i.e. it references other InstanceDeclarations with hierarchical References, these InstanceDeclarations are not further considered for instantiating the TypeDefinition.
For Methods, the ModellingRule OptionalPlaceholder is used to define the BrowseName where subtypes and instances provide more information. The Method definition with the OptionalPlaceholder only defines the BrowseName. An instance or subtype defines the InputArguments and OutputArguments. A subtype shall also change the ModellingRule to Optional or Mandatory. The Method is optional for instances. For example, a Device might have a Method to perform calibration however the specific arguments for the Method depend on the instance of the Device. In this example Device A does not implement the Method, Device B implements the Method with no arguments and Device C implements the Method accepting a mode argument to select how the calibration is to be performed. The example is shown in Figure 31.
For Object and Variable the ModellingRule MandatoryPlaceholder has a similar intention as the ModellingRule OptionalPlaceholder. It exposes the information that a TypeDefinition expects of instances of the TypeDefinition to add instances defined by the InstanceDeclaration. However, MandatoryPlaceholder requires that at least one of those instances shall exist.
For example, when the DeviceType requires that at least one DeviceParameter shall exist without specifying the BrowseName for it, it uses MandatoryPlaceholder as shown in Figure 32. Device A is a valid instance as it has the required DeviceParameter. Device B is not valid as it uses the wrong ReferenceType to reference a DeviceParameter (Organizes instead of HasComponent) and Device C is not valid because it does not provide a DeviceParameter at all.
The ModellingRule MandatoryPlaceholder requires that each instance provides at least one instance with the TypeDefinition of the InstanceDeclaration or a subtype, and is referenced with the same ReferenceType or a subtype as the InstanceDeclaration. It does not require a specific BrowseName and thus cannot be used for the TranslateBrowsePathsToNodeIds Service (see OPC 10000-4).
When the InstanceDeclaration is complex, i.e. it references other InstanceDeclarations with hierarchical References, these InstanceDeclarations are not further considered for instantiating the TypeDefinition.
For Methods, the ModellingRule MandatoryPlaceholder is used to define the BrowseName where subtypes and instances provide more information. The Method definition with the MandatoryPlaceholder only defines the BrowseName. An instance or subtype defines the InputArguments and OutputArguments. A subtype shall also change the ModellingRule to Mandatory. The Method is mandatory for instances.