OPC DA defines 3 elements in the address space: Branch, Item and Property. The COM UA Wrapper maps these types to the OPC UA types as described in Subclauses A.3.1.2to A.3.1.4.

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Figure A.1– Sample OPC UA Information Model for OPC DA

DA Branches are represented in the COM UA Wrapper as Objectsof FolderType.

The top-level branch (the root) should be represented by an Objectwhere the BrowseNameis the Server ProgId.

The OPC DA Address space hierarchy is discovered using the ChangeBrowsePosition from the Root and BrowseOPCItemIds to get the Branches, Items and Properties.

The name returned from the BrowseOPCItemIds enumString is used as the BrowseName and the DisplayName for each Branch. See also clause A.3.1.5.

The ItemId obtained using the GetItemID is used as a part of the NodeId for each Branch. See also clause A.3.1.5.

An OPC UA Folderrepresenting a DA Branch uses the Organizes Referencesto reference child DA Branches and uses HasComponent Referencesfor DA Leafs (Items). It is acceptable for customized wrappers to use a sub-type of these ReferenceTypes.

DA items (leafs) are represented in the COM UA Wrapper as Variables. The VariableType depends on the existance of special DA properties as follows:

Below are mappings that are common for all item types

Note that the same values are also set for the UserAccessLevel in the COM UA Wrapper.

Any Propertiesadded to a Node in the COM UA Wrapper are referenced using the HasProperty ReferenceType.

A property in the DA server is represented in the COM UA Wrapper as a Variablewith TypeDefinitionas PropertyType.

The properties for an item are retrieved using the QueryAvailableProperties call in the DA server.

Below are mappings of the property details to the OPC UA Property:

Table A.1shows the mapping between the common OPC COM DA properties to the OPC UA Node attributes/properties.

Table A.1– OPC COM DA to OPC UA Properties mapping

Property Name (PropertyID) of OPC COM DA

OPC UA Information Model

OPC UA DataType

Access Rights (5)

AccessLevel Attribute

Int32

EU Units (100)

EngineeringUnits Property

String

Item Description (101)

Description Attribute

String

High EU (102)

EURange Property

Double

Low EU (103)

EURange Property

Double

High Instrument Range (104)

InstrumentRange Property

Double

Low Instrument Range (105)

InstrumentRange Property

Double

Close Label (106)

TrueState Property

String

Open Label (107)

FalseState Property

String

Other Properties (include Vendor specific Properties)

PropertyType

Based on the DataType of the Property

As described above, both the OPC UA Browsename and Displayname for Nodes representing COM DA Branches and Leafs are derived from the name of the corresponding item in the COM DA Server.

This name can only be acquired by using the COM DA Browse Services. In OPC UA, however, the BrowseName and DisplayName are Attributes that Clients can ask for at any time. There are several options to support this in a Wrapper but all of them have pros and cons. Here are some popular implementation options:

  1. Allow browsing the complete COM DA Address Space and then build and persist an offline copy of it. Resolve the BrowseName by scanning this offline copy.
  2. Pro: The ItemID can be used as is for the OPC UA NodeId.
  3. Con: The initial browse can take a while and may have to be repeated for COM DA Servers with a dynamic Address Space.
  4. Create OPC UA NodeId values that include both the COM DA ItemID and the Item name. When the OPC UA Client passes such a NodeId to read the BrowseName or DisplayName Attribute, the wrapper can easily extract the name from the NodeId value.
  5. Pro: Efficient and reliable.
  6. Con: The NodeId will not represent the ItemId. It becomes difficult for human users to match the two IDs.
  7. A number of COM DA Servers use ItemIDs that consist of a path where the path elements are separated with a delimiter and the last element is the item name. Wrappers may provide ways to configure the delimiter so that they can easily extract the item name.
  8. Pro: Efficient and reliable. The ItemID can be used as is for the OPC UA NodeId.
  9. Con: Not a generic solution. Only works for specific COM-DA Servers.

For wrappers that are custom to a specific Server, knowledge of the COM DA server address space can result in other optimizations or short cuts (i.e. the server will always have a certain schema / naming sequence etc.).