A.4 COM UA proxy for DA Client
A.4.1 Guidelines

The Data Access COM UA Proxy is a COM Server combined with a UA Client. It maps the Data Access address space of UA Data Access Server into the appropriate COM Data Access objects.

Clauses A.4.1 through A.4.6 identify the design guidelines and constraints used to develop the Data Access COM UA Proxy provided by the OPC Foundation. In order to maintain a high degree of consistency and interoperability, it is strongly recommended that vendors, who choose to implement their own version of the Data Access COM UA Proxy, follow these same guidelines and constraints.

The Data Access COM Client simply needs to address how to connect to the UA Data Access Server. Connectivity approaches include the one where Data Access COM Clients connect to a UA Data Access Server with a CLSID just as if the target Server were a Data Access COM Server. However, the CLSID can be considered virtual since it is defined to connect to intermediary components that ultimately connect to the UA Data Access Server. Using this approach, the Data Access COM Client calls co-create instance with a virtual CLSID as described above. This connects to the Data Access COM UA Proxy components. The Data Access COM UA Proxy then establishes a secure channel and session with the UA Data Access Server. As a result, the Data Access COM Client gets a COM Data Access Server interface pointer.

A.4.2 Information Model and Address Space mapping
A.4.2.1 General

OPC UA defines 8 Node Class types in the address space Object, Variable, Method, ObjectType, VariableType, ReferenceType, DataType, View. The COM UA Proxy maps only the nodes of Node Class types Object, Variable to the OPC DA types as shown in the figure below. Only the nodes under the Objects node are considered for the COM UA Proxy address space and others such as Types, Views are not mapped.

Figure A.15 shows an example mapping of OPC DA to OPC UA information.

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Figure A.15 – Sample OPC DA mapping of OPC UA Information Model and Address Space

A.4.2.2 Object Nodes

A node of Object Node class in the OPC UA server is represented in the Data Access COM UA Proxy as a Branch.

The root of the Data Access COM UA Proxy is the Objects folder of the OPC UA Server.

The OPC UA Address space hierarchy is discovered using the Browse Service for the Objects Node using the following filters:

  • BrowseDirection as Forward
  • ReferenceTypeId as Organizes and HasChild.
  • IncludeSubtypes as True
  • NodeClassMask as Object and Variable

The DisplayName of the OPC UA node is used as the Name for each Branch in the Data Access COM UA Proxy

Each Branch in the Data Access COM UA Proxy is assigned 3 properties:

  • UA Browse Name (Property ID: 613): The value of the BrowseName attribute of the node in the OPC UA Server is assigned to this property.
  • UA Description (Property ID: 614): The value of the Description attribute of the node in the OPC UA Server is assigned to this property, if a Description attribute is provided.
  • Item Description (Property ID: 101): The value of the DisplayName attribute of the node in the OPC UA Server is assigned to this property.

NOTE COM DA Clients typically display the ItemID and the Item Description. Since the ItemID generated by the UA Proxy may be particularly difficult to read and understand, proxies may use the DisplayName as value for the Item Description Property as it will be easier to understand by a human user.

A.4.2.3 Variable Nodes

A node of Variable Node class in the OPC UA server is represented in the Data Access COM UA Proxy as an Item.

The DisplayName of the OPC UA node is used as the Name for each Item in the Data Access COM UA Proxy.

The NodeId of the OPC UA node is used as the ItemId for each Item in the Data Access COM UA Proxy. But the ‘=’ character is replaced with ‘-’ in the string. E.g. NodeId: ns=4,i=10, ItemID = “ns-4;i-10” or NodeId: ns=4,s=FL102, ItemID = “ns-4,s-FL102”

Each Item in the Data Access COM UA Proxy is assigned the following properties based on the node attributes or its references:

Standard Properties:

The other AccessLevel provided by OPC are ignored

  • Server Scan Rate (Property ID: 6): The value of the MinimumSamplingInterval attribute of the node in the OPC UA Server is assigned to this property.
  • Item EU Type (Property ID: 7): The EU Type value is assigned based on the references of the node in the OPC UA Server:

Analog(1) : if the node in the OPC UA Server references a EURange property node, then it is assigned the Analog EU Type.

Enumerated(2): if the node in the OPC UA Server references a EnumStrings property node, then it is assigned the Enumerated EU Type.

Empty(0): For a node in the OPC UA Server that does not meet above criteria, the type is set as 0 (Empty)

New Properties:

  • UA BuiltIn Type (Property ID: 610): The identifier value of the DataType node associated with the DataType attribute of the node in the OPC UA Server is assigned to this property.
  • UA Data Type Id (Property ID: 611): The complete NodeId value (namespace and identifier) of the DataType node associated with the DataType attribute of the node in the OPC UA Server is assigned to this property.
  • UA Value Rank (Property ID: 612): The value of the ValueRank attribute of the node in the OPC UA Server is assigned to this property.
  • UA Browse Name (Property ID: 613): The value of the BrowseName attribute of the node in the OPC UA Server is assigned to this property.
  • UA Description (Property ID: 614): The value of the Description attribute of the node in the OPC UA Server is assigned to this property.

A.4.2.4 Namespace Indices

For generating ItemIDs, the Proxy uses Namespace Indices. To assure that Clients can persist these ItemIDs, the Namespace Indices must never change. To accomplish this the Proxy has to persist its Namespace Table and only append entries but never change existing ones.

The Proxy also has to provide a translation from the current Namespace Table in the Server to the persisted Namespace Table.

If you move or copy the Proxy to another machine, the Namespace Table has to be copied to this machine as well.

A.4.3 Data and error mapping
A.4.3.1 General

In an OPC UA Server, Automation Data is represented as a Data Value and and status, in addition additional error data can be provided via Diagnostic Info for a tag

The COM UA Proxy maps the Data Value structure into VQT data and error code.

For successful operations(StatusCode of Good and Uncertain), the COM UA Proxy maps the Status Code of the DataValue to the OPC DA Quality But in case of error(StatusCode of Bad), the Status Code is mapped to the OPC DA Error code.

The StatusCode in the Diagnostic Info returned by the OPC UA Server are mapped to OPC DA Error codes. Figure A.16 illustrates this mapping.

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Figure A.16 – OPC UA to OPC DA data & error mapping

A.4.3.2 Value

The COM UA Proxy converts the OPC UA Data Value to the corresponding OPC DA Variant type. The mapping is shown in Table A.64. For DataTypes that are subtypes of an existing base DataType the conversion for the Base DataType is used.

Table A.64 – DataTypes and Mapping

OPC UA

Data type (Bin UA Server)

Variant Data

Type (In DA server)

Int16

VT_I2

Int32

VT_I4

Float

VT_R4

Double

VT_R8

Decimal

VT_DECIMAL

String

VT_BSTR

Boolean

VT_BOOL

Byte

VT_UI1

SByte

VT_I1

UInt16

VT_UI2

UInt32

VT_UI4

Int64

VT_I8

UInt64

VT_UI8

Guid

VT_BSTR

DateTime

VT_DATE

NodeId

VT_BSTR

XmlElement

VT_BSTR

ExpandedNodeId

VT_BSTR

QualifiedName

VT_BSTR

LocalizedText

VT_BSTR

StatusCode

VT_UI4

ExtensionObject

Array of VT_UI1

Array of above OPC UA types

Array of corresponding Variant type

A.4.3.3 Quality

The Quality of a Data Value in the OPC UA Server is represented as a StatusCode.

The COM UA Proxy maps the Severity, Subcode and the limit bits of the OPC UA Status code to the lower 8 bits of the OPC DA Quality structure (of the form QQSSSSLL). Figure A.17 illustrates this mapping.

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Figure A.17 – OPC UA Status Code to OPC DA quality mapping

The Severity field of the Status code is mapped to the primary quality. The SubCode is mapped to the Sub Status and the Limit Bits are mapped to the Limit field.

Table A.65 shows a mapping of the OPC UA status code to OPC DA primary quality

Table A.65 – Quality mapping

OPC UA Status Code

OPC DA Primary Quality (Quality & Sub status QQSSSS)

Good

GOOD

Good_LocalOverride

LOCAL_OVERRIDE

Uncertain

UNCERTAIN

Uncertain_SubNormal

SUB_NORMAL

Uncertain_SensorNotAccurate

SENSOR_CAL

Uncertain_EngineeringUnitsExceeded

EGU_EXCEEDED

Uncertain_LastUsableValue

LAST_USABLE

Bad

BAD

Bad_ConfigurationError

CONFIG_ERROR

Bad_NotConnected

NOT_CONNECTED

Bad_NoCommunication

COMM_FAILURE

Bad_OutOfService

OUT_OF_SERVICE

Bad_DeviceFailure

DEVICE_FAILURE

Bad_SensorFailure

SENSOR_FAILURE

Bad_WaitingForInitialData

WAITING_FOR_INITIAL_DATA

A.4.3.4 Timestamp

If available, the SourceTimestamp of the DataValue in the OPC UA Server is assigned to the Timestamp for the value in the COM UA Proxy. If SourceTimestamp is not available, then the ServerTimestamp is used.

A.4.4 Read data

The COM UA Proxy converts all the ItemIds in the Read into valid NodeIds by replacing the ‘-’ with ‘=’ and calls the OPC UA Read Service for the Value Attribute.

If the Read Service call is successful then DataValue for each node is mapped to the VQT for each item as shown in Figure A.16.

If the Read Service call fails or If there are errors for some of the Nodes, then the StatusCodes of these Nodes are mapped to the error code by the COM UA Proxy.

The mapping of the OPC UA Status code to OPC DA Read Error code (in the COM UA Proxy) is shown in Table A.66:

Table A.66 – OPC UA Read error mapping

OPC UA Status Code

OPC DA Error ID

Bad_OutOfMemory

E_OUTOFMEMORY

Bad_NodeIdInvalid

E_INVALIDITEMID

Bad_NodeIdUnknown

E_UNKNOWNITEMID

Bad_NotReadable

E_BADRIGHTS

Bad_UserAccessDenied

E_ACCESSDENIED

Bad_AttributeIdInvalid

E_INVALIDITEMID

Bad_UnexpectedError

E_FAIL

Bad_InternalError

E_FAIL

Bad_SessionClosed

E_FAIL

Bad_TypeMismatch

E_BADTYPE

A.4.5 Write data

The COM UA Proxy converts all the ItemIds in the Write into valid NodeIds by replacing the ‘-’ with ‘=’. It converts the Value, Quality and Timestamp (VQT) to a DataValue structure as per the mapping in Figure A.16. and calls the OPC UA Write Service for the Value Attribute.

If the Write Service call fails or if there are errors for some of the Nodes, then the StatusCodes of these Nodes are mapped to the error code by the COM UA Proxy.

The mapping of the OPC UA Status code to OPC DA Write Error code (in the COM UA Proxy) is shown in Table A.67:

Table A.67 – OPC UA Write error code mapping

OPC UA Status Code

OPC DA Error ID

Bad_TypeMismatch

E_BADTYPE

Bad_OutOfMemory

E_OUTOFMEMORY

Bad_NodeIdInvalid

E_INVALIDITEMID

Bad_NodeIdUnknown

E_UNKNOWNITEMID

Bad_NotWritable

E_BADRIGHTS

Bad_UserAccessDenied

E_ACCESSDENIED

Bad_AttributeIdInvalid

E_UNKNOWNITEMID

Bad_WriteNotSupported

E_NOTSUPPORTED

Bad_OutOfRange

E_RANGE

A.4.6 Subscriptions

The COM UA Proxy creates a Subscription in the OPC UA Server when a Group is created. The Name, Active flag, UpdateRate parameters of the Group are used while creating the subscription.

The COM UA Proxy Creates Monitored Items in the OPC UA Server when items are added to the Group.

Following parameters and filters are used for creating the monitored items:

  • The ItemIds are converted to valid NodeIds by replacing the ‘-’ with ‘=’.
  • Data Change Filter is used for Items with EU type as Analog:

Trigger = STATUS_VALUE

If DeadBand value is specified for the Group, the;

  • DeadbandType = Percent
  • DeadbandValue = deadband specified for the group.

The COM UA Proxy calls the Publish Service of the OPC UA Server periodically and sends any data changes to the client.