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.

image007.png

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.).

In a DA server, Automation Data is represented by Value, Quality and Time Stamp for a Tag.

The COM UA Wrapper maps the VQT data to the Data Value and Diagnostic Info structures.

The Error codes returned by the DA server are based on the HRESULT type. The COM UA Wrapper maps this error code to an OPC UA Status Code. Figure A.2illustrates this mapping.

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Figure A.2– OPC COM DA to OPC UA data and error mapping

The data values in the DA server are represented as Variant Data type. The COM UA Wrapper converts them to the corresponding OPC UA data type. The mapping is shown in Table A.2:

Table A.2– DataTypes and mapping

Variant Data

Type (In DA server)

OPC UA

Data type Mapping in COM UA Server (DataValue structure)

VT_I2

Int16

VT_I4

Int32

VT_R4

Float

VT_R8

Double

VT_BSTR

String

VT_BOOL

Boolean

VT_UI1

Byte

VT_I1

SByte

VT_UI2

UInt16

VT_UI4

UInt32

VT_I8

Int64

VT_UI8

UInt64

VT_DATE

Double

VT_DECIMAL

Decimal

VT_ARRAY

Array of OPC UA types

.

The Quality of a Data Value in the DA server is represented as a 16 bit value where the lower 8 bits is of the form QQSSSSLL (Q: Main Quality, S: Sub Status, L: Limit) and higher 8 bits is vendor specific.

The COM UA Wrapper maps the DA server to the OPC UA Status code as shown Figure A.3:

image009.png

Figure A.3– Status Code mapping

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

Please note that the Vendor quality is currently discarded.

Table A.3shows a mapping of the OPC COM DA primary quality mapping to OPC UA status code

Table A.3– Quality mapping

OPC DA Primary Quality (Quality & Sub status QQSSSS)

OPC UA Status Code

GOOD

Good

LOCAL_OVERRIDE

Good_LocalOverride

UNCERTAIN

Uncertain

SUB_NORMAL

Uncertain_SubNormal

SENSOR_CAL

Uncertain_SensorNotAccurate

EGU_EXCEEDED

Uncertain_EngineeringUnitsExceeded

LAST_USABLE

Uncertain_LastUsableValue

BAD

Bad

CONFIG_ERROR

Bad_ConfigurationError

NOT_CONNECTED

Bad_NotConnected

COMM_FAILURE

Bad_NoCommunication

DEVICE_FAILURE

Bad_DeviceFailure

SENSOR_FAILURE

Bad_SensorFailure

LAST_KNOWN

Bad_OutOfService

OUT_OF_SERVICE

Bad_OutOfService

WAITING_FOR_INITIAL_DATA

Bad_WaitingForInitialData

The Timestamp provided for a value in the DA server is assigned to the SourceTimeStamp of the DataValue in the COM UA Wrapper.

The ServerTimeStamp in the DataValue is set to the current time by the COM UA Wrapper at the start of the Read Operation.

The COM UA Wrapper supports performing Read operations to DA servers of versions 2.05a and 3.

For version 2.05a, the COM UA wrapper creates a Group using the IOPCServer::AddGroup method and adds the items whose data is to be read to the Group using IOPCItemMgmt::AddItems method. The Data is retrieved for the items using the IOPCSyncIO::Read method. The VQT for each item is mapped to the DataValue structure as shown in Figure A.2. Please note that only Read from Device is supported for this version. The “maxAge” parameter is ignored.

For version 3, the COM UA Wrapper uses the IOPCItemIO::Read to retrieve the data. The VQT for each item is mapped to the DataValue structure as shown in Figure A.2. The Read supports both the Read from Device and Cache and uses the “maxAge” parameter.

If there are errors for the items in the Read from the DA server, then these are mapped to the StatusCode of the DataValue in the COM UA Wrapper.

The mapping of the OPC COM DA Read Errors code to OPC UA Status code (in the COM UA Wrapper) is shown in Table A.4:

Table A.4– OPC DA Read error mapping

OPC DA Error ID

OPC UA Status Code

OPC_E_BADRIGHTS

Bad_NotReadable

E_OUTOFMEMORY

Bad_OutOfMemory

OPC_E_INVALIDHANDLE

Bad_NodeIdUnknown

OPC_E_UNKNOWNITEMID

Bad_NodeIdUnknown

E_INVALIDITEMID

Bad_NodeIdInvalid

E_INVALID_PID

Bad_AttributeIdInvalid

E_ACCESSDENIED

Bad_OutOfService

Others

Bad_UnexpectedError

The COM UA Wrapper supports performing Write operations to DA servers of versions 2.05a and 3.

For version 2.05a, the COM UA wrapper creates a Group using the IOPCServer::AddGroup method and adds the items whose data is to be written using IOPCItemMgmt::AddItems method. The value is written for the items using the IOPCSyncIO::Write method. Note that if the StatusCode or TimeStamps (Source or Server) is specified to be written for the item then the COM UA Wrapper returns a BadWriteNotSupported Status code for the item.

For version 3, the COM UA Wrapper uses the IOPCItemIO::WriteVQT data including StatusCode and TimeStamp. If a SourceTimeStamp is provided, this timestamp is used for the Write else the ServerTimeStamp is used.

If there are errors for the items in the Write from the DA server, then these are mapped to the StatusCode for the corresponding item.

The mapping of the OPC COM DA Write Errors code to OPC UA Status code (in the COM UA Wrapper) is shown in Table A.5:

Table A.5– OPC DA Write error code mapping

OPC DA Error ID

OPC UA Status Code

E_BADRIGHTS

Bad_NotWritable

DISP_E_TYPEMISMATCH

Bad_TypeMismatch

E_BADTYPE

Bad_TypeMismatch

E_RANGE

Bad_OutOfRange

DISP_E_OVERFLOW

Bad_OutOfRange

E_OUTOFMEMORY

Bad_OutOfMemory

E_INVALIDHANDLE

Bad_NodeIdUnknown

E_UNKNOWNITEMID

Bad_NodeIdUnknown

E_INVALIDITEMID

Bad_NodeIdInvalid

E_INVALID_PID

Bad_NodeIdInvalid

E_NOTSUPPORTED

Bad_WriteNotSupported

S_CLAMP

Good_Clamped

Others

Bad_UnexpectedError

A subscription is created in the DA server when a MonitoredItem is created in the COM UA Wrapper.

The SamplingInterval and the Deadband value are used for the subscription to setup a periodic data change call back on the COM UA Wrapper. Note that only the PercentDeadbandType is supported by the COM UA Wrapper.

The VQT for each item is mapped to the DataValue structure as shown in Figure A.2and published to the client by the COM UA Wrapper periodically.

The mapping of the OPC COM DA Read Errors code to OPC UA Status code (in the COM UA Wrapper) is the same as the Read mapping in Figure A.2.