An OPC-DA Property Point is a connectivity point that represents an OPC Data Access (OPC-DA) property.

Point, Parameters, and Arguments Types

For operations listed below, following classes are used for the point itself, the operation parameters and operation arguments:

Operation	Point Type	Parameters Type	Arguments Type
Read	DAPropertyPoint Class	Object (see Note 1)	DAPropertyPointReadArguments Class
Subscribe		Object (see Note 1)	DAPropertyPointReadArguments Class (see Note 2)

The Write operation is not supported. The Subscribe operation make a on-time Read (in OPC-DA terms, Property "Get").

Note 1: No specific parameters are needed.

Note 2: Arguments for the Subscribe operation reuse the type used with the Read.

Type Aliases

You can use following type aliases for this point type:

• opcdaproperty
• dp

Friendly Topic Syntax

In order to describe the point and its subscription parameters, use the following sequence:

   ServerDescriptorString, ItemId, { QualifiedName | PropertyId }

Where:

• ServerDescriptorString is a descriptor string for the server.
• ItemId is an OPC item identifier (empty string for the root).
• QualifiedName is a qualified name of the property (used with OPC XML-DA). Must contain a colon (:).
• PropertyId is a symbolic or numeric value of the property ID. Case insensitive.    

If the point or parameters differ from the default in other properties, the friendly topic syntax cannot be used.

Remarks 

The ServerDescriptorString may contain one of the following:

• A server class, i.e. ProgID (or {CLSID}) of the object.
• A string in the form "\\machineName\serverClass".
• A URI of the object.

For valid symbolic values of the PropertyId, see DAPropertyIds Enumeration Fields.

Examples

   "OPCLabs.KitServer.2", "Demo.Ramp", "DataType"

   "opcda://MY-PC/OPCLabs.KitServer.2", "Demo.Single", "5"

XML Topic Syntax

The XML topic syntax is capable to fully describe the point and parameters. It is currently not documented, and is subject to change.