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

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	DAItemPoint Class	DAItemPointReadParameters Class	DAItemPointReadArguments Class
Subscribe		DAItemPointSubscribeParameters Class	DAItemPointSubscribeArguments Class
Write		DAItemPointWriteParameters Class	DAItemPointWriteArguments Class

Type Aliases

You can use following type aliases for this point type:

• opcdaitem
• di

Friendly Topic Syntax

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

   ServerDescriptorString, ItemId [, RequestedUpdateRate]

Where:

• ServerDescriptorString is a descriptor string for the server.
• ItemId is an OPC item identifier (empty string for the root).
• RequestedUpdateRate is the requested update rate (how often should the updates be received). In milliseconds. Defaults to 1000 milliseconds.

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.

Examples

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

   "OPCLabs.KitServer.2", "Demo.Ramp", "5000"

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

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.