Item Properties

OPC-specification allows to add additional information to single OPC-items (opc-item means : a TwinCAT variable). This optional functionality is named "OPC-Item-Properties" in the OPC-spec and allows an opc-client to browse and read these additional properties.

Sample:
As an option the PLC variable "temperature" can hold addition informaton like comment (OPC-Property-ID 101) and also UNIT (OPC-Property-ID 100) which can be offered by the opc-server.

Configuration of OPC Propery ID in the PLC

Add a comment behind PLC-variable declaration. This comment will be interpreted by the opc-server and will define OPC-Item-Properties.

Today only global PLC-variables can contain item-properties. Elements of structures or arrays can not be configured. Future versions of OPC-Server will not have this limitation.

Sample of PLC variable-declaration:

All item-properties are optional.

lTemperatur : DWORD;
            (* ~
            (OPC : 1 : Make variable visible for
OPC-Server)
            (OPC_PROP[0005] : 3            :
OPC_PROP_RIGHTS, here Read AND Write Access)
            (OPC_PROP[0100] : Grad F           :
OPC_PROP_UNIT)
            (OPC_PROP[0101] : Demovariable     :
OPC_PROP_DESC)
            (OPC_PROP[0205] : We are the champions :
OPC_PROP_SND)
            (OPC_PROP[0206] : ..\..\info.html      :
OPC_PROP_HTML)
            (OPC_PROP[0207] : ..\..\service.avi    :
OPC_PROP_AVI)
            *)

Each time compiling the PLC-project the PLC-Control will create the file <PLC-projectname>.tpy. This XML based file contains information about PLC-variables and their link to the OPC-server.
The OPC server will analyze this information of the TPY file., so configure the OPC-Server to know the file <PLC-projectname>.tpy Details

List of supported OPC-item properties of OPC-Specification :