OPC Classic Configuration
OPC
Open Platform Communications. A communication interface based on DCOM and Web Services technology. Classic is an older communication interface based on DCOM Distributed Component Object Model and web services An application programming interface (API) that can be accessed using HTML. technology, which enables automation and control applications running on your PC to communicate with control equipment. In short, the OPC specification defines a set of interfaces for easy-to-use objects, and includes the functions and properties to enable the manipulation of these objects.
The basic transport layer for OPC is DCOM (Distributed Component Object Model). Because it is distributed, a human-machine interface (HMI) or supervisory control and data acquisition (SCADA) software package like VTScada can process and collect data from OPC Servers that are running on different computers on a network.
Configuration of OPC to use DCOM is not easy and recent changes by Microsoft (2023) add extra complexity. Trihedral recommends the use of an OPC tunneller instead.
For support of the tunneller software of your choice, please refer to the supplier.
The OPC server can produce error -1073479672 (0xC0040008) when referencing a tag that does not exist or is not available over OPC.
VTScada provides both an OPC Client driver and an OPC Server as tag types. Access to the server tag depends on whether your VTScada license key includes this optional feature. Please see your Trihedral account representative for details.
OPC Classic Client
The OPC Classic Client driver enables VTScada to act as a client to an OPC server, enabling the exchange of data between a VTScada application and the server. The advantage to using OPC is that it enables VTScada to communicate with new control devices without requiring a custom-written driver. The OPC server takes care of communications with the control device, and enables VTScada to access its data via an OPC automation DLL.
If you install third-party OPC software in some cases the VTScada OPC Client or Server may stop functioning correctly. Sometimes this can be solved by re-installing the OPC Core Components. Contact technical support for assistance.
The OPC Server (OPC Driver) enables VTScada to act as a server to an OPC client. This enables other programs (including another VTScada application with an OPC client driver) to exchange data with VTScada.
An OPC Server enables clients to retrieve information about objects. There are 3 basic levels of objects:
- OPC Server
- Group
- Items
The OPC Server object maintains information about the server, and acts as a container for OPC group objects. It handles data requests from clients.
The OPC Group object is a collection container for OPC Items. The OPC Group maintains information about itself and provides the mechanism both for containing and logically organizing OPC items.
The OPC items represent connections to data sources within the OPC Server. In the case where VTScada is providing the server, these items are tags. Each item provides the client with data and properties. Data presented to the client by each item is specified to contain one value, one time stamp and a set of quality flags. The data will be either numeric or text.
The OPC server object enables clients to browse within the server namespace to find what items are available to add to groups. The namespace is organized as a hierarchy and contains not only tag value identifiers, but property value identifiers that may be associated with a tag.
Trace Viewer Application - You can monitor OPC communications using the Trace Viewer. Note that communications will be slowed while tracing is enabled.
