The OPC Client driver allows VTS to act as a client to an OPC server, enabling the exchange of data between a VTS application and the server. The advantage to using OPC is that it enables VTS 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 VTS to access its data via an OPC automation DLL.
The OPC Server allows VTS to act as a server to an OPC client. This allows other programs (including another VTS application with an OPC client driver) to exchange data with VTS.
An OPC Server enables clients to retrieve information about objects. There are 3 basic levels of objects:
• The OPC Server,
• The Group, and
• The Items.
The OPC Server object maintains information about the server, and acts as a container for OPC group objects. The OPC Server object completely handles data requests from clients.
The OPC Group object is basically 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 VTS 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 timestamp and a set of quality flags. The data will be either numeric or text.
In order that the client can determine what items are available to be added to groups, the OPC server object provides a “browsing” capability within the server “namespace”. The namespace is organized as a hierarchy. The namespace contains not only tag value identifiers, but property value identifiers that may be associated with a tag.
An example of using an OPC Client driver tag to access data on an OPC Server is included in OPC Client Driver Addressing.