OPC fundamentals

OPC objects and interfaces

An OPC client can connect to OPC servers provided by one or more vendors. Vendor supplied code determines the devices and data to which each server has access, the data names, and the details about how the server physically accesses that data.

Where OPC fits

Although OPC is primarily designed for accessing data from a networked server, OPC interfaces can be used in many places within an application. At the lowest level they can get raw data from the physical devices into a SCADA or DCS, or from the SCADA or DCS system into the application. The architecture and design makes it possible to construct an OPC server that allows a client application to access data from many OPC servers provided by many different OPC vendors running on different nodes via a single object.

General OPC architecture and components

OPC specifications always contain two sets of interfaces; Custom Interfaces and Automation Interfaces. The OPC Specification specifies COM interfaces (what the interfaces are), not the implementation (not the how of the implementation) of those interfaces. It specifies the behavior that the interfaces are expected to provide to the client applications that use them.

Included are descriptions of architectures and interfaces that seemed most appropriate for those architectures. Like all COM implementations, the architecture of OPC is a client-server model where the OPC server component provides an interface to the OPC objects and manages them.

There are several unique considerations in implementing an OPC server. The main issue is the frequency of data transfer over non-sharable communication paths to physical devices or other data bases. Thus, OPC servers are either a local or remote EXE which includes code responsible for efficient data collection from a physical device or a database.

An OPC client application communicates to an OPC server through the specified custom and automation interfaces. OPC servers must implement the custom interface, and optionally may implement the automation interface. In some cases, the OPC Foundation provides a standard automation interface wrapper. This "wrapperDLL" can be used for any vendor-specific custom-server.