Understanding OPC

OPC Interoperability: Open Connectivity Through Open Standards

OPC is a client/server technology. One application acts as the server providing data, and the other acts as a client using data.

OPC is a widely accepted industrial communication standard that enables the exchange of data between multi-vendor devices and control applications without any proprietary restrictions. An OPC server can communicate data continuously among PLCs on the shop floor, RTUs in the field, HMI stations, and software applications on desktop PCs. Even when the hardware and software are from different vendors, OPC compliance makes continuous real-time communication possible.

OPC has led to improved cooperation between technology providers and users alike. OPC has helped automation suppliers provide solutions that are truly open, which in turn has given users more choices in their automation applications. This is an exciting time in the industry. Interoperability, open solutions, and freedom of choice have helped automation professionals around the globe realize the advantages of incorporating OPC into their industrial applications.

OPC is open connectivity in industrial automation and the enterprise systems that support the industry. Interoperability is assured through the creation and maintenance of non-proprietary open standards specifications. The first OPC standard specification resulted from the collaboration of a number of leading worldwide automation suppliers working in cooperation with Microsoft. Originally based on Microsoft's OLE COM and DCOM technologies, the specification defined a standard set of objects, interfaces, and methods for use in process control and manufacturing automation applications to facilitate interoperability. The COM/DCOM technologies provided the framework for software products to be developed. There are now hundreds of OPC Data Access servers and clients.

OPC standards are based on technologies of the general computing market. Products by OPC members offer a wide range of tools for people who prefer "off-the-shelf" products, to mid-level engineers who like the efficiency of building with a toolkit, to the developer who prefers to build the entire application.

OPC Data Access Overview

This specification describes the OPC COM objects and their interfaces implemented by OPC servers. Different vendors provide OPC servers, and 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. Specifics on naming conventions are supplied in a subsequent section.

At a high level, an OPC server is comprised of several objects: the server, the group, and the item. The OPC server object maintains information about the server and serves as a container for OPC group objects. The OPC group object maintains information about itself and provides the mechanism for containing and logically organizing OPC items.

The OPC groups provide a way for clients to organize data. For example, the group might represent items in a particular report or operator display. Data can be read and written. Exception based connections can also be created between the client and the items in the group, and can be enabled and disabled as needed. An OPC client can configure the rate that an OPC server should provide the data changes to the OPC client. There are two types of groups: public and local. Public allows sharing across multiple clients; local is local to a client. (Refer to the section on public groups for the intent, purpose, and functionality, and for further details.) There are also specific optional interfaces for the public groups. Within each group, the client can define one or more OPC items.

The OPC items represent connections to data sources within the server. An OPC item, from the custom interface perspective, is not accessible as an object by an OPC client. Therefore, there is no external interface defined for an OPC item. All access to OPC items is via an OPC group object that contains the OPC item, or simply where the OPC item is defined.

Associated with each item is a Value, Quality and Time Stamp. The value is in the form of a VARIANT, and the Quality is similar to that specified by Fieldbus.

Note that the items are not the data sources; they are just connections to them. For example, the tags in a DCS system exist regardless of whether an OPC client is currently accessing them. The OPC item should be thought of as simply specifying the address of the data, not as the actual physical source of the data that the address references.

OPC Data Access Details

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 make 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.

The OPC specification specifies COM interfaces, or what the interfaces are. It does not specify 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 communications paths to physical devices. Thus, we expect that the OPC server will either be a local or remote EXE that includes code that is responsible for efficient data collection from a physical device.

An OPC client application communicates to an OPC server through the specified OPC custom and automation interfaces. OPC servers must implement the custom interface, and optionally may implement the automation interface.

An inproc (OPC handler) may be used to marshal the interface and provide the additional item level functionality of the OPC automation interface.

It is also expected that the server will consolidate and optimize data accesses requested by the various clients to promote efficient communications with the physical device. For inputs (reads), data returned by the device is buffered for asynchronous distribution or synchronous collection by various OPC clients. For outputs (writes), the OPC server updates the physical device data on behalf of OPC clients.

The current OPC Data Access specification is version 3.0. KEPServerEX has fully supported OPC DA 3.0 since 2005.

The History of OPC

In 1994, a group of vendors representing a broad spectrum of disciplines in the industrial segment formed what is now known as the OPC Foundation. The OPC Foundation put forth the goal of developing a single client/server specification that would allow any vendor to develop software and applications that could share data in a fast, robust fashion, and do it in a way that would eliminate the proprietary schemes that forced these same vendors to duplicate development efforts.

The OPC Foundation developed the first specification, called Data Access Specification 1.0a, which was released in early 1996. Using this specification, vendors were able to quickly develop client/server software.

A major goal of the OPC Foundation and the Data Access specification was to eliminate the need of client application vendors to develop their own proprietary set of communications drivers. For many vendors, the effort required to develop numerous communications drivers outweighed the development effort involved in the client application itself. With the adoption of OPC technology, a vendor could now focus their efforts almost exclusively on the development of the client application. The Data Access specification defines how both the client and the server application interface must be constructed. If the specification is followed properly, a client vendor knows that any OPC server that exists for an industrial device can provide the connectivity needed for data access. Issues like time to market or reliability no longer restrict OPC applications.

OPC has given the end user the additional benefit of being able to select best-of-breed software to solve application problems. Historically, if the application software did not have the desired communication driver or if the available driver didn't perform adequately, the only solution was to try to persuade the application vendor to either develop the desired driver or repair an existing driver. The time required to complete either of these solutions was not typically short. With OPC, the end user is no longer tied to the resource limitations of the client application vendor. The user can now choose from a variety of OPC server vendors to address a new driver requirement or remedy a performance issue. Equally, the client application vendor can now focus on the continued improvement of their core product without the disruptive effort required to address communication issues and needs.

Kepware's goal within the OPC environment is to be a leading provider of the server component of the OPC equation and to do so by providing a product that is reliable and easy to use. KEPServerEX is built upon years of development efforts in communications driver development and OPC technology.

A lot has been said here about OPC but how does the OPC specification work?

To learn more about the OPC standard, visit the OPC Foundation or download Kepware’s Understanding OPC eBook.