What is OPC?

OPC is open connectivity via open standards. They fill a need in automation like printer drivers did for Windows. See the summary of current and emerging OPC Specifications and OPC Certification.

Here s a great picture of the OPC Architecture.

OPC is open connectivity in industrial automation and the enterprise systems that support industry. Interoperability is assured through the creation and maintenance of open standards specifications. There are currently seven standards specifications completed or in development.

Based on fundamental standards and technology of the general computing market, the OPC Foundation adapts and creates specifications that fill industry-specific needs. OPC will continue to create new standards as needs arise and to adapt existing standards to utilize new technology.

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OPC is a series of standards specifications. The first standard (originally called simply the OPC Specification and now called the Data Access 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 (component object model) and DCOM (distributed component object model) 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.

Everyone's favorite analogy for needing the original Data Access Specification is printer drivers in DOS and then in Windows. Under DOS the developer of each application had to also write a printer driver for every printer. So AutoCAD wrote the AutoCAD application and the printer drivers. And WordPerfect wrote the WordPerfect application and the printer drivers. They had to write a separate printer driver for every printer they wanted to support: one for an Epson FX-80 and one for the H-P LaserJet, and on and on. In the industrial automation world, Intellution wrote their Human Machine Interface (HMI) software and a proprietary driver to each industrial device (including every PLC brand). Rockwell wrote their HMI and a proprietary driver to each industrial device (including every PLC brand, not just their own).

Windows solved the printer driver problem by incorporating printer support into the operating system. Now one printer driver served all the applications! And these were printer drivers that the printer manufacturer wrote (not the application developer). Windows provided the infrastructure to allow the industrial device driver's solution as well. Adding the OPC specification to Microsoft's OLE technology in Windows allowed standardization. Now the industrial devices' manufacturers could write the OPC DA Servers and the software (like HMIs) could become OPC Clients.

The resulting selfish benefit to the software suppliers was the ability to reduce their expenditures for connectivity and focus them on the core features of the software. For the users, the benefit was flexibility. They could now choose software suppliers based on features instead of "Do they have the driver to my unique device?" They don't have to create a custom interface that they must bear the full cost of creating and upgrading through operating system or device vendor changes. Users were also assured of better quality connectivity as the OPC DA Specification codified the connection mechanism and compliance testing. OPC interface products are built once and reused many times; hence, they undergo continuous quality control and improvement.

The user's project cycle is shorter using standardized software components. And their cost is lower. These benefits are real and tangible. Because the OPC standards are based in turn upon computer industry standards, technical reliability is assured.

The original specification standardized the acquisition of process data. It was quickly realized that communicating other types of data could benefit from standardization. Standards for Alarms & Events, Historical Data, and Batch data were launched.

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Current and emerging OPC Specifications include:

	OPC Data Access
	The originals! Used to move real-time data from PLCs, DCSs, and other control devices to HMIs and other display clients. The Data Access 3 specification is now a Release Candidate. It leverages earlier versions while improving the browsing capabilities and incorporating XML-DA Schema.
	OPC Alarms & Events
	Provides alarm and event notifications on demand (in contrast to the continuous data flow of Data Access). These include process alarms, operator actions, informational messages, and tracking/auditing messages.
	OPC Batch
	This spec carries the OPC philosophy to the specialized needs of batch processes. It provides interfaces for the exchange of equipment capabilities (corresponding to the S88.01 Physical Model) and current operating conditions.
	OPC Data eXchange
	This specification takes us from client/server to server-to-server with communication across Ethernet fieldbus networks. This provides multi-vendor interoperability! And, oh by the way, adds remote configuration, diagnostic and monitoring/management services.
	OPC Historical Data Access
	Where OPC Data Access provides access to real-time, continually changing data, OPC Historical Data Access provides access to data already stored. From a simple serial data logging system to a complex SCADA system, historical archives can be retrieved in a uniform manner.
	OPC Security
	All the OPC servers provide information that is valuable to the enterprise and if improperly updated, could have significant consequences to plant processes. OPC Security specifies how to control client access to these servers in order to protect this sensitive information and to guard against unauthorized modification of process parameters.
	OPC XML-DA
	Provides flexible, consistent rules and formats for exposing plant floor data using XML, leveraging the work done by Microsoft and others on  SOAP and Web Services.
	OPC Complex Data
	A companion specification to Data Access and XML-DA that allows servers to expose and describe more complicated data types such as binary structures and XML documents.
	OPC Commands
	A Working Group has been formed to develop a new set of interfaces that allow OPC clients and servers to identify, send and monitor control commands which execute on a device.
	OPC Unified Architecture
	A new set of specifications that are not based on Microsoft COM that will provide standards based cross-platform capability.

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OPC Product Certification and Interoperability

	OPC Compliance
	The vision of interoperability in multi vendor systems has become a reality, via the OPC standards. Certification is the process of ensuring that applications meet the standards. OPC Certification programs include Self-Certification, Interoperability Workshops and 3rd party testing by Independent Certification Test Labs.

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