OPC Unified Architecture Unveiled

About 10 years ago, representatives of five automation companies met with Microsoft Corp., the Redmond, Wash., software supplier, to define a method for data exchange among computers and controllers from various suppliers. The result, released in 1996, was OLE for Process Control, or OPC. “OLE” stood for object linking and embedding—then the Microsoft technology for moving information from application to application within its Windows operating system environment.

Microsoft replaced OLE with component object model (COM) and distributed COM (DCOM). Users discovered that the data exchange specification worked for more applications than process control. So then the name became simply OPC. Its guardian organization, the OPC Foundation, refers to it as “open connectivity via open standards.”

OPC is perhaps the most successful cooperative effort in automation. It is used by an overwhelming number of users, as suggested by a survey undertaken by the OPC Foundation. But it has evolved into an unwieldy conglomeration of specifications. These include data access (DA, the original), alarms and events, batch (corresponding to the Instrumentation, Systems and Automation Society’s ISA-88.01), data exchange, historical data access, security, XML DA (for eXtensible Markup Language DA) and complex data.

Rashesh Mody, OPC chief architect, as well as chief technology officer of Lake Forest, Calif., software supplier Wonderware, expands on the results of a survey, citing requests for improved robustness, replacement of DCOM and adding failure detection at the top of users’ wish lists.

Mody details the important goals of the new Unified Architecture (UA) upgrade to the specification. "The new thing is XML messaging. This will replace DCOM. Next, we will provide a wrapper for legacy COM objects so that they will interoperate with the new XML-based messaging. The 'old' OPC came in various flavors such as data access and alarming. The Unified Architecture will bring them together into one specification. Improving security and robustness is a priority. Finally, we will add the ability to talk to applications at the manufacturing execution system (MES) and enterprise resource planning (ERP) levels. The key words in this work are reliable, robust and secure."

User input

Another change is the process used to create the new specifications. Previously, the work was done by a group of engineers from the technology supplier community. This time, the Foundation is involving many users of the technology, as well. Says Mody, "We are working with OMAC's (Open Modular Architecture Control users group) Microsoft Manufacturers Users Group (MSMUG). Input from the group goes directly to OPC Foundation Executive Director Tom Burke and me."

The UA specification will rely on open standards such as XML and simple object access protocol (SOAP), as well as Ethernet standards transmission control protocol/Internet protocol (TCP/IP) and hypertext transfer protocol (HTTP), rather than on Microsoft protocols.

Burke says, "The whole world, including Microsoft, is embracing open, Internet-based communication standards. Not being tied solely to Microsoft platforms makes OPC UA highly scalable. So expect to see OPC UA implemented on everything from embedded field devices through to Unix or mainframe-based enterprise applications."

Describing how the Unified Architecture will break down the walls separating all the current specifications, Burke states, "Clients and servers based on the Unified Architecture will blur the distinction among the various specifications, because the transfer of data, no matter what the type or purpose, all use the same base services. This will be coupled with support of complex data (arrays, binary structures, XML documents and the like), such that server vendors are not forced into a subset of data in an OPC-defined format."

OPC Foundation is also working with several other standards bodies in order to assure even greater enterprise interoperability in the future. Some of the other organizations include the ISA's SP-95 and SP-88 committees, the Machinery Information Management Open Systems Alliance (MIMOSA), the Fieldbus Foundation's enhanced device description language (EDDL), the Open Applications Group and technical committee 57 of the International Electrotechnical Commission.

For more information, please visitwww.opcfoundation.org.