OPC brings XML-DA to the factory floor

Control Solutions - July 2002
Jim Luth
Chairman of the OPC Alarm & Events Committee
Consulting Engineer
Iconics
Foxborough, MA 02035

Two years ago, the OPC Foundation (http://opcfoundation.org) undertook the task of creating a new XML (eXtensible Markup Language) standard to augment its existing, widely successful COM/DCOM standards. Programmable logic controller (PLC), distributed control system (DCS), human-machine interface (HMI), and other factory-floor software vendors use the OPC-COM standards to move real-time data between field devices, control systems and other applications in a standard way, promoting multivendor compatibility and interoperability.

The OPC XML Committee work is now complete. At the time of this writing, the XML-DA (Data Access) specification is in release-candidate form. By the time you read this, the specification should be released and available to the public (as all OPC specifications are).

Why XML?
In the last few years, XML has become a preferred format for encoding and moving data in an open, system-independent way. XML- a highly structured text format-is both machine- and human-readable. (For more on XML, see "XML: A key enabler on the road to factory-to-enterprise integration" in the December 2000 issue of Control Solutions.)

The OPC Foundation chose XML as the enabling technology to move the exact same data as the existing OPC-COM interface, with the following expected benefits:

Easy integration with Internet applica tions-Existing OPC-COM applications work fine on the typical LAN (local area network) environment of the factory floor. However, if you try to use DCOM over the Internet, you will likely be thwarted by DCOM's tight coupling with MicrosoftRegistered Windows NTRegistered security and its use of dynamically allocated TCP/IP ports (not typically allowed through corporate firewalls).

Conversely, XML is a World Wide Web Consortium (W3C) specification designed specifically with Internet protocols in mind. It is often transported via HTTP just like ordinary HTML Web pages, but is also easily transported via other Internet protocols, such as FTP and SMTP.

Implementation in nonMicrosoft environ ments-Microsoft designed COM/DCOM as a modern, object-based RPC (Remote Procedure Call) mechanism to facilitate cross-component, cross-process and cross-machine communications used in the Microsoft Windows environment. Although Microsoft has opened the door to nonMicrosoft implementations, COM/DCOM never really caught on outside of Microsoft, and is not available on most nonMicrosoft platforms.

XML as a W3C standard is platform agnostic and fully supported on all modern platforms. Even the newer SOAP and Web Services standards that build on top of the XML standard (on which OPC XML-DA is based) have widespread support across many platforms and computing environments.

Better connectivity to enterprise ap pli cations-Manufac turing enterprise applications like MRP need real-time plant floor data that is often available via servers that implement the OPC-COM interfaces. The problem is, most of these higher-level applications do not implement the OPC-COM interfaces necessary to talk to OPC-COM servers. Many times these applications are on nonMicrosoft platforms, making it all but impossible to communicate via COM.

FIG. 1: OPC XML provides factory floor to manufacturing enterprise integration.

Connectivity is the key to most of these enterprise systems, and many of these applications now embrace XML as the preferred method of data exchange. With OPC also embracing XML, the real-time OPC data will now be within easy reach of manufacturing applications outside the realm of the factory floor (Fig. 1).

SOAP, Web Services & Microsoft .NET
The OPC-XML Committee decided early on to limit the scope of the first OPC-XML specification to the same ground covered by the existing OPC-DA specification. Hence, this would be an XML-DA specification only, and other OPC-COM interfaces like Alarm & Events (AE) and Historical Data Access (HDA) would also have companion XML specifications that would follow sometime after the release of XML-DA. With the scope limited to the well-known DA interface, it did not take the committee long to come up with XML schemas to define the DA data. So why did it take more than two years to release the specification? Two reasons: the limited capabilities of existing tools and interoperability.

FIG. 2: A single XML-DA wrapper can front-end multiple existing OPC-COM servers.

Defining XML schemas is pretty easy, but OPC is all about interoperability, and defining XML schemas is not enough to ensure multi-vendor compatibility. The committee would also have to specify communication protocols, discovery mechanisms, error handling and all of the rest of the "plumbing" that came for free when OPC designed its DCOM interfaces. The committee also found that working with the limited XML tools of two years ago (DOM and SAX XML parsers) was far more tedious than the high-level COM programming that had been accepted by the OPC community.

Fortunately, the OPC Committee was not alone in dealing with these XML growing pains. Major platform vendors like Microsoft were hard at work layering new standards and new tools on top of the existing XML standards. First, SOAP (Simple Object Access Protocol) is layered on top of XML, then WSDL (Web Services Description Language) is layered on top of SOAP. Together, these specifications define the plumbing that is needed for true interoperability.

FIG 3: OPC provides members with source code for sample clients and servers.

With the Web Services specification in place and the high-level tools available to easily create them, the OPC XML Com mit tee chose to complete the reference implementation (sample code) using Microsoft's Visual Studio .NET. As the author of the original XML-DA Sample Client, I can attest to the ease of use and high level of programmer productivity using the .NET tools to create and consume Web Services like XML-DA.

Aids to early adoption
With a new specification based on new technology that is best implemented with brand new tools and platforms, it would seem that XML-DA is geared for the future, but not for now. However, there are several factors that should speed the adoption of the interface:

Software vendors in the manufacturing arena will adopt the use of the new programming tools like .NET quickly-not because of OPC, but for the gains in programmer productivity. Once the tools with built-in Web Services support are adopted, it will be simple to support industry-standard Web Services like XML-DA.
If all of the existing OPC vendors had to upgrade each of the hundreds of OPC-COM servers to XML-DA in order to gain a critical mass, we could be in for a very long wait. The XML-DA standard was designed specifically to allow existing OPC-COM servers to be "wrapped" by XML-DA. An XML-DA Wrapper can be a generic, standalone process that front-ends any number of OPC-COM servers from any software vendor. Thus, a vendor that provides such a generic wrapper (see Figure 2 on page 35) can instantly XML-enable every OPC-COM server in the world! I expect one or more of the many OPC vendors are working on this killer app right now.
The OPC is committed to providing true production-quality sample code (Fig. 3) to assist OPC members with their development efforts. The first sample server and client, released at the same time as the specification, are written in C# for the Microsoft .NET platform.

Future OPC-XML specifications
The release of XML-DA is the first in a long line of Web Services specifications that will be released by OPC. The other functional areas addressed by the other OPC-COM speci fi cations (Alarm & Events, Historical Data Access, Batch etc.) will all need to be brought into the world of Web Services. As with XML-DA, these new specifications will allow for wrappers so that the existing base of COM servers can be instantly upgraded.

An important new paradigm
XML Web Services are an important new programming paradigm complete with built-in support in the latest tools from major computer hardware and software vendors such as Microsoft, IBM, Sun Microsystems and others. The XML-DA Standard from OPC is a Web Service interface designed to move factory-floor data between software components across a LAN or across the Internet. Through the use of wrappers, hundreds of existing OPC-COM Servers can be exposed to the world via XML-DA, speeding its viability and acceptance as a standard.

About the author

Jim Luth is chairman of the OPC Alarm & Events Committee and a member of the OPC Technical Steering Committee. He has been a key contributor to many of the OPC working groups, including XML-DA.

Mr. Luth is a consulting engineer at ICONICS, Inc., responsible for the architecture and design of many of the GENESIS32 components. His career spans more than 20 years in the automation industry. He has previously held engineering positions with The Foxboro Co., Taylor Instruments and General Motors.

He has a BS in electrical engineering from Rensselaer Polytechnic Institute (RPI) in Troy, New York.

For more information...
Jim Luth will be available to answer any questions you may have about this article. He can be reached via e-mail at JimL@iconics.com.

ICONICS Background Information

Founded in 1986, ICONICS is an award winning leader in the development of Web-enabled industrial automation and manufacturing intelligence software for Microsoft® Windows® operating systems. Our solutions are certified for Windows 7, Windows Server 2008 and Windows Vista. ICONICS has successfully deployed more than 250,000 solutions in over 60 countries worldwide. Its solutions meet diverse customer needs in a variety of industries including Automotive, Building Management, Food & Beverage, Oil/Gas/Petrochemical, Machine Builders, Pharmaceutical/Biotech, Security, Water/Wastewater, Utilities, Government Infrastructures and more.

ICONICS’ corporate headquarters is located in Foxborough, Massachusetts (USA) near Boston. The company has offices throughout the United States, as well as in Australia, China, France, India, Italy, Germany, Netherlands and United Kingdom. ICONICS also has a joint software development and sales office in the Czech Republic.

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