processanalyticaltechnology

1. OPC in Process Control and Automation

OPC Express Interface (OPC Xi) is the newest OPC specification from the OPC Foundation. This article is written by Tom Burke (OPC Foundation), Lee Neitzel (Emerson) and Randy Kondor (OPCTI) briefly compares OPC Xi with Classic OPC and OPC UA, and provides an overview of OPC Xi.

2. OPC Standards Satisfy Different Plant Requirements

All OPC specifications have the same main objective: securely transport industrial data between automation applications on separate computers. However, each specification has a slightly different objective. Comparing OPC technologies is not a matter of the difference between apples and oranges, but rather between different types of apples. The OPC set of specifications that is right for an individual plant depends on the specific application objectives, communication vision, vendor product availability/cost, and integration restrictions.

3. Putting Express in OPC

As the name implies, the motivation for the creation of OPC Xi is simplicity and speed. Programmers can quickly develop automation applications, integrators can apply existing knowledge for efficient deployment, and end users can use OPC in any plant network environment, no matter how complex.

3.1 Rapid Application Development

While Classic OPC relies on DCOM, OPC Xi relies on WCF (Windows Communication Foundation) to transport data. This is a key design concept for OPC Xi because vendors strive to increase application reliability while simultaneously decreasing product development time. For this reason, programmers are developing client applications that execute inside Managed Code. A Managed Code environment enables Windows to capture application errors without crashing (thus avoiding the dreaded “blue screen of death”). Microsoft’s .NET set of tools provide this Managed Code environment, and they use WCF (Windows Communication Foundation) to transport data. The COM/DCOM communication infrastructure of Classic OPC uses Unmanaged Code and so it becomes time-consuming to integrate newly developed .NET clients with existing OPC DCOM servers. To solve this problem, OPC Xi was developed to provide a standard .NET wrapper for existing OPC servers.

3.2 Secure Network Communication

Security is a key concern for plants that need to transport data across a network. WCF automatically provides applications with a secure communication infrastructure that provides all the necessary security services for Authentication, Authorization, and Encryption. Programmers do not need to make any special considerations for these security services, and they can leave the decision to either allow or deny access to specific User Accounts and Groups to the plant itself. Therefore, network architects and integrators can make security-related decisions at deployment time and can easily modify security settings afterwards to accommodate new requirements without having to ask their vendors to make any software changes.

OPC Xi layers additional security on top of the WCF security mechanisms. This additional security provides for restricting read, write, and subscribe access to servers based on the location and identity of the client application and the protocol it uses to access the server. For example, the server may disallow write access to the server if the client is accessing the server across the web using http.

3.3 Deploy across large networks

Plants can quickly deploy OPC Xi applications across networks of any size. WCF works with standard network equipment and protocols, which enables IT (Information Technology) personnel to comprehend and approve architectures. IT and Automation personnel do not require specialized or proprietary knowledge to assemble their systems; they can work together to meet their respective goals.

From small open networks to large networks that must comply with the most stringent of federal security regulations, Microsoft designed WCF to traverse all existing network equipment. Plants can configure OPC Xi to communicate across network equipment and appliances such as routers, switches, firewalls and NAT (Network Address Translation).

3.4 Non-Windows Operating Systems

WCF provides programmers with the ability to use XML web services. So, they can design applications to work on any operating system, such as UNIX, Mac, or even VxWorks. This will enable vendors to develop OPC Xi clients on the platform of their choice (like a PLC, Analyzer, RTU, Intelligent Electronic Device, etc). However, since XML web services are slower than native WCF binary communications (which can transfer thousands of points per second), high performance applications should not be designed to use web services. Therefore, if speed is an issue in cross-platform applications, OPC UA (OPC Unified Architecture) is an outstanding alternative.

4. Conclusion

With the release of OPC Xi, the OPC Foundation is securing the future of OPC. OPC Xi enables plants to replace their existing reliance on DCOM with WCF. OPC Xi builds on the success of Classic OPC and the innovative architecture of OPC UA. OPC Xi enables vendors to quickly develop reliable automation applications, and plants to sharply increase the penetration of OPC in their facilities.

5. Resources

OPC Xi Specification Information: http://www.opcti.com/opc-xi-specification.aspx

OPC Xi Webinar: http://www.opcti.com/Webinars-Schedule.aspx