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Compare OPC DA, OPC Xi and OPC UA standards


Posted 2010-08-24 17:09:06 by OPC Foundation

Compare OPC DA, OPC Xi and OPC UA standards OPC DA (Data Access) is a group of standards that provides specifications for communicating real-time data (or historical data with OPC HDA) from data acquisition devices such as PLCs. There are three attributes associated with OPC DA tags: value, the quality of the value and timestamp. This is the first standard resulted from the collaboration of a number of leading worldwide automation suppliers working in cooperation with Microsoft. Originally based on Microsoft 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.

OPC UA is much more complex than previous OPC specifications and is designed to: use cross-platform capable Web Services instead of DCOM, combine the OPC DA, A&E, HDA functionality into a single set of services, model complex data structures for collaboration with other standards organizations, be implementable on different platforms, from embedded systems to enterprise systems.

The ExpressInterface Xi specification is designed to deliver secure and reliable real-time and historical data communication. Xi delivers fast and secure communication through firewalls and for simple implementation and use. OPC Xi is the newest technology added to the OPC Foundation technology portfolio complementing OPC Unified Architecture (UA) and COM-based OPC Classic technologies. Key features of OPC Xi: security, simplicity, plug-and-play, backward compatibility. Industrial plants have been successfully using OPC for the secure transportation of process control and automation data since 1996. To build on the success of Classic OPC, the OPC Foundation has been working diligently on new specifications to help secure the future adoption of OPC. In 2009, the OPC Foundation released OPC UA (OPC Unified Architecture), which enabled companies to embed OPC Servers on PLCs, ensure Operating System platform independence, and provide a rich industrial information model. Then, to help with rapid application development using .NET, they released OPC Xi (OPC Express Interface) in 2010. The existing OPC COM based specifications have served the OPC Community well over the past 10 years, but as technology moves on so must our interoperability standards. Here are the factors that influenced the decision to create a new architecture:
  • Microsoft has deemphasized COM in favor of cross-platform capable Web Services and SOA (Service Oriented Architecture)
  • OPC Vendors want a single set of services to expose the OPC data models (DA, A&E, HDA etc)
  • OPC Vendors want to implement OPC on non-Microsoft systems, including embedded devices
  • Other collaborating organizations need a reliable, efficient way to move higher level structured data
The Unified Architecture (OPC-UA) is described in a layered set of specifications broken into Parts. It is purposely described in abstract terms and in later parts married to existing technology on which software can be built. This layering is on purpose and helps isolate changes in OPC-UA from changes in the technology used to implement it. The structure and depth of material to absorb in learning OPC-UA is harder than the OPC COM Specifications.

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.
OPC DA | HDAOPC XiOPC UA
Client/Server Architecture
Transfer real-time and historical data
WindowsAny operation System
Microsoft communication componentsVendor Components
No complex objectsInformation model can define objects
Windows security modelAddition security levelOPC UA security model
Microsoft COM/DCOM APIMicrosoft WCF APIOPC Foundation protocol
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.

While OPC DA 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.

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.

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).

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.

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.

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