By Randy Kandor, B.Sc. Eng. CompEOPC HAS QUICKLY become the de facto open communication standard in the world of industrial connectivity. It offers improved data connectivity, while dramatically lowering the cost of data transfer between devices and applications.OPC communication is layered on top of various Microsoft operating system components. These components function very well under typical LAN conditions (high bandwidth and reliable connections), but under less favorable conditions, their behavior can lead to unreliable data delivery, and even data loss. OPC Tunnelling provides a solution to this problem.OPC (OLE for Process Control) standardizes data sharing between plant floor devices (DCSs, PLCs, analyzers, etc) and software applications (such as HMIs, Process Historians, trenders, etc). No matter what the device is, data is always shared with an application in a standardized format. Of course, standards-based communication is only half the task – the other half deals with the actual method by which the data moves across the network.COM/DCOM: The infrastructure for OPC
OPC is built on Microsoft technology called COM, which stands for Component Object Model. It is a framework that defines how applications communicate with each other and share data. When OPC applications are installed on the same computer they use COM to exchange data. However, when installed on 2 separate PCs, then the COM messages are wrapped in a Microsoft security layer called DCOM (Distributed COM).Most OPC configurations involve the use of DCOM. There are circumstances where DCOM can experience timeouts that can lead to unreliable data delivery and even data loss. Such circumstances can include:
OPC Tunnelling provides an alternative approach that eliminates the DCOM risk altogether. This technology uses standards-based TCP/IP communications instead of DCOM for carrying OPC messages. It provides users with configurable timeouts that are appropriate for the specific installation.In a typical setup, an OPC Tunneller application is installed on each of the two PCs in use (see the diagram below). Each OPC Tunneller communicates with the locally installed OPC application with OPC (using COM). The two OPC Tunneller applications then pass the OPC messages using TCP/IP, which eliminates the requirement for DCOM.
OPC is built on Microsoft technology called COM, which stands for Component Object Model. It is a framework that defines how applications communicate with each other and share data. When OPC applications are installed on the same computer they use COM to exchange data. However, when installed on 2 separate PCs, then the COM messages are wrapped in a Microsoft security layer called DCOM (Distributed COM).Most OPC configurations involve the use of DCOM. There are circumstances where DCOM can experience timeouts that can lead to unreliable data delivery and even data loss. Such circumstances can include:
- Hardware problems, such as a faulty network card, router or switch
- System issues, such as an overloaded network
- Inherently unstable network architectures that use satellite links, WAN, radio communication, etc.
OPC Tunnelling provides an alternative approach that eliminates the DCOM risk altogether. This technology uses standards-based TCP/IP communications instead of DCOM for carrying OPC messages. It provides users with configurable timeouts that are appropriate for the specific installation.In a typical setup, an OPC Tunneller application is installed on each of the two PCs in use (see the diagram below). Each OPC Tunneller communicates with the locally installed OPC application with OPC (using COM). The two OPC Tunneller applications then pass the OPC messages using TCP/IP, which eliminates the requirement for DCOM.
OPC Tunnelling offers other advantages over DCOM:
- Users avoid the initial DCOM configuration problems
- OPC applications can share data across different domains
- Allows for tighter security in firewall configuration
- Reduce bandwidth requirements considerably
Looking back
OPC is an accepted standard within the process control industry. Whenever possible, users should use the existing infrastructure: COM/DCOM. However, in cases where the underlying technology cannot accommodate the project’s specific needs, OPC Tunnelling provides a viable and reliable alternative.
Since its inception in 1996, the OPC Foundation has been working to build awareness and offer an alternative to exchanging data with proprietary drivers and application. The demand for OPC drivers and tools is increasing exponentially. More and more vendors and developers are working towards making their products OPC compliant. The ability for OPC to now tunnel its way across connections where DCOM may be inappropriate is just another step towards creating a truly open, robust and scalable solution for the process control industry.
Additional Resources:
Multimedia Tutorial
Matrikon OPC
