PLCopen and OPC UA introduce Communications Model | Automation.com

PLCopen and OPC UA introduce Communications Model

April 19, 2010 - OPC Foundation and PLCopen combine their technologies to platform and manufacturer-independent information and communication architecture. This mix from OPC Unified Architecture (UA) and IEC 61131-3 creates interesting options for the realization of automation structures.

Objective is to increase the reusability of controllers and visualization modules and their communication - in the long run the efficiency of the engineering process clearly increases.

As an example imagine a PLC project running on different control platforms from different suppliers which seen from the outside by a visualization tool or MES/ERP from the axis look exactly the same: The IEC 61131-3 software model with all PLC objects and their detailed descriptions are illustrated in the OPC UA namespace by the work of the common working group by OPC UA.

Today there are still many old historically grown data exchange protocols being used in various industries. They offer little possibility to transport complex data and the extensibility is very limited. OPC Unified Architecture however, offers the basis for universal, secure and reliable network communication, i.e. provides the monitoring of configurable timeouts and connecting interruptions and encrypted communication. To use this communication as standard for the IEC 61131 environment will lead in many industries to a completely new form of information exchange: If an industry has defined a specific profile e.g. a data structure or a function block, then the question of secured and efficient data exchange and the reusability of the visualization objects is already solved. With the combination of PLCopen and OPC UA, an additional level of interoperability is standardized on top of IEC 61131-3.

The first step was to map the IEC 61131-3 software model to an OPC UA information model. This standardizes the way how controllers with integrated OPC UA server technology expose data structures and function blocks to OPC UA clients like HMIs. This first step was achieved in a very short time followed by first demonstrations.

When today an IEC 61131-3 control program is loaded on different control platforms from different control suppliers one can communicate with these controllers by using OPC UA and access process variables. The representation in the namespace of the OPC UA servers is however different from each platform: A visualization program must each time be adapted for each controller although the control code is identical.

However, the expectation from the customers is that an identical control project also is accessed via OPC UA in the way. Besides the accessibility to the instances of the controller variables, further metadata needs to be available. Descriptions how complex data structures are constructed, the type of function blocks used, and which variable are in or out parameters. Other metadata can be the number of tasks and their cycle times. The entire IEC 61131-3 software model and the content of the controller programs are mapped into the OPC UA namespace. This namespace can be provided by an OPC UA server which is integrated into an embedded controller.

But why use OPC UA? With the Unified Architecture, OPC Foundation answers questions like:
  • How to find a communication partner?
  • How are data, metadata, and functionalities of the communication partner made available?
  • How can communication be reliable, performant and independent of the operating system or the programming language?
  • How can security aspects such as authentication and encrypting be ensured?

  • Unified Architecture offers the basis for universal, secure and durable network communication, i.e. provides the monitoring of configurable timeouts and connecting interrupts, encrypted communication and different communication protocols, are part of OPC UA. Many other organizations use OPC UA as a modern and efficient transport layer. PLCopen quickly recognized the possibilities, and adds to OPC UA by describing the contents, thus "what ", which should be exchanged: not only variables but also functions and metadata of the control logic.

    The results from the working group are an important basis for the future: A HMI for example can now provide a picture template for a function block - this graphic element can now communicate directly with all controllers on which the function block is present. This clearly simplifies the engineering process. The results of this first step are demonstrated at the Hannover Messe 2010 with controllers and HMIs from different vendors.

    After this basic work on the information model, further steps will include communication function blocks linking the controller directly to the OPC UA environment. For many branches this could mean an enormous simplification: nowadays control programmers often need to work with branch specific protocols. These protocols are each time adapted and extended to be able to support security and further data objects. With the combination of PLCopen and OPC UA, an additional level of interoperability is standardized on top of IEC 61131-3.

    About OPC Foundation
    The OPC Foundation defines standards for online data exchange between automation systems. They address access to current data (OPC DA), alarms and events (OPC A&E) and historical data (OPC HDA). Those standards are successfully applied in industrial automation.

    About PLCopen
    PLCopen was founded in 1992 as an independent worldwide association for industrial suppliers and users. It is the leading association resolving topics related to industrial control programming. PLCopen creates concepts to reduce the costs of industrial automation. These cost savings are realized in areas such as engineering, training, operation and maintenance. Together with its members, PLCopen creates specifications to materialize these concepts.

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