TC4 - Communication
PLCopen and OPC Foundation create transparent communication
with mapping of the software model and releasing
OPC-UA Client Function Blocks for IEC61131-3
In October 2008 the cooperation between PLCopen and OPC Foundation started with a kick-off to map the IEC 61131-3 Software Model to the OPC UA information model, which was released in March 2010 by both organizations. The basis was provided by the OPC UA Client Server architecture for which the server is integrated in the controller with a link to the extended program and controller information as provided by the PLCopen / IEC 61131-3 mapping.
From the beginning it was clear that integrating the OPC UA server in the controller was only the first step. So in 2011 the working groups initiated the next step by adding the OPC UA client functionality in the controller by defining a set of Function Blocks for IEC61131-3. This specification has been released on April 3rd 2014.
With this specification the controller becomes an intelligent part in the IT communication. The interaction between IT and the industrial automation level is often represented by the established model of the automation pyramid:
In terms of communication, this model represents the assumption that a controller, as a main component of the automation system, is dumb towards the higher level, and only responds to requests from these higher levels. The higher level is always the client and initiates the data requests the lower layer is always the server and courteously responds to the requests. In the modern world this strict separation of levels, as well as the top-down approach of the information flow, is softened and more and more a flat models appear. For instance, in a smart network every device or service must be able to initiate independent communication with all other services.
The result of the working group is about OPC-UA client functionality out of the IEC61131-3 controller: A controller can exchange complex data structures horizontally with other controllers independently from fieldbus system or vertically with other devices using an OPC-UA server call in an MES/ERP system in order to collect data or write new production orders to the cloud. It allows a production line to be independently active in combination with integrated OPC UA Security features.
If one looks to a production line one can easily see where this functionality fits in. For example in the production line hereunder the different machines can communicate to each other via the client server architecture. Moreover, with the Method call they can start a method in a system on a higher level, and for instance get a production recipe for the product at hand.
An OPC-UA client functionality in a controller does not provide hard deterministic real time and so its not a deterministic fieldbus but OPC UA provides fast and secure communication, and provides modelling mechanism for information models.
Status of the PLCopen TC4 Communication work
PLCopen together with the OPC Foundation worked on a new edition of the definition of a set of Function Blocks providing OPC-UA client functionality. With this functionality implemented on a controller it becomes possible to initiate a communication session to any other available OPC UA Server. The controller can exchange complex data structures horizontally with other controllers independently from fieldbus system or vertically with other devices using an OPC-UA server call in an MES/ERP system in order to collect data or write new production orders to the cloud. It allows a production line to be independently active in combination with integrated OPC-UA Security features.
The original version 1.0 was published 2 years ago, and during several implementations it became clear that additional as well as different functionality was needed, esp. concerning a List feature where one can group information in a list, and work with the function blocks on these lists (versus individual parameters).
This new edition is now published as Release for Comments. You are herewith invited to give your feedback till April 8, 2016 so we can make this version better suitable for your usage.
You can download this document PLCopen_OPC_UA_ClientFunctionBlocks_V1 1_RfC as .pdf
Read more about the cooperation between PLCopen and OPC Foundation: