The ability to exchange function block and application models between different software tools and platforms is a very important aspect of the IEC 61499 standard. Being able to exchange function blocks between different products and systems will bring significant end-user benefits. This will include major cost savings from being able to re-use function block designs in different hardware and system configurations.
The ability to exchange software designs between different vendors’ products has been a serious limitation of the IEC 61131-3 PLC Software standard because IEC 61131 failed to include a definition of a file exchange format. It has not been possible for graphical PLC software designs using IEC 61131-3 languages to be exchanged between different PLC software support tools.
There is now an option in IEC 61499 Part 2 to use the Extended Markup Language XML as a file exchange format for IEC 61499 library elements. XML is the next generation language on from the hypertext markup language (HTML) that is used for all World Wide Web page definitions. The use of XML will bring some exciting benefits and should have a major impact on how quickly IEC 61499 is accepted by the industrial community. This will mean that IEC 61499 function block designs can be transferred across the Internet and viewed on Web pages using the next generation of Web browsers. Part 2 defines Document Type Definitions (DTDs) for use with XML to provide additional attributes to the various library elements. For example, there is the provision for adding attributes such as Version, Author, Date and Remarks to function block type specifications.
Part 2 also allows XML to be used to express function block type specifications that use both textual and graphical languages for algorithm definitions. For example, there are extensions to allow function block algorithms to be defined using the
IEC 61131-3 Ladder Diagram language. This means that a function block can be designed graphically and then stored and transferred using the XML file format. The complete graphical design can then be viewed on XML compliant Web browsers as shown in Figure 7.1.
Note: XML browsers will display the hierarchical structure and attributes of IEC
61499 library elements using a tree view. Browsers will require additional add-ins or applets to show the various graphical views of IEC 61499 library elements.
As use of the Internet and the World Wide Web becomes more commonplace we will undoubtedly start to see the influence of this technology in the industrial control domain. Dr. Christensen from Rockwell Automation has already demon- strated the use of Java as a viable control language for expressing function block
Figure 7.1 Using XML for function block definitions IEC 1499 Function
Block Specification
VoterBlk
In 1 In 2 Out 1
In 1 In 3
Algorithm defined using IEC 1131-3 Ladder Diagram
Complete function block definition in XML format <?xml version=“1.0”encoding=“UTF-8”?> <!DOCTYPE System SYSTEM “FBType.dtd”> <FBType Name=“VoterBlk” Comment=“2 out of 4 voter”> <Identification Function=“VOTER” Classification=“Safety” Type=“Boolean”/> <VersionInfo Organisation=“LewisTechnology” Version=“1.0” … VoterBlk
XML viewed using Web browsers
algorithms. If we link the use of Java along with XML as a method for storing function block definitions, we are very close to having an Internet centred control system design methodology. Perhaps we will start to see IEC 61499 being applied in completely novel control system designs where functionality is distributed across an Intranet or even world-wide across the Internet.
Summary
As this is the end of the last chapter, we have reached a point where we have considered all the main aspects of the new IEC 61499 function block standard. We have seen how the various elements defined in Part 1 fit together to create a system architecture for function block execution. The standard covers the key features of the function block specification including formal definition of its external interface including the inputs and outputs of the block and their association with input and output events.
We have seen that the standard includes a special form of state machine, the Execution Control Chart, to define how events and algorithms are related within a function block. There are also special forms of function block for dealing with the interfaces to external services, such as accessing device I/O values or communi- cating with other function blocks in other resources.
In chapter 6 we reviewed how these concepts can be applied to some industrial applications. We have seen that the function block approach is particularly applic- able to Fieldbus devices. In this final chapter we have reviewed IEC 61499 Part 2 and considered some of the capabilities of engineering tools that are required to manage and create function block oriented system models. This included a discussion on the use of a file exchange format based on XML which will allow function block designs to be viewed in the future using Web browsers.
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