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1、Visualization of PLC Programs using XMLM. Bani Younis and G. Frey Juniorprofessorship Agentenbased AutomationUniversity of KaiserslautemP. 0. Box 3049, D-67653 Kaiserslautem, GermanyAbstract- Due to the growing complexity of PLC programsthere is an increasing interest in the application of formalmet
2、hods in this area. Formal methods allow rigid proving ofsystem properties in verification and validation. One way to apply formal methods is to utilize a formal design approach inPLC programming. However, forexisting software that has tobe optimized, changed, or ported to new systems .There is thene
3、ed for an approach that can start from a given PLC program.Therefore,formalization ofPLC programs is a topic of current research. The paper outlines a re- engineering approach based on the formalization of PLC programs. Thetransformation into a vendor independent format and the visualization of the
4、structure of PLC programs is identified as an important intermediate step in this process. It is shownhow XML and corresponding technologies can be used for theformalization and visualization of an existing PLC program.I. INTRODUCTION Programmable Logic Controllers PLCs are a specialtype of computer
5、s that areused in industrial and safetycritical applications. The purpose of a PLC is to control aparticular process, or a collection of processes, by producing electrical control signals in responseto electrical process- related inputs signals. The systems controlled by PLCs vary tremendously, with
6、 applications in manufacturing,chemical process control, machining, transportation, power distribution, and many other fields. Automation applications can range in complexity froma simple panel to operate the lights and motorizedwindowshades inaconference roomtocompletelyautomated manufacturing line
7、s.With the widening of their application horizon ， PLC programs are beingsubject to increased complexity and highquality demands especially for safety-critical applications.The growing complexity of the applications within the compliance of limiteddevelopment time as wellas the reusability of existi
8、ng software orPLC modules requires a formal approach to be developed I.Ensuring the highquality demands requires verificationand validation procedures as wellas analysis and simulation of existing systems to be carried out2. One of the important fields for the formalization of PLC programs that have
9、 been growing up in recent time is Reverse- engineering 3.Reverse Engineering isa process ofevaluating something to understand how it works in order to duplicate or enhance it. While the reuse of PLC codes is being established as a tool for combating the complexity of PLC programs, Reverse Engineeri
10、ng is supposed to receive increased importance in the coming years especially ifexiting hardware has to be replaced by new hardware with different programming environmentsVisualization of existing PLC programs is an important intermediate step of Reverse Engineering. The paper provides an approach t
11、owards the visualization of14 / 20PLC programs using XMLwhich is an important approach for the orientation and better understanding for engineers working with PLC programs.The paper is structured as follows. First, a short introduction to PLCs and the corresponding programming techniques according t
12、o the IEC 61131-3 standard is given. In Section an approach for Re-engineering based on formalization of PLC programs isintroduced. Thetransformation ofthe PLCcode intoa vendor independent format is identified as an important first step in this process. XMLand corresponding technologies such as XSLa
13、nd XSLTthat can be used inthis transformation are presented in Section IV . Section V presents the application of XMLfor the visualization of PLC programs and illustrates the approach with an example. The final Section summarizes the resultsand gives an outlook on future work in this ongoing project
14、.PLC AND IEC61131Since its inception in the early 70s the PLC received increasing attention due to its successin fulfilling the objective of replacing hard-wired control equipments atmachines. Eventually it grew up as a distinct field of application, research and development, mainly for Control Engi
15、neering.IEC 61 131 is the first real endeavour to standardize PLC programminglanguages for industrial automation. In I993 the International Electrotechnical Commission 4 published the IEC 61131 Intemational Standard for Programmable Controllers. Before the standardization PLC programminglanguages we
16、re being developed as proprietary programming languages usable to PLCs of a special vendor.But in order to enhance compatibility, opennessand interoperability among different products as well asto promote the development of tools and methodologies with respect to a fixed set of notations the IEC 611
17、31standard evolved. The third part of this standard defines a suit offive programming languages:Instruction List IL is a low-level textual language with a structure similar to assembler. Originated in Europe IL is considered to be the PLC language in which all other IEC61 131-3 languages can be tran
18、slated.Ladder Diagram LO is a graphical language that has its roots in the USA. LDs conform to a programming style borrowed from electronic and electrical circuits for implementing control logics.Structured Text STJ is a very powerful high-level language. ST borrows its syntax from Pascal, augmentin
19、g it with some features from Ada. ST contains all the essential elements of a modem programming language.Function Block Diagram FBD is a graphical languageand it is very common to the processindustry. In this language controllers are modelled assignal and data flows through function blocks. FBD tran
20、sforms textual programming into connecting function blocks and thus improvesmodularity and software reuse.SequentialFunction Chart SFC is a graphical language. SFC elements are defined for structuring the organization of programmable controller programs.One problem with IEC 61 131-3 is that there is
21、 no standardized format for the project information in a PLC programming tool. At the moment there are only vendor specificformats. This is also one reason for the restriction of formalization approachesto single programs or algorithms.However, recently the PLC users organization PLCopensee http:/ww
22、w.plcopen.org started a Technical Committeeto define an XML based format for projects accordingto IEC 61131-3. This new format will ease the access offormalization tools to all relevant information of a PLC project. . RE-ENGINEERING APPROACHThe presented approach towards re-engineering cf. Fig.1 is
23、based upon the conception that XML can be used as amedium in which PLC codes will be transformed.This transformation offers the advantage ofobtaining avendor independent specification code. Even if the PLCopen succeeds in defining a standardized format for PLC applications, there will remain a lot o
24、f existing programsthat do not conform to this standard.Based on this code a step-wise transformation to a formalmodel automata is planned. This model can then beused for analysis, simulation, formal verification and validation,and finally for the re-implementation of the optimizedalgorithm on the s
25、ame or another PLC.Since re-engineering of complete programs will, in mostcases, be only a semi- automatic process, intermediate visualizationof the code is an important point. At differentstages of the process different aspects of the code and/orformal model have to be visualized in a way that a de
26、signercan guide the further work. XMLwith its powerful visualizationand transformation tools is an ideal tool for solvingthis task.IV. XML AS A TOOL FOR VISUALIZATIONXMLextensible Markup Language is a simple and flexible meta-language, i.e， a language for describing other languages. Tailored by the
27、World Wide Web Consortium W3C asa dialect of SGML S , XML removes two constraints which were holding back Web developments 6.The dependenceon a single, inflexible document typeHTML which was being much abused for tasks it wasnever designed for on one side； and the complexity of fullSGML,whose syntax
28、 allows many powerful but hard-to-programoptions on the other side.While HTML describeshow data should be presented,XML describes the data itself. A number of industries andscientific disciplines-medical records and newspaper publishingamong them-are already using XMLtoexchangeinformation across pla
29、tforms and applications. XMLcan betailored to describe virtuallyany kind of information in aform that the recipient of the information can use in a varietyof ways.It is specifically designed to support informationexchange between systems that use fundamentally differentforms of data representation,
30、as for example betweenCAD and scheduling applications.Using XML with its powerful parsers and inherent robustnessin terms of syntactic and semantic grammar ismore advantageous than the conventional method of usingalexical analyzer and a validating parser cf. Fig. 2,7.The conventional method of analy
31、sis of program coderequires a scanner lexical analyser which generatesa setof terminal symbols tokens followed by a parser thatchecks the grammatical structure of the code and generatesan object net. In the object net the internal structure of theprogram is representedby identified objects and the r
32、elationsbetween them. Both the scanner and the parser to beused in this method are document oriented which impliesthat analysis of different types of documents requires rewritingthe generated code forthe scanner and the parser. Anexample ofan application of this method can be foundin8.The most promi
33、singaspect ofusingXMLinstead isthatXMLand itscomplementary applications for transformationsare standardized so as to provide maximum flexibilityto its user.The XMLbased method is advantageous,since the lexicalspecification is an invariant component of XML ； thereforethe well-formedness is independen
34、t from the respectiveindividual application.Hence, an XML-Parser also can transfer well-shapedXML documents in an abstract representation called DocumentObject Model DOM without using a grammar.DOM is an application programming interface APII forvalid HTML and well-formed XML documents. It definesth
35、e logical structure of documents and theway a documentis accessedand manipulated. In the DOMspecification, theterm document is used in a broad sense increasingly. XMLis used as a way of representing many different kind of information that may be stored in diverse systems, andmuch ofthiswouldtraditio
36、nallybe seen as data rather thanasdocuments. Nevertheless, XMLpresents this data asdocuments,and the DOM can be used to manage this data5.XSLT, the transformation language for XML is capableof transforming XML notonly to another XML or HTMLbutto many other user-friendly formats. Before the adventof
37、XSLT, the transformation of XMLto any other formatwas only possible through custom applications developedin a procedural language such asC+, Visual Basic or,Java. This procedure lacked the generality with respect tothe structural variation of XML documents. Capitalizing onthe concept that the custom
38、 applications for the transformationsare all very similar, XSLT evolved as a high-leveldeclarative language9.XSLTfunctionsintwosteps. Inthe firststep, itperformsa structuraltransformation so as to convert the XMLintoa structure that reflects the desiredoutput. The secondstage is formatting the new s
39、tructure into the required format,such as HTML or PDF cf. Fig. 3 . The most importantadvantage of this transformation is that it allows a simpleand easily-conceivable representation of the documentor data structure embedded inside the well-structured buthard-to-understand XMLto be produced. When HTM
40、Lischosen as the format of the transformed produce it is possibleto use the extensive ability of HTMLto produce aneasily-conceivable and attractive visualization of a program.Every XML document has its own syntax and vocabulary.Therefore, in additionto being well-formed,the XMLdocumentneeds to confo
41、rm to a set ofrules. According toW3C recommendations this set of rules has to be definedeither through a Document Type Definition DTD or anXML Schema. The rules defined in a DTD or an XMLSchema state the hierarchical and structural constraints ofthe XML document. The DTD is for defining the document
42、 grammars； morerecently a number ofalternativelanguages have been proposed.The W3CXMLSchema language replicates theessential functionality of DTDs, and adds a number of features:the use of XMLinstance syntax rather than an ad hocnotation, clear relationships between schemas and namespaces,asystemati
43、c distinction between element types anddata types, and a single-inheritance form of type derivation.In other words schemas offer a richer and more powerfulwayofdescribing informationthan what is possible withDTDs. Fig. 4shows the XMLtechnologies discussedaboveand the connection between them.V. AN AP
44、PROACH FOR THE VISUALIZATION OFPLC PROGRAMSA. OverviewSince Instruction List IL is the most commonly usedPLC language in Europe, the presented approach is basedon this language. The proprietary IL dialect Siemens STEP5and the standardized version according to IEC 61131-3are considered.The generation
45、 of XML documents showing differentaspects of a PLC program is realized in the following threesteps cf. Fig. 5:1. Transformation of the PLC program to an XMLdocument2. Validation of the XML against the XML Schemawhich sets the syntax of the XML 3.Identification of the Instruction elements of thetran
46、sformed XML according to the instruction setof the source PLCThese three steps are discussed in sub-sections B to Drespectively. Sub-section Eexplains the visualization of thedifferent XMLs obtained during the preceding steps.Throughout this Section an example is used to illustrate the presented con
47、cepts. Fig. 6shows a PLC code written in Instruction List Siemens S5. The PLC code is written in atabular form where each row element is either a delimited list consisting of address, label, instruction, operand and descriptionor a comment.Kommentar:AutorErstellt :15.07.2003 Geaendert am: B1B:O NETZ
48、WERK 1 EMPFANGEN SLAVE 3 VON MASTER NAME :EMPEMAST0005:UM98.7 ABFRAGE OB EMPFANG MOEGLICH00060007 :SPB= MOOl00080009 :A DB140 EMPFANGSFACH IST DB 140 OOOA :L KF+20 LAENGE DES DATENPAKETSoooc :T DLOOOOD :L KF+O ZIELNUMMER O=MASTER OOOF :TDRO00100011 :UNM98.7 FANGEN WIEDER ERLAUBEN0012 :S M98.70013 MOOl :NOP 000140015 :BE BAUSTEIN ENDEFig. 6 APLC program written in SiemensS5 Instruction ListB. Conversion of a PLCProgram inio a well-formedXMLGiven a PLC program in ASCII forma