1、Visualization of PLC Programs using XML M. Bani Younis and G. Frey University of Kaiserslautem P. 0. Box 3049, D-67653 Kaiserslautem, Germany Abstrac: Due to the growing complexity of PLC programs there is an increasing interest in the application of formal methods in this area. Formal methods allow
2、 rigid proving of system properties in verification and validation. One way to apply formal methods is to utilize a formal design approach in PLC programming. However, for existing software that has to be optimized, changed, or ported to new systems .There is the need for an approach that can start
3、from a given PLC program. Therefore, formalization of PLC programs is a topic of current research. The paper outlines a re-engineering approach based on the formalization of PLC programs. The transformation into a vendor independent format and the visualization of the structure of PLC programs is id
4、entified as an important intermediate step in this process. It is shown how XML and corresponding technologies can be used for the formalization and visualization of an existing PLC program. I. INTRODUCTION Programmable Logic Controllers (PLCs) are a special type of computers that are used in indust
5、rial and safety critical applications. The purpose of a PLC is to control a particular process, or a collection of processes, by producing electrical control signals in response to electrical process- related inputs signals. The systems controlled by PLCs vary tremendously, with applications in manu
6、facturing, chemical process control, machining, transportation, power distribution, and many other fields. Automation applications can range in complexity from a simple panel to operate the lights and motorized window shades in a conference room to completely automated manufacturing lines. With the
7、widening of their application horizon， PLC programs are being subject to increased complexity and high quality demands especially for safety-critical applications. The growing complexity of the applications within the compliance of limited development time as well as the reusability of existing soft
8、ware or PLC modules requires a formal approach to be developed I. Ensuring the high quality demands requires verification and validation procedures as well as analysis and simulation of existing systems to be carried out 2. 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 is a process of evaluating 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 Engineer
10、ing is supposed to receive increased importance in the coming years especially if exiting hardware has to be replaced by new hardware with different programming environments Visualization of existing PLC programs is an important intermediate step of Reverse Engineering. The paper provides an approac
11、h towards the visualization of PLC programs using XML which 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 to
12、 the IEC 61131-3 standard is given. In Section an approach for Re-engineering based on formalization of PLC programs is introduced. The transformation of the PLC code into a vendor independent format is identified as an important first step in this process. XML and corresponding technologies such as
13、 XSL and XSLT that can be used in this transformation are presented in Section IV. Section V presents the application of XML for the visualization of PLC programs and illustrates the approach with an example. The final Section summarizes the results and gives an outlook on future work in this ongoin
14、g project. PLC AND IEC 61131 Since its inception in the early 70s the PLC received increasing attention due to its success in fulfilling the objective of replacing hard-wired control equipments at machines. Eventually it grew up as a distinct field of application, research and development, mainly fo
15、r Control Engineering. IEC 61 131 is the first real endeavour to standardize PLC programming languages for industrial automation. In I993 the International Electrotechnical Commission 4 published the IEC 61131 Intemational Standard for Programmable Controllers. Before the standardization PLC program
16、ming languages were being developed as proprietary programming languages usable to PLCs of a special vendor. But in order to enhance compatibility, openness and interoperability among different products as well as to promote the development of tools and methodologies with respect to a fixed set of n
17、otations the IEC 61131 standard evolved. The third part of this standard defines a suit of five 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 13
18、1-3 languages can be translated. 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
19、 syntax from Pascal, augmenting it with some features from Ada. ST contains all the essential elements of a modem programming language. Function Block Diagram (FBD) is a graphical language and it is very common to the process industry. In this language controllers are modelled as signal and data flo
20、ws through function blocks. FBD transforms textual programming into connecting function blocks and thus improves modularity and software reuse. Sequential Function Chart (SFC) is a graphical language. SFC elements are defined for structuring the organization of programmable controller programs. One
21、problem with IEC 61 131-3 is that there is no standardized format for the project information in a PLC programming tool. At the moment there are only vendor specific formats. This is also one reason for the restriction of formalization approaches to single programs or algorithms. However, recently t
22、he PLC users organization PLCopen (see http:/www.plcopen.org) started a Technical Committee to define an XML based format for projects according to IEC 61131-3. This new format will ease the access of formalization tools to all relevant information of a PLC project. . RE-ENGINEERING APPROACH The pre
23、sented approach towards re-engineering (cf. Fig.1) is based upon the conception that XML can be used as a medium in which PLC codes will be transformed. This transformation offers the advantage of obtaining avendor independent specification code. (Even if the PLCopen succeeds in defining a standardi
24、zed format for PLC applications, there will remain a lot of existing programs that do not conform to this standard.) Based on this code a step-wise transformation to a formal model (automata) is planned. This model can then be used for analysis, simulation, formal verification and validation, and fi
25、nally for the re-implementation of the optimized algorithm on the same or another PLC. Since re-engineering of complete programs will, in most cases, be only a semi-automatic process, intermediate visualization of the code is an important point. At different stages of the process different aspects o
26、f the code and/or formal model have to be visualized in a way that a designer can guide the further work. XML with its powerful visualization and transformation tools is an ideal tool for solving this task. IV. XML AS A TOOL FOR VISUALIZATION XML (extensible Markup Language) is a simple and flexible
27、 meta-language, i.e， a language for describing other languages. Tailored by the World Wide Web Consortium (W3C) as a dialect of SGML S, XML removes two constraints which were holding back Web developments 6. The dependence on a single, inflexible document type (HTML) which was being much abused for
28、tasks it was never designed for on one side; and the complexity of full SGML, whose syntax allows many powerful but hard-to-program options on the other side. While HTML describes how data should be presented, XML describes the data itself. A number of industries and scientific disciplines-medical r
29、ecords and newspaper publishing among them-are already using XML to exchange information across platforms and applications. XML can be tailored to describe virtually any kind of information in a form that the recipient of the information can use in a variety of ways. It is specifically designed to s
30、upport information exchange between systems that use fundamentally different forms of data representation, as for example between CAD and scheduling applications. Using XML with its powerful parsers and inherent robustness in terms of syntactic and semantic grammar is more advantageous than the conv
31、entional method of using a lexical analyzer and a validating parser (cf. Fig. 2, 7). The conventional method of analysis of program code requires a scanner (lexical analyser) which generates a set of terminal symbols (tokens) followed by a parser that checks the grammatical structure of the code and
32、 generates an object net. In the object net the internal structure of the program is represented by identified objects and the relations between them. Both the scanner and the parser to be used in this method are document oriented which implies that analysis of different types of documents requires
33、rewriting the generated code for the scanner and the parser. An example of an application of this method can be found in 8. The most promising aspect of using XML instead is that XML and its complementary applications for transformations are standardized so as to provide maximum flexibility to its u
34、ser. The XML based method is advantageous, since the lexical specification is an invariant component of XML; therefore the well-formedness is independent from the respective individual application. Hence, an XML-Parser also can transfer well-shaped XML documents in an abstract representation called
35、Document Object Model (DOM) without using a grammar. DOM is an application programming interface (APII) for valid HTML and well-formed XML documents. It defines the logical structure of documents and the way a document is accessed and manipulated. In the DOM specification, the term “document“ is use
36、d in a broad sense increasingly. XML is used as a way of representing many different kind of information that may be stored in diverse systems, and much of this would traditionally be seen as data rather than as documents. Nevertheless, XML presents this data as documents, and the DOM can be used to
37、 manage this data 5 . XSLT, the transformation language for XML is capable of transforming XML not only to another XML or HTML but to many other user-friendly formats. Before the advent of XSLT, the transformation of XML to any other format was only possible through custom applications developed in
38、a procedural language such as C+, Visual Basic or, Java. This procedure lacked the generality with respect to the structural variation of XML documents. Capitalizing on the concept that the custom applications for the transformations are all very similar, XSLT evolved as a high-level declarative lan
39、guage 9. XSLT functions in two steps. In the first step, it performs a structural transformation so as to convert the XML into a structure that reflects the desired output. The second stage is formatting the new structure into the required format, such as HTML or PDF (cf. Fig. 3 ). The most importan
40、t advantage of this transformation is that it allows a simple and easily-conceivable representation of the document or data structure embedded inside the well-structured but hard-to-understand XML to be produced. When HTML is chosen as the format of the transformed produce it is possible to use the
41、extensive ability of HTML to produce an easily-conceivable and attractive visualization of a program. Every XML document has its own syntax and vocabulary. Therefore, in addition to being well-formed, the XML document needs to conform to a set of rules. According to W3C recommendations this set of r
42、ules has to be defined either through a Document Type Definition (DTD) or an XML Schema. The rules defined in a DTD or an XML Schema state the hierarchical and structural constraints of the XML document. The DTD is for defining the document grammars; more recently a number of alternative languages h
43、ave been proposed. The W3C XML Schema language replicates the essential functionality of DTDs, and adds a number of features: the use of XML instance syntax rather than an ad hoc notation, clear relationships between schemas and namespaces, a systematic distinction between element types and data typ
44、es, and a single-inheritance form of type derivation. In other words schemas offer a richer and more powerful way of describing information than what is possible with DTDs. Fig. 4 shows the XML technologies discussed above and the connection between them. V. AN APPROACH FOR THE VISUALIZATION OFPLC P
45、ROGRAMS A. Overview Since Instruction List (IL) is the most commonly used PLC language in Europe, the presented approach is based on this language. The proprietary IL dialect Siemens STEP 5 and the standardized version according to IEC 61131-3 are considered. The generation of XML documents showing
46、different aspects of a PLC program is realized in the following three steps (cf. Fig. 5): 1.Transformation of the PLC program to an XML document 2.Validation of the XML against the XML Schema which sets the syntax of the XML 3.Identification of the Instruction elements of the transformed XML accordi
47、ng to the instruction set of the source PLC These three steps are discussed in sub-sections B to D respectively. Sub-section E explains the visualization of the different XMLs obtained during the preceding steps. Throughout this Section an example is used to illustrate the presented concepts. Fig. 6
48、 shows 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 description or a comment. Kommentar : Autor Erstellt :15.07.2003 Geaendert am: B1B:O NETZWERK 1
49、EMPFANGEN SLAVE 3 VON MASTER NAME :EMPEMAST 0005 :U M98.7 ABFRAGE OB EMPFANG MOEGLICH 0006 0007 :SPB= MOOl 0008 0009 :A DB140 EMPFANGSFACH IST DB 140 OOOA :L KF+20 LAENGE DES DATENPAKETS oooc :T DLO OOOD :L KF+O ZIELNUMMER O=MASTER OOOF :T DRO 0010 0011 :UNM98.7 FANGEN WIEDER ERLAUBEN 0012 :S M98.7 0013 MOOl :NOP 0 0014 0015 :BE BAUSTEIN ENDE Fig. 6 A PLC program written in Siemens S5 Instruction List B. Conversion of a PLC Program inio a well-formed XML Given a PLC program in ASCII format and in a tabular structure with separate columns for addresses, labels, in
