【文章內(nèi)容簡介】 is capable of transforming XML not only to another XML or HTML but to many other userfriendly formats. Before the advent of XSLT, the transformation of XML to any other format was only possible through custom applications developed in 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 highlevel declarative language [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 important advantage of this transformation is that it allows a simple and easilyconceivable representation of the document or data structure embedded inside the wellstructured but hardtounderstand XML to be produced. When HTML is chosen as the format of the transformed produce it is possible to use the extensive ability of HTML to produce an easilyconceivable and attractive visualization of a program.Every XML document has its own syntax and vocabulary. Therefore, in addition to being wellformed, the XML document needs to conform to a set of rules. According to W3C remendations this set of rules 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 have 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 types, and a singleinheritance 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 PROGRAMSA. Overview Since Instruction List (IL) is the most monly 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 611313 are considered. The generation of XML documents showing different aspects of a PLC program is realized in the following three steps (cf. Fig. 5): of the PLC program to an XML document of the XML against the XML Schema which sets the syntax of the XML of the Instruction elements of the transformed XML according to the instruction set of the source PLC These three steps are discussed in subsections B to D respectively. Subsection 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 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 ment.Kommentar :AutorErstellt : Geaendert am: B1B:ONETZWERK 1 EMPFANGEN SLAVE 3 VON MASTERNAME :EMPE39。MAST0005 :U ABFRAGE OB EMPFANG MOEGLICH00060007 :SPB= MOOl00080009 :A DB140 EMPFANGSFACH IST DB 140OOOA :L KF+20 LAENGE DES DATENPAKETSoooc :T DLOOOOD :L KF+O ZIELNUMMER O=MASTEROOOF :T DRO00100011 : FANGEN WIEDER ERLAUBEN0012 :S 0013 MOOl :NOP 000140015 :BE BAUSTEIN ENDEFig. 6 A PLC program written in Siemens S5 Instruction ListB. Conversion of a PLC Program inio a wellformed XML Given a PLC program in ASCII format and in a tabular structure with separate columns for addresses, labels, instructions, operands and descriptions delimited by whitespaces, XSLT can convert it into a wellformed XML document. The XML document obtained through this transformation is a hierarchically structured document. Fig. 7 shows the XML document obtained through the transformation of the PLC code of Fig. 6. The XML document is structured in a hierarchy in which the root element is the IL Code Block representing the whole PLC code. Each of the rows of the PLC code is contained within a corresponding ILRow element which is M e r smtctured into child elements. Note: The structure chosen for the XML representation of ILCode is oriented at the working proposal of the PLCopen.C. XML Validation against the XML Schema The XML obtained as a result of the previous processing can be validated using a validating parser that confirms that the XML document in addition to being wellformed conforms to the set of syntactic rules defined in context of the PLC programming language. D. rdenhpcation of instructions This step in the process of visualization of PLC programs using XML ensures that the XML document to be used for visualization contains only valid can be used to transform the wellformed and valid Xh4L to another XML which as a result of identification on instructions has an additional attribute appende