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Therefore, formalization of PLC programs is a topic of current research. The paper outlines a reengineering 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 identified 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. INTRODUCTIONProgrammable Logic Controllers (PLCs) are a special type of puters that are used in industrial 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 manufacturing, chemical process control, machining, transportation, power distribution, and many other fields. Automation applications can range in plexity from a simple panel to operate the lights and motorized window shades in a conference room to pletely automated manufacturing lines.With the widening of their application horizon， PLC programs are being subject to increased plexity and high quality demands especially for safetycritical applications. The growing plexity of the applications within the pliance of limited development time as well as the reusability of existing software 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 been growing up in recent time is Reverseengineering [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 bating the plexity of PLC programs, Reverse Engineering is supposed to receive increased importance in the ing years especially if exiting 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 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 the IEC 611313 standard is given. In Section Ⅲ an approach for Reengineering 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 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 ongoing project.Ⅱ PLC AND IEC 61131Since its inception in the early ‘70s the PLC received increasing attention due to its success in fulfilling the objective of replacing hardwired control equipments at machines. Eventually it grew up as a distinct field of application, research and development, mainly for 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 programming languages were being developed as proprietary programming languages usable to PLCs of a special vendor. But in order to enhance patibility, openness and interoperability among different products as well as to promote the development of tools and methodologies with respect to a fixed set of notations the IEC 61131 standard evolved. The third part of this standard defines a suit of five programming languages:Instruction List (IL) is a lowlevel textual language with a structure similar to assembler. Originated in Europe IL is considered to be the PLC language in which all other IEC61 1313 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 highlevel language. ST borrows its 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 mon to the process industry. In this language controllers are modelled as signal and data flows through function blocks. FBD