【正文】 controller Programmable Logic Controller is used to replace relays For the implementation of the logical judgment timing sequence number and other control functions The concept is presented Plc General Motors Corporation Plc and the basic design is the puter functional improvements flexible generic and other advantages and relay control system simple and easy to operate such as the advantages of cheap prices bined controller hardware is standard and overall According to the practical application of target software in order to control the content of the user procedures memory controller the controller and connecting the accused convenient target In the mid1970s the Plc has been widely used as a central processing unit microprocessor import export module and the external circuits are used largescale integrated circuits even when the Plc is no longer the only logical IC judgment functions also have data processing PID conditioning and data munications functions International Electro technical Commission IEC standards promulgated programmable controller for programmable controller draft made the following definition programmable controller is a digital electronic puters operating system specifically for applications in the industrial design environment It used programmable memory used to implement logic in their internal storage operations sequence control timing counting and arithmetic operations such as operating instructions and through digital and analog input and output the control of various types of machinery or production processes Programmable controller and related peripherals and industrial control systems easily linked to form a whole to expand its functional design Programmable controller for the user is a noncontact equipment the procedures can be changed to change production processes The programmable controller has bee a powerful tool for factory automation widely popular replication Programmable controller is useroriented industries dedicated control puter with many distinctive features First high reliability antiinterference capability Secondprogramming visual simple Third adaptability good Fourth functional improvements strong functional interface TWOHistory of PLC Programmable Logic Controllers PLC a puting device invented by Richard E Morley in 1968 have been widely used in industry including manufacturing systems transportation systems chemical process facilities and many others At that time the PLC replaced the hardwired logic with softwired logic or socalled relay ladder logic RLL a programming language visually resembling the hardwired logic and reduced thereby the configuration time from 6 months down to 6 days [Moody and Morley 1999] Although PC based control has started to e into place PLC based control will remain the technique to which the majority of industrial applications will adhere due to its higher performance lower price and superior reliability in harsh environments Moreover according to a study on the PLC market of Frost and Sullivan [1995] an increase of the annual sales volume to 15 million PLCs per year with the hardware value of more than 8 billion US dollars has been predicted though the prices of puting hardware is steadily dropping The inventor of the PLC Richard E Morley fairly considers the PLC market as a 5billion industry at the present time Though PLCs are widely used in industrial practice the programming of PLC based control systems is still very much relying on trialanderror Alike software engineering PLC software design is facing the software dilemma or crisis in a similar way Morley himself emphasized this aspect most forcefully by indicating If houses were built like software projects a single woodpecker could destroy civilization Particularly practical problems in PLC programming are to eliminate software bugs and to reduce the maintenance costs of old ladder logic programs Though the hardware costs of PLCs are dropping continuously reducing the scan time of the ladder logic is still an issue in industry so that lowcost PLCs can be used In general the productivity in generating PLC is far behind pared to other domains for instance VLSI design where efficient puter aided design tools are in practice Existent software engineering methodologies are not necessarily applicable to the PLC based software design because PLCprogramming requires a simultaneous consideration of hardware and software The software design bees thereby more and more the major cost driver In many industrial design projects more than of the manpower allocated for the control system design and installation is scheduled for testing and debugging PLC programs In addition current PLC based control systems are not properly designed to support the growing demand for flexibility and reconfigurability of manufacturing systems A further problem impelling the need for a systematic design methodology is the increasing software plexity in largescale projects The objective of this thesis is to develop a systematic software design methodology for PLC operated automation systems The design methodology involves highlevel descri