logic programs. Though the hardware costs of PLC are dropping continuously, reducing the scan time of the ladder logic is still an issue in industry so that low-cost PLC can be used.

In general,the productivity in generating PLC is far behind compared to other domains, for instance, VLSI design, where efficient computer aided design tools are in practice.Existent software engineering methodologies are not necessarily applicable to the PLC based software design because PLC-programming requires a simultaneous consideration of hardware and software. The software design becomes,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 complexity in large-scale projects.

The objective of this thesis is to develop a systematic software design methodology for PLC operated automation systems. The design methodology involves high-level description based on state transition models that treat automation control systems as discrete event systems, a stepwise design process,and set of design rules providing guidance and measurements to achieve a successful design.The tangible outcome of this research is to find a way to reduce the uncertainty in managing the control software development process, that is, reducing programming and debugging time and their variation,increasing flexibility of the automation systems, and enabling software reusability through modularity. The goal is to overcome shortcomings of current programming strategies that are based on the experience of the individual software developer.

Three：now of PLC

From the structure is divided into fixed PLC and Module PLC, the two kinds of PLC including CPU board,I/O board, display panel, memory block, power, these elements into a do not remove overall.