Programmable controller is user-oriented industries dedicated control computer, with many distinctive features.

First, high reliability,anti-interference capability;

Second，programming visual, simple;

Third,adaptability good;

Fourth functional improvements,strong functional interface. TWO：History of PLC

Programmable Logic Controllers(PLC), a computing 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 soft-wired logic or so-called relay ladder logic(RLL),a programming language visually resembling the hardwired logic, and reduced thereby the configuration time from 6 months down to 6days[Moody and Morley, 1999].

Although PC based control has started to come 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 PLC per year with the hardware value of more than8 billion US dollars has been predicted,though the prices of computing hardware is steadily dropping. The inventor of the PLC, Richard E Morley, fairly considers the PLC market as a5-billion 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 trial-and-error.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