【正文】 has strong antijamming and antivibration and impact resistance, so in a harsh working environment has been widely Applications. I also mean freedom of munication S7300 type PLC39。 play for spare parts ? Open system architecture ? Scalable redundancy ? The intelligent terminal TM 1703 The Base Unit AK 1703 ACP with Peripheral Elements has one basic system element CP2020/CPC25 (Master control element) and CP2020/PCCE25 (Processing and munication element) ,one bus line with max. 16 peripheral elements can be connected. CP2020/CPC25 Features and Functions System Functions: ? Central element,coordinating all system services Central hub function for all connected basic system elements ? Time management Central clock of the automation unit Setting and keeping the own clock`s time with a resolution of 10ms Synchronization via serid munication via LAN or local ? Redundancy Voting and changeover for redundant processing and munication elements of the own automation unit Supports voting and changeover by an external SCARS redundancy switch Supports applicational voting and changeover by an external system, control system ? SAT TOLLBOX|| connection Storing firmware and parameters on a Flash Card Communication: ? Communication via installable protocol elements to any superior or subordinate automation unit ? Automatic data flow routing ? Priority based data transmission (priority control) ? Own circular buffer and process image for each connected station(data keeping) ? Redundant munication routes Communication with redundant remote stations ? Special application specific functions for dialup traffic Test if stations are reachable Process Peripherals: ? Transmission of spontaneous information objects from and to peripheral elements, via the serial Ax 1703 peripheral bus Functions for Automation: ? Open/closedloop control function for the execution of freely definable user programs which are created with CAEX plus according to IEC 611313, function diagram technology 512KB for user program Approx variables and signals, of them retained Cycle of 10ms or a multiple thereof Online test Loadable without service interruption ? Redundant open/closedloop control functions Synchronization via redundancy link Transmission of periodic process information between the open/closedloop control function and the peripheral elements,via the serial Ax 1703 peripheral bus. Six、 SIEMENS PLC SIMATIC S7300 series PLC applied to all walks of life and various occasions in the detection, monitoring and control of automation, its power to both the independent operation of, or connected to a work able to achieve plex control. The photoelectric products with isolation, high electromagic patibility。 ISO 157451, 1999] have been formed to fix and disseminate stateoftheart design methods, but they normally cannot participate in advancing the knowledge of efficient program and system design. A systematic approach will increase the level of design automation through reusing existing software ponents, and will provide methods to make largescale system design manageable. Likewise, it will improve software quality and reliability and will be relevant to systems high security standards, especially those having hazardous impact on the environment such as airport control, and public railroads. The software industry is regarded as a performance destructor and plexity generator. Steadily shrinking hardware prices spoils the need for software performance in terms of code optimization and efficiency. The result is that massive and less efficient software code on one hand outpaces the gains in hardware performance on the other hand. Secondly, software proliferates into plexity of unmanageable dimensions。 IEC611313, 1993。 Fourth functional improvements, strong functional interface. TWO、 History 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