【正文】 are design bees, thereby, more and more the major cost driver. In many industrial design projects, more than 50% 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 reconfigure ability of manufacturing systems. PLC is welladapted to a range of automation tasks. These are typically industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation, and where changes to the system would be expected during its operational life. PLC contains input and output devices patible with industrial pilot devices and controls。 millions of units are built each year, and very few endusers alter the programming of these controllers. However, some specialty vehicles such as transit busses economically use PLC instead of customdesigned controls, because the volumes are low and the development cost would be uneconomic. Very plex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even highperformance PLC. Very highspeed or precision controls may also require customized solutions。 where processes required hundreds or thousands of loops, a distributed control system (DCS) would instead be used. As PLC has bee more powerful, the boundary between DCS and PLC applications has bee less distinct. PLC has similar functionality as Remote Terminal Units. An RTU, however, usually does not support control algorithms or control loops. As hardware rapidly bees more powerful and cheaper, RTU, PLC and DCS are increasingly beginning to overlap in responsibilities, and many vendors sell RTU with PLClike features and vice versa. The industry has standardized on the IEC 611313 functional block language for creating programs to run on RTU and PLC, although nearly all vendors also offer proprietary alternatives and associated development environments. 2 The structure of PLC system Structurally divides, PLC divides into the stationary type and the bined type (module type) two kinds. Stationary PLC including the CPU board, the I/O board, demonstrated the kneading board, the memory block, the power source and so on。s role, wraps PLC to have CPU at least every time, it the function which entrusts with according to the PLC system program receives and stores the user program and the data, with scanning way gathering the condition or the data which sends by the scene input device, coexisting enters the stipulation in the register, 5 simultaneously, diagnoses the power source and in the PLC internal circuit active status and the programming process grammatical error and so on. After enters the movement, one by one reads the instruction from the user program memory, the duty which stipulated according to the instruction produces the corresponding control signal again after the analysis, directs the related control circuit. CPU mainly by the operator, controller, register and implementation of data link between them, control and state BUS, CPU unit also includes peripheral chips, bus interface and related circuitry. Memory is mainly used for storing programs and data, is an integral unit PLC. In the user view, unnecessarily detailed analysis of the CPU39。s I/O circuits。s point of view argument. PLC is a service specifically for the industrial production control device, usually do not need to take measures, it can be directly used in industrial environments. However, when the production environment is too harsh, especially strong electromagic interference, or improper installation, can not guarantee the normal operation of PLC, and therefore should pay attention to the following questions using the. 7 First, the working environment 1. PLC required temperature in the temperature 0 ~ 55 ℃ , heating installation, large ponents can not be placed below the space around the ventilation and cooling should be large enough, the basic unit and expansion unit interval between the need for more than 30mm。 if the surrounding Stresses above 55 ℃ , to install electric fan forced ventilation. 2. PLC humidity in order to ensure the insulation performance, air relative humidity should be less than 85% (no condensation). 3. PLC should be made from a strong shock vibration source, to prevent the vibration frequency of 10 ~ 55Hz frequent or continuous vibration. When the environment is inevitable when using vibration, shock absorption to take measures, such as glue, such as using shock absorption. 4. Air to avoid corrosion and flammable gases, such as hydrogen chloride, hydrogen sulfide, etc.. The more the air of dust or corrosive gas environment can be installed in a closed PLC good control room or control cabi, and install air cleaning devices. 5. Power PLC power supply is 50Hz, 220 (1 177。s output devices are packaged in printed circuit board and connected to the terminal board that if the load shortcircuits connecting the output ponents, printed circuit boards will be burned, thus, applied fuse protected output devices. (3) The relay output, inductive load borne by the size, will affect the life of the relay, therefore, choose to use the relay inductive load longer working life. (4) PLC may interfere with the output load, so to take measures to control, such as the DC output of the continued flow of control protection, exchange of the output RC snubbed circuit, transistors and resistors Triad output bypass the protection. Third, the external safety circuit In order to ensure that the entire system to work reliably in a safe condition to avoid failure due to external power supply, PLC abnormalities, misuse, and the output error caused m