【正文】 pbystep cycle of operation. Each PLC pleted a cycle of the time required for the operation known as a scanning cycle. Different models of the PLC, scanning cycle to cycle in a microsecond between the scores of microseconds. PLC ladder program used in calculating logic, to show the advantages of quick, microsecond in the order of magnitude, calculating logic 1K procedures less than 1 ms. It all have input as a switch to handle the volume, 16bit (also 32bit) for a simulated traffic. PLC largescale use of a CPU to plete the simulation of the amount of puting. The results sent to the PLC same I / O points system, with PLC than DCS, the cost should be lower (about 40% of the province to around). PLC special operation did not stop it with software and hardware are mon, so maintenance costs much lower than the DCS. One of the PLC controller, you can receive thousands of I / O points (up to more than 8000 I / O). If the object is a chain of equipment, small loop, the PLC to adopt more appropriate. As a result of General PLC control software in the design of management information systems, to be easier. Over the past 10 years, with the PLC to lower prices and growing user demand, a growing number of small and mediumsized equipment to start using PLC control, PLC in China grew rapidly. With China39。s rapid economic development and the basis of the level of automation and continuous improvement in the next period of time in the PLC will continue to maintain our growth momentum. General PLC can be used in special equipment that it is an embedded controller, but in general the relative PLC and embedded controller to have higher reliability and better stability. Practical work encountered a number of users of the original use of embedded controllers are now being used or customGeneral PLC to replace the embedded controller PLC.PLC control system design main point in the modernized industrial production equipment, has the massive digital quantity and the simulation quantity control device, for example the electrical machinery stops, solenoid valve39。s make and break, the product counting, the temperature, the pressure, the current capacity hypothesis and the control and so on, in the industry scene39。s these automatic control question, the programmable controller (PLC) has bee one of solution most effective tools. When PLC control system design should pay attention to the following several spots. one, programmable controller (PLC) and programmer39。s selecting and purchasing: in the present market39。s PLC product is numerous, besides the domestically produced brand, the overseas brand includes: Japanese OMRON, MITSUBISHI, FUJI, IDEC, HITACHI, do Matushita, Germany39。s Simens, South Korea39。s LG and so on, how select and purchase the PLC product? 1. The system first should determine that the system uses PLC with the PLC single plane control to form the network, from this calculates the input, the output (I/O) the points, and when selects and purchases PLC must in the actual need points foundation reserve 10% remainders. 2. The definite load type acts according to the PLC outport to have the load is the cocurrent or the exchange, is the big electric current or the undercurrent, as well as PLC output movement frequency and so on, thus determined that the outport uses the relay to output the transistor to output, perhaps thyristor output. The different load selects the different output mode to system39。s steady operation is very important. 3. The storage capacity and the instruction carry out the speed are the PLC shaping important targets, generally the reserves is bigger, the speed quicker PLC price to be higher, although overseas various factories product more or less the same, but also has certain difference. “COM choice, different PLC product, it COM a quantity is dissimilar, some COM belt 8 output spots, some belt 4 output spots, also has the belt 1 or 2 output spot. When the load type are many, and the electric current is big time, uses one COM a belt 12 output product, when the load type minority quantity are many, uses one COM a belt 48 output product. 5. Because each Manufacturer39。s development software is different, the patibility is also systematically selects and purchases when the key point, at present had not discovered that the pletely patible product, should select the PLC product reasonably according to the system. 6. Programmer39。s selecting and purchasing: the PLC programming may adopt three ways: First, with the general handhold programmer, it can only use in the sentence table sentence programming which the factory stipulated. Center way easy scene debugging, and the volume small cost is low, but its efficiency low adaptation aircraft type type are few, pared with is suitable is small in the system capacity, in amount used few systems. Second, the graph programmer programming, this way uses the graphic mode programming, facilitates intuitively, generally the electrical personnel shortterm may apply freely, but the programmer price is high. Third, programs with IBM and the patible personal puter +PLC software package, this way is the efficiency highest one way, is also the most monly used one way, but the majority of software package price is expensive. 　　 Selects the big pany39。s product as far as possible, because its product quality has the safeguard, and the technical support, the postsale service is also generally good, will be advantageous in the later product expansion and the software and hardware promotion. Second, input, output circuit39。s design 1. power source return route 　　 The PLC power supply is generally AC85240V (also has DC24V), adapts the power source scope to be wide, but for the antijamming, should install the power source to purify the part (for example power source filter, 1:1 insulating transformer and so on) on DC24V power source39。s use 　　 On various panies PLC product has the DC24V power source generally, but this p