【正文】 (3) RAM module to remove or insert the 。 (6) Installed in reverse order。 (4) Connected to the power supply rack power cord clear line number keep in mind and remove the link location, then remove the power rack and rack screws connected, power rack can be removed。 (2) The way the CPU on the front panel select switch from the Run to Stop position。 (3) Maintenance scheduling and operator contact before and well, be linked to overhaul licensing office hung maintenance card。3. Maintenance preparation, maintenance of order(1) Ready before maintenance tools。 2. The provisions of cleaning equipment on a regular basis(1) Every six months or a quarter of the PLC to clean, cut off power to the PLC power supply to the power rack, CPU board and input / output board in turn be removed to purge, clean and then in turn installed in situ, will all link to resume evacuation call and start the host PLC. PLC box carefully sweeping health。 PLC ConnectionsWhen a process is controlled by a PLC it uses inputs from sensors to make decisions and update outputs to drive actuators, as shown in Figure . The process is a real process that will change over time. Actuators will drive the system to new states (or modes of operation). This means that the controller is limited by the sensors available, if an input is not available, the controller will have no way to detect a condition.The control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and then changing the outputs. Like any puter this does not happen instantly. Figure shows the basic operation cycle of a PLC. When power is turned on initially the PLC does a quick sanity check to ensure that the hardware is working properly. If there is a problem the PLC will halt and indicate there is an error. For example, if the PLC backup battery is low and power was lost, the memory will be corrupt and this will result in a fault. If the PLC passes the sanity checks it will then scan (read) all the inputs. After the inputs values are stored in memory the ladder logic will be scanned (solved) using the stored values not the current values. This is done to prevent logic problems when inputs change during the ladder logic scan. After this the system goes back to do a sanity check, and the loop continues indefinitely. Unlike normal puters, the entire program will be run every scan. Typical times for each of the stages are in the order of milliseconds.Questions Should Pay Attention to When PLC Is in the UsePLC which should pay attention to in the application is specially the control device which serves for the industrial production, usually does not need to take any measure, may use directly in the industry environment. But, when the production environment is too bad, the electromagnetic interference is special intense, or installs uses improper, cannot guarantee PLC the normal operation, therefore should pay attention to the following questions in the use. First, Siemens PLC maintenance 1. Maintenance procedures, equipment, periodic testing, adjustment provisions (1) Halfyearly or quarterly inspection PLC cabinet terminal connections, and if time to reloose strong local connection。 REPEAT N7:0 := N7:0 + 1。 if it is 0 the output will be turned off. The ladder logic program in Figure , is equivalent to the mnemonic program. Even if you have programmed a PLC with ladder logic, it will be converted to mnemonic form before being used by the PLC. In the past mnemonic programming was the most mon, but now it is unmon for users to even see mnemonic programs. Sequential Function Charts (SFC) have been developed to acmodate the programming of more advanced systems. These are similar to flowcharts, but much more powerful. The example seen in Figure is doing two different things. To read the chart, start at the top where is says start. Below this there is the double horizontal line that says follow both paths. As a result the PLC will start to follow the branch on the left and right hand sides separately and simultaneously. On the left there are two functions the first one is the power up function. This function will run until it decides it is done, and the power down function will e after. On the right hand side is the flash function。[10] . 北京: 機(jī)械工業(yè)出版社, 2005 [11] 西門子公司. S7200可編程控制器系統(tǒng)手冊(cè), 2002[12] 西門子公司. S7200可編程控制器產(chǎn)品目錄, 2005 [13] 北京亞控自動(dòng)化軟件科技有限公司. “組態(tài)王”使用手冊(cè), 2002[14] . 西安: 西安電子科技大學(xué)出版社, 2004[15] 汪曉平. PLC可編程控制器系統(tǒng)開發(fā)實(shí)例導(dǎo)航. 北京: 人民郵電出版社, 2004[16] 張萬(wàn)忠. 可編程控制器應(yīng)用技術(shù). 北京: 化學(xué)工業(yè)出版社, [17] 周美蘭等. PLC電氣控制與組態(tài)設(shè)計(jì). 北京: 科學(xué)出版社, 2000[18] . 北京: 電子出版社, 2006[19] 李長(zhǎng)久. PLC 原理及應(yīng)用. 北京: 機(jī)械工業(yè)出版社, [20] 周美蘭等. PLC電氣控制與組態(tài)設(shè)計(jì). 北京: 科學(xué)出版社, 2000外文資料翻譯 PROGRAMMABLE LOGIC CONTROLLERS INTRODUCTION Control engineering has evolved over time. In the past humans was the main method for controlling a system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is mon to use relays to make simple logical control decisions. The development of low cost puter has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has bee the most mon choice for manufacturing controls. PLC have been gaining popularity on the factory floor and will probably remain predominant for some time to e. Most of this is because of the advantages they offer. Cost effective for controlling plex systems. Flexible and can be reapplied to control other systems quickly and easily. Computational abilities allow more sophisticated control. Trouble shooting aids make programming easier and reduce downtime. Reliable ponents make these likely to operate for years before failure. LADDER LOGIC Ladder logic is the main programming method used for PLC. As mentioned before, ladder log