【正文】 ch time the program goes through this loop N7:0 will increase by 1 until the value reaches 10. N7:0 := 0。 REPEAT N7:0 := N7:0 + 1。 UNTIL N7:0 = 10 END_REPEAT。 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。(2) Of the cabinet to the host power supply voltage of the monthly resurvey work。 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。 (2) Every three months to replace the power rack below the filter。3. Maintenance preparation, maintenance of order(1) Ready before maintenance tools。 (2) To protect the ponents and the template does not function no fault damage, must be protection devices and serious preparations for antistatic。 (3) Maintenance scheduling and operator contact before and well, be linked to overhaul licensing office hung maintenance card。 4. Equipment and method of disassembly sequence (1) Downtime, must be two or more monitoring operation。 (2) The way the CPU on the front panel select switch from the Run to Stop position。 (3) Close the PLC power supply mains, and then close the other to the power supply module Osaka。 (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。 (5) CPU motherboard and I / 0 boards in the bottom of the screw rotation after template changed。 (6) Installed in reverse order。5. Maintenance process and technical requirements (1) measure voltage, use a digital voltmeter or accuracy of 1% of the multimedia, measure (2) the power rack, CPU board can only be removed when the main power cut off。(3) RAM module to remove or insert the CPU from the CPU prior to disconnect the PC39。s power supply, so as to ensure the data is not confused。(4) before removing RAM modules, check the battery module is working correctly, if the battery fault light is removed when the PAM module content will be lost。(5) input / output board should be removed before the first turn off the mains, but if the production of necessary I / 0 boards can be removed at runtime programmable controllers, but the CPU board QVZ (overtime) lights。 (6) pluggable template, to be more careful, gently, and the goods shipped out of static electricity。(7) the replacement of ponents not live action。(8) Maintenance must be placed after the template installed in place Second, installment and wiring1. The power line, the pilot wire as well as the PLC power line and the I/O line should distinguish the wiring, between the insulating transformer and PLC and I/O should use the double rubberized wire connection.2. PLC should be far away from the strong jamming source like electric welding machine, the high efficiency silicon rectifier unit and the largescale power equipment, cannot install with the highpressured electric appliance in the identical switch cabinet. 3. The input and output of PLC are best separate the line, the switch quantity and the simulation quantity must separate the placing. The simulation quantity signal39。s transmission should use the shielded wire, the shielding layer should an end or the both sides earth, and the earth resistance should be smaller than the shielding layer resistance 1/10. 4. The PLC elemental area and the expansion unit as well as functional module39。s ties cable should lie alone, prevents the outside signal the disturbance. exch