The PLC user program execution is what?

For users, in writing the user program or choosing equipment, must be clear the following three stages, namely the principle of user program execution. PLC adopts the method of centralized processing, namely to scan input signal, execute user programs, and output refresh all adopt centralized way batch work. ( 1) Input scan at this stage, PLC controller to scan read in all the input terminal of the input signal, and the input signal in the input image area, the input image memory be refreshed. In program execution stage and output refresh stage, the input image memory isolation with the outside world, its content remains the same, until the next scan input scan stage of the cycle is refreshed to read in the input signal. Visible, PLC in the execution procedure and process the data, not directly use the scene at the time of the input signal, and using the sampling data from the input to the image area. If the input device can make the PLC input form a closed loop, corresponding to the input number within the internal input relay is saved as a '1', which is equivalent to relay coil conduction. In the process of program execution, the serial number corresponding to the contact action; If the input device can make the input open, the corresponding input number within the internal input relay is saved as a '0', which is equivalent to relay coil conduction, in the process of program execution, the corresponding contact number is not action. If the PLC in the input scan stage, PLC controller input devices outside the state changed, the internal input relay also won't change, will have to wait until the next input scan stage to refresh according to input state at this time. So, for less than 10 milliseconds of the input signal, often can't collect. ( 2) Execution procedures in the process of execution of user programs, PLC ladder diagram program order from top to bottom, from left to right scan performed individually, as mnemonic instruction table of order execution. But met jump instruction program, to jump the decision process according to jump condition meets the address. The execution of a program, PLC from the input image area to retrieve the current state of the input variables, and then to be determined by the program logic operations or other operations, according to the program instructions will result in the corresponding internal relay, including internal output relays, auxiliary relays, timers, counters, etc. Relay output signal is stored in the output image area, that is, the output relay with PLC controller external to the number of dots. In the process of program execution, the same period, logic result in front of the back of the contact, after the execution of the program may be used in front of the latest middle operation result; But the same cycle, the result does not affect the front behind logical relationship. Other than the scan cycle input relay all internal relay final state ( Conduction or not) , will be under the influence of a scan cycle on and off of each contact. ( 3) Output refresh program execution phase of the operation result is placed into the output image area, and not to the output port. In the output refresh stage, the PLC will output the output variable in the image area into the output latch, and then produced by the latch through the output module to control the output of this cycle. If the internal output relay status to '1', then the output relay contact closure. All the state of the output device to keep a scan cycle.
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