When a PLC is put into operation, its working process is generally divided into three phases, i.e., the three phases of input sampling, user program execution, and output refreshing. The completion of the above three stages is called a scanning cycle. During the entire operation period, the PLC CPU repeats the above three phases at a certain scanning speed.
1.3.1 Input Sampling Phase During the input sampling phase, the PLC reads in all the input states and data sequentially in a scanning manner and stores them in the corresponding units in the I/O image area. After the input sampling is finished, it moves to the user program execution and output refreshing phase. During these two phases, even if the input states and data change, the states and data of the corresponding cells in the I/O image area do not change. Therefore, if the input is a pulse signal, the width of this pulse signal must be greater than one scanning cycle to ensure that the input can be read in any case.1.3.2 User Program Execution StageIn the user program execution stage, the P; ground scans the user program (ladder); 1.3.3 Output Refresh StageWhen the scanning of the user program is completed, between PL;, the CPU scans the user program (ladder) in accordance with the I/O mapping area. Corresponding state and data refresh; 1.4 Features of Siemens PLC; SIMATICS7-200 series PLC is suitable for all walks of life; S7-200 series excels in the following aspects: * Extremely high; * Rich built-in integrated functions * Real-time features * Strong communication capabilities *; 1.5P
1.3.2 User Program Execution Stage In the user program execution In the user program execution phase, the PLC always scans the user program (ladder diagram) sequentially
in a top-to-bottom order. When scanning each ladder diagram, it always scans the control line consisting of each contact on the left side of the ladder diagram first, and performs logical operation on the control line consisting of the contacts in the order of left first, right first, up first, and then according to the result of the logical operation, it refreshes the state of the corresponding bit of the logic coil in the system RAM memory area; or refreshes the state of the corresponding bit of the output coil in the I/O image area; or determining whether or not to execute a special function instruction specified in the ladder diagram. That is, during the execution of the user program, only the state and data of the input points in the I/O image area will not change, while the state and data of other output points and soft devices in the I/O image area or the system RAM memory area may change, and the ladder diagrams which are arranged on the top, and the results of the program execution of the ladder diagrams thereof will have an effect on the ladder diagrams which are arranged below wherever the coils or data are used; on the contrary, the ladder diagrams which are arranged On the other hand, a ladder at the bottom of the ladder, the state or data of the logic coils that are refreshed will not be available to the program at the top of the ladder until the next scan cycle. If an immediate I/O instruction is used during program execution, I/O points can be accessed directly. That is, if an I/O instruction is used, the value of the input process image register is not updated, the program takes the value directly from the I/O module, and the output process image register is updated immediately, which is somewhat different from immediate input.
1.3.3 Output Refresh Phase When the scanning user program is finished, the PLC enters the output refresh phase. During this period
The CPU refreshes all the output latch circuits according to the corresponding states and data in the I/O image area, and then drives the corresponding peripherals through the output circuits. At this point, it is the real output of the PLC.