The working process of the PLC, PLC operation mode

Initially the development and production of PLC controller is mainly used to replace the traditional composed of relay contactor control device, but the two operation mode is not the same: ( 1) Relay logic control unit adopts hard run in parallel manner, namely if the relay coil electricity or without electricity, all the relay contact ( Including their normally open or normally closed contacts) On the relay control circuit which position will immediately action at the same time. ( 2) PLC of the CPU, the use of the sequential logic scan user program operation mode, if an output coil or logical coil is connected or disconnected, the coil all contact ( Including their normally open or normally closed contacts) No immediate action, must wait for scan will move to the contact. In order to eliminate the difference caused by different operation mode between the, considering the action time of the relay control device of all kinds of contact generally in more than 100 ms, and PLC scan user program time is less than 100 ms, therefore, PLC USES a different from general microcomputer operation mode - - - Scanning technology. On the occasion of low requirements for the I/O response, PLC and relay control device on the processing results of no difference. 1, scanning techniques when PLC after put into operation, its working process is generally divided into three stages, namely the input sampling, the user refreshes the three stages of program execution and output. Complete the above three stages as a scan cycle. During the whole operation, scanning speed of PLC CPU in a repeat the above three stages. ( 1) The input sampling stage in the input sampling stage, PLC to scan mode in order to read all the input state and data, and put them into the I/O image area of the corresponding cell. After the input sampling, transferred to the user program execution, and the output refresh stage. In these two stages, even if the input state and data changes, the relevant unit I/O image area of the status and the data will not change. Therefore, if the input is the pulse signal, the pulse signal must be greater than the width of a scan cycle, can ensure in any case, this input all can be read. ( 2) User program execution stage in the user program execution stage, PLC is always in top-down order, in turn, to scan the user program ( Ladder diagram) 。 When scanning each ladder diagram, and always scan ladder diagram on the left side of the control circuit, composed of various contact and press the left right after the first, after the first order of composed of contact control circuit logic operations, and then according to the result of logic operation, refresh the logic coil in the system RAM memory area corresponds to a state; Or refresh the output coil in the I/O image area corresponds to a state; Or to determine whether or not to perform the function of the trapezoidal diagram of special instructions. Namely, in the execution of a user program, the only input point in the I/O image in the state and the data will not change, while the other dots and soft devices in the I/O image area or the state of system RAM memory area and the data are likely to change, and in the ladder diagram above, the program execution results to row below those who use these coils or data of ladder diagram work; On the contrary, the ladder diagram below, the competition for the logic state of the coil or data only on to the next scan cycle in the above procedure. ( 1) After the output refresh stage when scanning the user program, PLC into the output refresh stage. During this period, the CPU according to the I/O image in the corresponding state and data refresh all the output latch circuit, then the output circuit drive corresponding peripherals. At this moment, is the real output of PLC. Compare the similarities and differences between two procedures: procedures 1: program 2: the results of the two program execution exactly the same, but in the process of PLC in the execution. Program 1 only one scan cycle, can be completed for % of the M4 refresh; Program 2 will scan cycle with the four times, to complete the % of the M4 refresh. These two examples: the same article number of the ladder diagram, the order is different, the result of the execution is different also. In addition, also can see: the scan of the user program run results hard logic run in parallel with the relay control results. Of course, if the scan cycle time occupied for the entire operation can be ignored, so have no difference between the two. In general, PLC scan cycle including self-diagnosis, communications, etc. , as shown in the figure below, that is, a scan cycle is equal to the diagnosis, communication, input sampling, user program execution, and the output refresh, etc. The sum total of all time. 2, PLC I/O response time in order to enhance the PLC controller anti-jamming ability, improve the reliability of every switch input and PLC adopts photoelectric isolation technology, etc. In order to implement parallel control relay control circuit of hardware logic, PLC have adopted different from general microcomputer operation mode ( Scanning technology) 。 Above, two main reasons to make PLC I/O response than the general industrial control system consisting of a microcomputer is full, the response time is equal to at least one scan cycle, generally not less than one scan cycle or even longer. So-called from PLC I/O response time refers to an input signal changes to the system about the time needed for the change of the output signal.
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