The programming of the MCU and the PLC is very different. All the programs of the MCU are designed by the user. The PLC program includes the system program (Siemens called the operating system) and the user program. The operating system is used to organize all CPU functions that are not related to specific control tasks. Operating system tasks include processing the boot process, refreshing the input/output process image, invoking user programs, detecting interrupt events and invoking interrupted organization blocks, detecting and processing errors, managing memory, and handling communication tasks. Many of the PLC's functions, such as scan mode, timer function, and interrupt handling, are done by the operating system. This should be considered when drawing a flowchart. The program of the PLC's switching quantity control system is described in a simple and concise manner. It is also possible in principle to describe the PLC user program in a flow chart, but it should be distinguished which functions are implemented by the operating system and which are implemented by the user program. For example, someone asked if a jump instruction is needed to wait for the rising edge of a signal. This is a kind of thinking of microcontroller programming. The scanning mode of the PLC reads all the digital inputs at the beginning of each scanning cycle, and judges whether there is a rising edge according to the change of the state of the same input point in the two scanning cycles. So there is no need to use a jump instruction to skip back to wait for an input signal to appear. If you use this method to query the state change of the signal, the PLC can't do anything else, because its full energy is used in the query! This kind of query is performed in one scan cycle, which will make the scan cycle very long, and it is easy to cause the monitoring timer (watchdog) to time out. In the PLC user program, the rising edge of the pulse signal can be captured with only one instruction that detects the rising edge. If a fast response is required, the interrupt mode can be used, and the interrupt program written by the user is called by the system program on the rising edge of the pulse signal.
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