Siemens PLC's loop processing method - Database & Sql Blog Articles

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Siemens PLCs operate using a cyclic execution model known as the scanning mode. During runtime, the user program is executed repeatedly in a loop. Here’s what happens during each phase of the scan cycle: 1. The operating system initiates cycle time monitoring to ensure consistent execution. 2. The CPU updates the output modules by writing data from the process image output area. 3. It then reads input data from the input modules, updating the process image input area. 4. After completing the main program, the operating system handles other background tasks like block downloading or deletion. 5. The CPU returns to the beginning of the cycle, restarting the monitoring and continuing the loop. 6. Finally, it processes the user program to perform control operations. A common question arises: why do Siemens PLCs write the process image output data to the output module at the start of the next scan cycle, rather than at the end? This might seem to introduce a delay compared to some other PLC brands. However, the reason lies in the nature of the scan cycle itself. Like a clock that moves from 23:59:59 to 00:00:00, the transition between cycles is nearly instantaneous. Therefore, even though the data is written at the start of the next cycle, the actual time difference between this and writing at the end of the current cycle is negligible—essentially just a fraction of a scan period. In practical terms, this difference has no real impact on system performance. This approach ensures consistency and reliability in the control process, making it well-suited for real-time applications. Mitsubishi PLCs also follow similar scanning logic, emphasizing the importance of predictable and repeatable operation in industrial automation.