3/14/2018 1:53 AM | |
Joined: 9/27/2006 Last visit: 9/24/2024 Posts: 12282 Rating: (2685) |
Hello gahlul; Please refer to the screenshot below, taken from this site: https://www.slideshare.net/indira_kundu/basics-of-automation-plc-and-scada Th silde presents a feature that is at the base of all PLCs: the Scan Cycle. It shows that inside a PLC's central processor, the OS executes a pattern that repeats itself continuously, known as the scan cycle. The scan starts by updating the physical inputs attached to the CPU, then uses these refreshed input values to run the programmed functions. At this point a small period of the scan cycle allows the processor to exchange data using communications, run diagnostics and proceed to maintenance and cleanup. Finally the results of the program instructions are used to update the physical outputs on the CPU rack. And the process starts over again. if you program your instructions in a way that is acceptable to the CPU (no major programming fault) then your program will repeat itself automatically without intervention on your part, about every 20 ms (on average). If I may, Siemens proposes a series of PLC tutorials that have been useful to many beginners on their platforms: http://w3.siemens.com/mcms/sce/en/advanced_training/training_material/pages/default.aspx If you plan to work with Step 7 v5.x, then open the Classic Modules; Module A is the best pace for your to start your training. Hope this helps, Daniel Chartier |
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