In this article, we will be discussing the function of forcing and modifying in the PLC programming and its effects on the Siemens program logic.
Forcing and Modifying is the software facility (feature) that works differently in s7- 300 and s7- 400 PLCs.
In the process industries, processes run for long hours 24×7 365 days, in such a situation if any fault occurs then the whole process interrupts or stops depends on the criticality of the fault occurs.
Now let us consider that there is a fault in the pump (say some failure) and unable to run the pump. Due to this fault, PLC won’t allow other processes to run (as depends on pump run feedback), in such condition we can force the feedback of the pump to ON, so PLC understands that there is no fault now and allow other processes to complete the task. (Take all precautions before forcing logics, if we do forcing without cross-checking then it may lead to equipment failure or production loss)
In our example, there is some failure in the pump, so it is not starting. Because of this we are not receiving the pump run feedback. Say, we have a situation that, we want to run the remaining process even though the pump may not run but this remaining process runs only when we receive this pump run feedback. So now we are forcing the pump run feedback, thus making the PLC believe that the pump is running in the field by forcing. In actual condition, the pump is not running.
Read the detail:
https://glrassin.com/blogs/news/forcing-and-modifying-plc-logic
