A engineer needs to write a PLC program to control a water pump driven by an electric motor. This water pump will be manually started and stopped by pushbutton switches, and shutdown automatically by any one of several “permissive” switches.
The Stop_PB contact instruction should be drawn as normally-closed rather than normally-open as shown in the engineer’s first draft of the PLC program.
The program is designed to shutdown the motor when that pushbutton is pressed (i.e. the Stop_PB contact instruction becomes uncolored (i.e. fails to “conduct” virtual power).
We have been told that the real-world Stop pushbutton switch is NO, which means its contact closes when pressed.
This means pressing the Stop pushbutton causes that bit to be 1, which necessitates an NC contact instruction so that it will un-color under that condition.
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View Comments
Why would you show an example of stop being NO and not fail safe. You would never see it wired this way in an individual setting?
Hi , i believe that there are two ways that you can program a stop button . this is one way that you mention
and the other way ,you have stop button NC (hardwired) and you program it as NO in the plc ,fail safe .so there is no wrong in this i believe .great work