They have used a N/O stop button instead of N/C
Even the question setter has not got the importance of N/C stop buttons.
arnabbanik,
Your program is correct for the way the system is defined. Ronnie's objection is with the instructor's design of the system.
Because the instructor has specified a NO contact on the stop pushbutton, if one of the wires to the button should become disconnected, the pump could be started, but not stopped by the stop button. If the instructor had specified a NC contact, a disconnected wire to the stop button would prevent starting as well as stopping a running pump.
...should I put a check on the inputs PB's and the O/P? If the O/P(motor) is on, then jump and ignore the other steps? Is that what you meant? if you can explain little further and give me an example, it will be easier for me to understand...
In this case that is wrong in the program, because the definition of the physical STOP button in the problem is backwards (Normally Open), compared to the way real STOP buttons should be configured (Normally Closed).And as noted before I did make the Stop PB a NC that always has to be on for the motor to run.
Here's a picture of the order I would put the checks in, thus improving scan time for the PLC. And as noted before I did make the StopPB a NC that always has to be on for the motor to run.
If you do, your instruction for the Stop PB (as given in the problem) will be incorrect.Thanks a lot, much appreciated! I will incorporate it in my program, so that I don't forget.
Here's a picture of the order I would put the checks in, thus improving scan time for the PLC. And as noted before I did make the StopPB a NC that always has to be on for the motor to run.