arnabbanik
Member
I am attaching here a problem and the logic I did. If anyone can kindly check.
If someone can kindly check the problem
- Thread starter arnabbanik
- Start date
Mickey
Lifetime Supporting Member
Looks good.
The only thing I would do different is check the output being on first, then branch down to check the push button - that way the scan of the PLC won't have to check the PB(that's only on once every few million scans) then branch down & see that the output is already on to keep it on.
On a large program the additional scan steps can add up - best to start off in the right mindset.
I always check the most-common-on first & branch down to the least-common-on, and for checking the run conditions after the branch I start with the least-common-on & progress to the most-common-on so that when the least-common-on item drops the rest of the line is not scanned - it jumps right to the output to turn it off.
On a large program the additional scan steps can add up - best to start off in the right mindset.
I always check the most-common-on first & branch down to the least-common-on, and for checking the run conditions after the branch I start with the least-common-on & progress to the most-common-on so that when the least-common-on item drops the rest of the line is not scanned - it jumps right to the output to turn it off.
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arnabbanik
Member
OP
Thanks Mickey and Aabeck.
So , 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.
Thanks!
So , 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.
Thanks!
Ronnie Sullivan
Member
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.
Even the question setter has not got the importance of N/C stop buttons.
arnabbanik
Member
OP
Hi Ronnie,
Hence I used XIC check for the stop button. Can you please explain little but what can be done better and what is the harm of doing it the way I did.
Thanks!
Steve Bailey
Lifetime Supporting Member + Moderator
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.
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.
arnabbanik
Member
OP
Thanks Steve,
I was thinking on the same lines. How can I take care of this problem.If I assume it is NC button, then should I check for XIC? In that way, if wire is broken, it stops the motor?
Steve Bailey
Lifetime Supporting Member + Moderator
You can't assume a NC pushbutton. The instructor specifically stated that the X1 signal is true when the button is pressed. That's a NO contact. It is an inherent property of the system as designed that a likely mode of failure will result in an undesirable condition.
It is certainly possible for a NC pushbutton to fail such that the pump can no longer be stopped. For instance, if the NC contact block on the stop pushbutton became disengaged with the pushbutton mechanism, the pump could not be stopped by the button.
It is certainly possible for a NC pushbutton to fail such that the pump can no longer be stopped. For instance, if the NC contact block on the stop pushbutton became disengaged with the pushbutton mechanism, the pump could not be stopped by the button.
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.
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).
It is difficult to fix a basic error by introducing another error.
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arnabbanik
Member
OP
Thanks a lot, much appreciated! I will incorporate it in my program, so that I don't forget.
Also, can you or anyone here tell me, even though I am performing division with integer function, the math register bit(S13 and S14) is not changing? I read about it from the rockwell manual and it says, remainder and quotient are stored there but I can not see anything but 0.Any ideas?
I was trying to do a even/odd check for a problem.
gbradley
Lifetime Supporting Member
In my attached drawing, Will Rung 0 scan faster than Rung 1?
I realize you wouldn't actually put two rungs like this together in a program.
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