Jump to content

Recommended Posts

Hi everyone!

Is there any way to set the PLC to RUN/STOP mode via ladder code?

I want to have a button (it could be a physical one using one input) that once you press it, the PLC goes to STOP mode (every output turns off), and if you press it again the PLC goes to RUN mode and keeps on working like it was before stopping it.

Thanks in advance,

Gonzalo

Share this post


Link to post
Share on other sites

No, that isn't possible. If you think about it - how would the PLC know you pressed the button to return to Run mode if it was stopped? You can place the PLC into Stop or Run mode via INFO Mode if you want.

If your purpose in placing the PLC into Stop Mode is to prevent certain I/O, then you can program that into ladder with a button for your pseudo-stop function.

Share this post


Link to post
Share on other sites

Thanks Flex, yes you're right about that..

I think I'll have to do a pseudo-stop function as you said, but I have many I/Os that I'll have to back-up their previous values (1 or 0).

I'll try to find the best way to do it.

Thanks again Flex!

Share this post


Link to post
Share on other sites

I would use an open contact in series with every single output you use like a Master Control Relay - MCR -.  Then, when contact is closed again (RUN mode), all logic you use to control your outputs, remains without requiring backing up previous states.

 

Share this post


Link to post
Share on other sites

Or  you could replace the output coils with MB coils, then map the MB coils  to the associated output coils,

then make an MB that you set for your stop condition, and put an inverted contact for that MB before each output you want off.

JohnR

 

 

Share this post


Link to post
Share on other sites

Gabriel Franco & JohnR: Keep in mind that your suggestions only work with direct coils. Set coils will require the extra logic the OP is referring to.

Share this post


Link to post
Share on other sites

Flex....

you are correct if you were to just put a contact in series with the output.

my suggestion is to replace the outputs with MB's,  set/reset, toggle, or any latching contacts are now done in the logic with the MB

that MB then controls the physical output, with the inhibit/start/stop/MCR/whatever-you-want-to-call-it contact before the  physical output.

 

JohnR

 

Share this post


Link to post
Share on other sites
21 hours ago, Flex727 said:

The vector copy function would make that fairly easy.

Yes Flex, that's really useful too. I'll think about it. Thanks!

 

21 hours ago, Gabriel Franco said:

I would use an open contact in series with every single output you use like a Master Control Relay - MCR -.  Then, when contact is closed again (RUN mode), all logic you use to control your outputs, remains without requiring backing up previous states.

 

20 hours ago, JohnR said:

Or  you could replace the output coils with MB coils, then map the MB coils  to the associated output coils,

then make an MB that you set for your stop condition, and put an inverted contact for that MB before each output you want off.

JohnR

 

Thanks Gabriel and John!! I think I'll do that. I've already map the MB coils to the associated output coils in my program, so I think it'll be the best and easiest option.. 

 

Cheers people and thank you very much!!

Gonzalo

Share this post


Link to post
Share on other sites

Now, we also do not know what the OP's reason is for wanting to do this, but my first guess is that he wants to create a "safety" of sorts, isolate the outputs so  the user can perhaps make adjustments to a machine without shutting down.

Now we have to bear in mind that just because the outputs are turned off, the program itself is still running, and any inputs that can change the state of the logic, WILL!  And when the outputs are turned back on, the machine may not be in the same state it was. SURPRISE!

So, now we could take that same "MCR" contact and inhibit inputs so the program sits still where it was.....

And that gets us to the old saying;

"JUST BECAUSE YOU CAN, DOESN'T MEAN YOU SHOULD"

If this is indeed a pseudo-safety circuit, then we have opened up another can of worms.

At least here in the US, a safety circuit is not part of the running program, but instead a separate mechanical (usually redundant) safety relay that takes power away from the PLC's I/O.

And that leads to a whole different conversation........

JohnR

Share this post


Link to post
Share on other sites

Hey John!.. 

As you said,  it's just to isolate the outputs so the user can make adjustments to the machine without shutting down.

I'll consider what you're saying about the inputs..

The safety-circuit is not part of the running program.. It's done, as you mentioned, with a mechanical (and redundant) relay that takes power away from the PLC and everything connected to it..

Cheers,

Gonzalo

 

Share this post


Link to post
Share on other sites

Hi Gonzalo,

I follow what you are trying to do, and appreciate the fact that you do have a true safety circuit, but I would still be concerned that by not using the safety circuit when making adjustments you could be putting the user in harms way....

I don't pretend to know exactly what you are controlling, or what hazards are present, but please be mindful of the users surroundings and what could happen........

Is there any stored energy (air, hyd, water), that you need to release before entering the area?

If you are going to pursue this, I would at least make the "MCR" a keyed switch (and one that the key can't be removed in the Run position), and the user must have possession of the key while making adjustments.

Regards,

JohnR

  • Like 1

Share this post


Link to post
Share on other sites

Hi John!

Thanks for the willingness.

I agree with you. But there's no harm for the user if he need to do some adjustments to the machine and it turns to RUN mode accidentally.

That STOP mode, will be activated at the same time that a mechanical relay will cut off the power of a pump and a solenoid valve, once the user opens one door.

The only thing that could hurt him once the door is open, is that pump and the liquid that comes trough the solenoid valve (NC), and it'll be power off by software and hardware.. 

I need to stop the process as well, in order to keep it running as before (timers and outputs), after the user makes the changes to the pump. 

Regards,

 

Gonzalo

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×