Was Bit, Was On, Was Off, Was Detected, Sensor Detected, Switch Detected,

[Mark Buskell]

I refers to those type of bits as memory bits. Example: Part Detected Memory, Part Released Memory. How you set them is up to you using a timer, seal, latch, etc.

 

[widelto]

This is a situation where you need to know if something has been ON then goes OFF (or vice versa) to enable the next rung/step of the operation.

I did this in the past maybe can help you

 

[Peter Nachtwey]

The term you are looking for is state machine.

Here is some ancient forum history. The forum has history folks.
http://www.plctalk.net/qanda/showthread.php?t=1563&highlight=Moore+diagram

No one did very well on this problem. I think Terry is guilty of Kmap abuse. I am sure I got it right the second time but the picture or solution doesn't display. It must have been a link to something on my site that isn't there any more.

Search the internet for mealy moore state machine.
https://www.cs.umd.edu/class/spring2003/cmsc311/Notes/Seq/impl.html

 

[rsdoran]

I disagree, what is new.

Are all or should all plc programs be state machines? Should they all use a sequencer because in reality you have to always do a sequence?

I was offering, and hoping others would too, the HOW TO in understand how to handle events, does not matter how you program.

Why do y'all have to make it so hard for some to understand?

This was simple, X occured, not that X WAS you can do Z.

Does not matter whether you program in Ladder, IL, STL, or BS. It just offers an idea to help comprehend.

Such a simple concept but now I understand why so many have problems with it.

 

[allscott]

Are all or should all plc programs be state machines? Should they all use a sequencer because in reality you have to always do a sequence?

I was offering, and hoping others would too, the HOW TO in understand how to handle events, does not matter how you program.

Why do y'all have to make it so hard for some to understand?

This was simple, X occured, not that X WAS you can do Z.

Does not matter whether you program in Ladder, IL, STL, or BS. It just offers an idea to help comprehend.

Such a simple concept but now I understand why so many have problems with it.

I don't think all programs have to be state machines. If you have more than a couple of states then using state programming will obviously be the cleaner way to go.

I think this is one of those problems you don't appreciate until you actually have to do it.

The attached file is a VERY basic example of why you would need the "was" bit and a simple way to illustrate the concept. It has no timers.

A box is travelling down a conveyor, you want to eject the box with a cylinder when the box PASSES a photoeye. Why not move the photoeye down the line and activate the cylinder when the box blocks the photoeye?...., maybe my boxes are different lengths.

I'm sure this will get picked apart and yes you could use a falling one shot to do the same thing in this example.

I think Ron was trying to find a way to illustrate a simple (yet important) concept.

Or maybe I am missing something deeper?

 

[rsdoran]

That is exactly what my program does. A product, on a conveyor, has to go PAST a sensor before you can do anything further.

Has nothing to do with the whole program, just a concept that may be part of a program.

 

[allscott]

That is exactly what my program does. A product, on a conveyor, has to go PAST a sensor before you can do anything further.

Has nothing to do with the whole program, just a concept that may be part of a program.

I know, I wrote my program in RSlogix500 because I think that most beginners are either familiar with this or Logixpro. I wanted to illustrate the concept without any other programming.

I don't know if my explanation is any better than anyone elses here. Maybe we should let Ron Beaufort take care of this kind of thing

 

[allscott]

I was just informed discretely by a fellow member that my example wouldn't work becuase of the unlatching .

I've edited the example, here it is.

Thanks

 

[djbillyd007]

not sure what to call it

I did a very simular setup on a old shanklin L sealer. I used a dl-06 and set up a positive edge belt go and then a negative edge fire one shot to run belt until time to seal. not sure what a good name for this would be but it worked. By the way are u working in GA by any chance??

 

[mellis]

Are all or should all plc programs be state machines? Should they all use a sequencer because in reality you have to always do a sequence?

Of course not. There are plenty of applications out there that work just fine with basic "relay logic". In gereral, if you can describe everything in your system with two states (off-on, open-closed), it probably fits that catagory. If it's easy to understand and troubleshoot, why make it more difficult?

Similarly not everything requires a sequence. But some problems cry out to be a sequence and are a whole lot more reliable if you treat them as a sequence. On the other hand, I've seen plenty of examples where a "sequencer" instruction made a program unnecessarily difficult to understand. My rule of thumb is: If you keep saying "the next step" when you are describing the problem, you're talking about a sequence. I did a program project recently where I replaced a brand new program someone else had written from scratch. The equipment wouldn't run reliably and was difficult to get going again once it got "confused". The only thing really wrong with it, the guy just didn't get the concept of programming a sequence.

Using the right tools for the problem at hand is what separates the best from the rest. Or to illustrate the opposite condition "If all you have is a hammer, then all your problems look like nails". Something I've learned is, if I'm working on a problem and my solution just keeps getting more and more complicated. That's a good indication that I've either misunderstood the problem or I'm using the wrong tool to solve it. Ideally, once you have solved a problem, the solution should seem simple and obvious.

I think I'm pretty much agreeing with you here, but feel free to straighten me out if I'm not.

 

[rsdoran]

I did a very simular setup on a old shanklin L sealer. I used a dl-06 and set up a positive edge belt go and then a negative edge fire one shot to run belt until time to seal. not sure what a good name for this would be but it worked. By the way are u working in GA by any chance??

If asking me I work for myself and located in Birmingham, Alabama.

I think the L-sealer(s) I did this too were Shanklin. If the main electrical components are in the bottom below the outgoing conveyor then its probably the same type.

 

[croakus]

I agree with Mark B. I think the best way to describe it is "memory" It's a word that implies that something has happened. "Part Was Here Memory" sounds pretty descriptive - it was here but now it's gone (predicated upon the logic conditions). If that bit isn't true, it should be because the part hasn't arrived yet or hasn't left yet.
Does this fit your first paragraph?
"Maybe someone has a different term to express this concept. It applies to a situation where a sensor has activated but has to deactivate before the next step activates."

Otherwise it sounds like a One-shot Falling instruction (except for the one-shot part ): "Use the OSF whenever an event must start based on the change of state from true-to-false, not on the resulting rung status."
Rockwell Software 2000

Mike

 

[ndzied1]

I would agree it is memory which in ladder is sometimes called a latch or holding coil.

  EVENT TO       FORGET

  REMEMBER        EVENT            WAS BIT

-----] [-----+-----]\[----------------( )-----

             |

   WAS BIT   |

-----] [-----+

 

The Event turns on the WAS BIT. The holding coil maintains the ON status of the WAS BIT until the time pass when I no longer need/want to remember that the event happened. I can FORGET the event.

This could also be done with true LATCH/UNLATCH --(L)--/--(U)-- instructions.

In practice I've always found the most critical part of this is knowing when to reset or forget the "WAS BIT". This depends a lot on how the WAS BIT is used in the rest of the program.

 

[mrtweaver]

Its to bad it was not an EBERLE PLC509S because in these units they are programmed using what is refered to as statement list but it is more like assembler language. It also runs in a step mode where you can monitor certain bits or logic to determine the status. You could have it monitor this bit and say if on goto step 7 then in step 7 you have to make sure it is off then proceed to step 8. It is a nice and easy process. I have used it on a number of retrofits and it works wonderful, no problems.

I obtained these PLC's from some equipment that we obtained from over the seas that has since been retired so I had saved these units. Last I hear GE Fanuc bought the EBERLE product line. I dont know if this is still the case or not. But you might want to google EBERLE and see what you come up with.

I have several Shanklin A27A L bar sealers that operate exactly like this. My older ones run about 28 ppm and the new ones about 35. The older ones use simple photoeye initiate and off delays with SLC 100. The newer ones have SLC 5/03 still with a PE. Same basic programming though. Settings through a pot on several. Several have DTAMs or PV550 with adjustment there.
I'll look later when I get some time and if still relevant shoot you the ladder stuff.

 

[brucechase]

Next zone detect

Ron, this is a sample of how I track a product past a photoeye and into a diverter station on an AB SLC 5/04. The photoeye is before the diverter and I have to wait a small time delay after the photoeye clears to actuate the diverter. I can include more if you want to see about some of the other unlatch conditions or how I track the products down a conveyor system or how the diverter works. I track about 25 items down a system that has about 8 diverter zones. I use latch and unlatch (generously) and pretty much ignore all the arguments of how no one should use these (We can argue this in a different thread if anyone wants to bring this up again). I'm not sure the nomenclature of what this would be. I never really thought about it. Hope this helps.