Question from a newbie

I disagree. Picture a limit switch that is mounted very near the top of the door opening with an arm that rides on the door itself. It is a normally open switch held closed by the door. It will energize the physical input point any time the door is NOT FULLY OPEN. When the door is fully open the limit switch arm comes off the door and the switch will open.

I contend this is more "failsafe" than a normally closed limit switch. If the limit switch arm falls off or the limit switch itself falls off the door stops. If you use a normally closed switch that wouldn't happen.

The REAL lesson this teaches is don't necessarily count on the 8-character symbol in a plc program. If there is supporting documentation, go to it.

Keith

After carefully considering that angle, I guess it's plausible. I'm guessing since our lab doesn't have a garage door and we are very low budget, we likely won't be programming that. We have a couple of air cylinders with limit switches but, they don't ride on anything. They are either contacted or, they aren't.
 
daba, I'm not looking to be overly cantankerous here, but I disagree (again). There is no ambiguity about the text description of the system. I'm not sure about everywhere else, but relay ladder diagrams were littered with NOHC (normally open held closed) and NCHO references. It was the cue to the user about the orientation of the switch so there was no confusion about how it was intended to operate. I do agree with Steve that the switch drawing should have had the NOHC reference and should have been shown as such. But other than that I don't see a problem with it. This is probably because I have seen it my fair share of this in "the wild".

Which brings up the next point. If I have seen this in the field, at some point so will Rocguy. Better he sees it in class than sitting on a 100 degree shop floor with a plant manager hanging over his shoulder. Furthermore, one of the recurring complaints that ends up on this site is how new automation professionals can pass any book test put in front of them but they can't think their way out of a paper bag. The common consensus is that teachers aren't challenging them with real world problems. So we have this case. A problem was put in front of a class. Rocguy evaluated the information given, found what he thought was a discrepancy and searched out information to help him clear it up. Isn't that what we want our education system to force our students to do? The only unknown to me at this point is what the instructor would do with this case given an incorrect evaluation.

Originally posted by Rocguy:

However, I must go back to the point above the physical switches that says they are shown Unactuated. If the arm is riding on the door, wouldn't that be considered at actuated?

I do agree that the unactuated comment is unnecessary. But I would also say, look back at post #5 at your own description of what "actuated" and "unactuated" mean. By your own definition, the switches are shown in the unactuated position. Its just that in this case the up limit switch is unactuated when the door is in the full open position.

Which, by the way, highlights another lesson. Don't infer unless you absolutely have to. You allowed a personal bias to influence your evaluation; that bias being that the up switch would be "actuated" when the door was fully open. Nothing in any of the information given to you said that was the case. You inferred that the up switch was actuated by the opening door based on your personal experience, like most of us would. But fight the urge. It can sometimes lead you down a rabbit hole.

Keith
 
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daba, I'm not looking to be overly cantankerous here, but I disagree (again). There is no ambiguity about the text description of the system. I'm not sure about everywhere else, but relay ladder diagrams were littered with NOHC (normally open held closed) and NCHO references. It was the cue to the user about the orientation of the switch so there was no confusion about how it was intended to operate. I do agree with Steve that the switch drawing should have had the NOHC reference and should have been shown as such. But other than that I don't see a problem with it. This is probably because I have seen it my fair share of this in "the wild".

Which brings up the next point. If I have seen this in the field, at some point so will Rocguy. Better he sees it in class than sitting on a 100 degree shop floor with a plant manager hanging over his shoulder. Furthermore, one of the recurring complaints that ends up on this site is how new automation professionals can pass any book test put in front of them but they can't think their way out of a paper bag. The common consensus is that teachers aren't challenging them with real world problems. So we have this case. A problem was put in front of a class. Rocguy evaluated the information given, found what he thought was a discrepancy and searched out information to help him clear it up. Isn't that what we want our education system to force our students to do? The only unknown to me at this point is what the instructor would do with this case given an incorrect evaluation.



I do agree that the unactuated comment is unnecessary. But I would also say, look back at post #5 at your own description of what "actuated" and "unactuated" mean. By your own definition, the switches are shown in the unactuated position. Its just that in this case the up limit switch is unactuated when the door is in the full open position.

Which, by the way, highlights another lesson. Don't infer unless you absolutely have to. You allowed a personal bias to influence your evaluation; that bias being that the up switch would be "actuated" when the door was fully open. Nothing in any of the information given to you said that was the case. You inferred that the up switch was actuated by the opening door based on your personal experience, like most of us would. But fight the urge. It can sometimes lead you down a rabbit hole.

Keith

Nice points Keith. I hope I can ask this in a way that's easily understood. Since it says the LS are shown unactuated. Does that mean the program instructions should reflect that at the current time or, should the two be separated? In other words, is that just showing you the unactuated state but, has nothing to do with how they are currently reflected on the screen? They are both shown unactuated yet one has an XIO and the other, an XIC and both are indicating a true state.

It sounds like you have seen plenty of diagrams and I know there is no substitute for experience so if I can glean any information from experienced people, I will certainly listen.

I know textbooks are not immune to mistakes. I've seen obvious ones in other classes.
 
Humm.. some rambling thoughts. kamenges is correct. when I read it again I can see that is indeed the correct description.

Now, my opinion; it's absolutely bad way to describe something in the logic. In the logic I would call it the "Not Up Input" or similar. I been in this business to long to see how many time people get tripped up by TOF, for example. We had a long thread here talking about "what can be done but shouldn't". It's the art of programming, not the science, that sometime is the most important part.

We don't have the entire book to judge, so maybe this specific terminology has been discussed earlier in the book, if not, then my opinion is that it's needless confusing to new students (not to mention someone like me).
 
Originally posted by Rocguy:

In other words, is that just showing you the unactuated state but, has nothing to do with how they are currently reflected on the screen?

That is correct. The device diagrams were there just to show you the devices present and, in effect, how they were wired (NO or NC).

Ron Beaufort has a huge number of posts on this site and has a knack for explaining things like this. But I will say it this way. Any given input path into a plc program is made up of three things that are really only incidentally related.
The "contacts" you see in the logic only tell you the logic state of the input: either 1 or 0.
This is separate from the wiring of the device connected to the input: either normally open or normally closed.
This is further separated from how the device is actuated relative to what you are concerned with detecting.
All three of these are separate and distinct and are only interrelated in that you need to analyse the entire input path to know what that little colored XIC in a program really means. You will most certainly run into a whole slew of different drawing and programming styles as you go along. Try to keep an open mind. You don't have to like what someone else left behind but you do need to understand it.

Having said all that I was being somewhat pedantic in my posts. In the strictest sense everything that was shown and stated in the example was correct. However, as others have said, I wouldn't have necessarily described the programmed inputs as they were. Many programmers will comment and describe inputs, outputs and memory bit in terms of their logic level 1 (or "on") state. So, as others have said, the up limit input in the plc would more correctly be labeled as N_UP_IN (Not_Up_Input) trying to keep under the 8-character symbol limit the old SLC programming software used to enforce. Most would also add a comment, which allows more text. Regardless of any other information, that symbol text change on that one input contact would have made a huge difference in how you evaluated the program. So, yes, comments are important.

Keith
 
That is correct. The device diagrams were there just to show you the devices present and, in effect, how they were wired (NO or NC).

Ron Beaufort has a huge number of posts on this site and has a knack for explaining things like this. But I will say it this way. Any given input path into a plc program is made up of three things that are really only incidentally related.
The "contacts" you see in the logic only tell you the logic state of the input: either 1 or 0.
This is separate from the wiring of the device connected to the input: either normally open or normally closed.
This is further separated from how the device is actuated relative to what you are concerned with detecting.
All three of these are separate and distinct and are only interrelated in that you need to analyse the entire input path to know what that little colored XIC in a program really means. You will most certainly run into a whole slew of different drawing and programming styles as you go along. Try to keep an open mind. You don't have to like what someone else left behind but you do need to understand it.


Keith

A couple of months or so ago before I started this class, I went searching videos about it on youtube. I watched a whole series of Ron Beauforts videos. I learned quite a bit doing so.

I use his method of looking at the XIC and XIO instructions as simply as he stated in the videos. XIC tells the processor to "go look for a 1" and the XIO tells the processor to "go look for a 0" simple as that. Of course in my class the teacher is using the terms XIO and XIC. I know what they are but, I inherently think of the simple thing Rons video taught me.

Last semester I had the industrial motor control class so I catch myself sometimes thinking about those diagrams. I know they are similar but, not interchangeable. The way a N.C. stop button is represented in each is a perfect example. That's where the "go look for a 1 or 0" can be really helpful. (Among many other ways)

To add context for that diagram on the door. It was one of three examples of deriving a program from a narrative description. I'm only 5 classes in out of a total of 16.

I appreciate all of the responses here.
 
The way the up limit and down limit switches are drawn are as NO switches. In the rungs for O:4/0 and O:4/3 the I:3/0 contacts should be NC for this logic to work.

Jerry
 
JerryH, are you telling me you read through this whole thread and still come to that conclusion?

Keith

Not knowing the mechanics of how these switches are used, and only looking at the schematic show, yes I would assume the Up Limit switch contacts are shown wrong in the logic diagram.

If the Up Limit switch was held on while the door was not UP then this would make sense. Nothing I see here points me into that direction. I'm not sure why the Up/Down switches would work differently.

Jerry
 
The logic is correct as are the devices shown. Probably the only argument is that the normally open switch should have been drawn normally open held closed to make it clearer. The switch is held closed by the door as a fail safe so that if the wire fails the door stops opening. But regardless it is still a door open limit switch. Conversely the door close switch is not held by the door because the switch is at the bottom of the door and once the door starts to rise there's no door to hold state.

Now it could be argued that closing the door should be failsafe also....that any wire that breaks should stop movement.
 
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Not knowing the mechanics of how these switches are used, and only looking at the schematic show, yes I would assume the Up Limit switch contacts are shown wrong in the logic diagram.

If the Up Limit switch was held on while the door was not UP then this would make sense. Nothing I see here points me into that direction. I'm not sure why the Up/Down switches would work differently.

Jerry

Except that it clearly states the open switch is normally open held closed....I suggest you Google the difference between NO, NOHC, NC and NCHO.
 

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