nc no whats the deal

I also do away with the "normally open and normally closed" monikers at times. When I'm in the zone and churning out ladder, I use this mental shorthand syntax:

-| |- "IF you have"

and

-|/|- "if you DO-NOT have"


so as I type the line of ladder my train of thought actually defines what is happening.

Ultra simple example:

IF you have "light on" AND if you DO-NOT have "light off" THEN turn ON the "light".

-| |-----|/|---------------------------( )-

"light "light .................... "light"
..on" .. off" ......................output

It lets you convert the elements and the boolean operations into a flowing, descriptive text.....

"the world is flat" ...

for thousands of years that was a simple way of explaining things – and it was perfectly acceptable ... in spite of the fact that it's completely WRONG, it still worked OK – just as long as no one traveled too far in any one direction ...

BUT ...

then Ferdinand Magellan set off sailing toward the east – and came back home again sailing in from the west ... reality set in – and suddenly a new way of looking at the shape of the world was required ...

the way that most people look at XIC and XIO instructions is like that ... in other words, you can use all manner of simple ways of explaining their actions – and most of those ways will be perfectly acceptable in day-to-day applications ...

BUT ...

sometimes TROUBLES arise ... sometimes PROBLEMS present themselves ... and in those situations, "troubleshooting" and "problem solving" skills are required ...

for one very simple example: a switch in the field might indeed be closed, but for some reason or other, the associated bit/box on the input table might NOT contain the expected value of ONE ... and in that case, the PLC will NOT evaluate an XIC for that switch as a TRUE condition as we would normally expect it to ...

SO ...

I submit that the absolutely BEST way to interpret the XIC and the XIO instructions is as follows:

--] [-- "go look for a ONE" ...

--]/[-- "go look for a ZERO" ...

if the processor DOES find the specific condition that it's looking for, then the instruction will be evaluated as TRUE ...

conversely, if the processor does NOT find the condition it's looking for, then the instruction will be evaluated as FALSE ...

NOW THEN ...

the actual ONE or ZERO status of any particular bit/box becomes another (separate) topic for conversation ... specifically, exactly WHY the bit/box contains a ONE – or WHY it contains a ZERO – becomes a completely distinct and individual step in the "troubleshooting" or "problem solving" exercise ...

SUMMING UP ...

as long as everything is running along smoothly, the method suggested by Stationmaster is generally adequate for interpreting most ladder logic ...

IF you have "light on" AND if you DO-NOT have "light off" THEN turn ON the "light".

Click to expand...

on the other hand, whenever troubleshooting or problem solving is required, the ability to break each and every step of the PLC's signals - and its logic – into completely separate and distinct components becomes an essential part of a technician's skill set ...

for a few simple examples, people who believe (incorrectly) that:

(a) an XIC instruction "Examines a SWITCH" to see if it's ON ...
(b) "green on the screen means TRUE" ...
(c) "power-flow through the rung will cause an OTE to energize an output" ...

and other such misconceptions are at a significant disadvantage whenever a PLC-controlled system isn't working the way it's expected to ...

Ron Beaufort said:

....people who believe (incorrectly) that:

(a) an XIC instruction "Examines a SWITCH" to see if it's ON ...
(b) "green on the screen means TRUE" ...
(c) "power-flow through the rung will cause an OTE to energize an output" ...

and other such misconceptions

Click to expand...

Ron didn't offer an explanation as to why these statements may be incorrect, and I feel that without such explanation, his comments may cause confusion in the minds of the less experienced....

I offer the following, hopefully it covers all the bases.....
(a) an XIC instruction "Examines a SWITCH" to see if it's ON ...

The only thing I can see wrong with this statement is the use of the word SWITCH. A switch can be wired NO, or NC, so the concept of ON or OFF is ambiguous - the switch may be ON or OFF (as indicated by its legend or position), but its contacts may be closed, or open, in that ON position.

Boolean (bit) instructions in PLCs examine the state of the memory bit used to store data. That memory bit may be data from input modules/cards, or it can be an "internal" memory bit, set or reset by other logic.

However, if we take the word SWITCH to mean any memory bit that can be "switched" on or off, then statement (a) is perfectly OK.

(b) "green on the screen means TRUE" ...

Mostly this is a correct assumption. When this isn't the case, then there is invariably a timing issue involved.

The programming software highlights (green) an XIC instruction when it sees the attached bit location is ON, logic 1, or true.

It may not be continuously true (in the PLC), but at the time that the data was read from the PLC, it was.

The visual interpretation by the software is that it will display what it sees at the time it sees it.

(c) "power-flow through the rung will cause an OTE to energize an output" ...

Two things here -

1. An OTE will both Energize, and De-Energize "an output" (see below).

I dislike the use of Energize/De-Energize in this context.

"power flow through the rung" is known as "rung logic continuity".

When the rung evaluates as TRUE, an OTE will write a "1" to the addressed bit location, but it will also write a "0" when rung logic continuity is false.

2. "an output" is simply a copy of an internal memory location to an output module.

There are cases where the internal representation of an output value (digital or analog), do not represent what is actually happening at the physical output. Forcing may be in use, or there may be multiple "writes" to the same location.


I tried to keep this as short as possible - and I bet I've missed a few things - the most important thing to grasp is that a PLC is just a machine, a computer program that interprets and acts on the data that is presented to it

Ron Beaufort said:

as long as everything is running along smoothly, the method suggested by Stationmaster is generally adequate for interpreting most ladder logic ...

on the other hand, whenever troubleshooting or problem solving is required, the ability to break each and every step of the PLC's signals - and its logic – into completely separate and distinct components becomes an essential part of a technician's skill set ...

Click to expand...

Wow Ron,

You make it sound as if my skill set is inadequate for "troubleshooting"". Not true. I find being able to think in boolean to be VERY useful in examining ladder, and the sensors and devices connected to the inputs.

You DO realized don't you, that I was SIMPLIFYING on purpose for a rather inexperienced poster who obviously needed a very basic way to relate to TWO commands? I even STATED that in my post.

"Go look for a one" is at best an INCOMPLETE description of what the input is doing anyway. You THEN had to add "report as true" ... to complete the thought.

How exactly does "if you have" NOT work on a round earth? The troubleshooting logic that follows (intuitively, for most) would be "why do I NOT have?" then using our technical skill set, we follow the wiring or print back to the device that is NOT providing the signal we need....

JEEESH!

Greetings daba ...

thanks for filling in for me – my time was very short earlier ... here's a little more detail on what I had in mind ...

(a) an XIC instruction "Examines a SWITCH" to see if it's ON ...

Click to expand...

that's a misconception – because (as you said) an XIC works ONLY on a BIT (a "box" in the PLC's memory) ... specifically, an XIC instruction NEVER examines a switch in the field ...

(b) "green on the screen means TRUE" ...

Click to expand...

that's a misconception – because (as you said) timing issues related to the screen's update can play all sorts of tricks with the green highlights ... another example is an ONS instruction in a PLC-5 or a ControlLogix platform ... this will be highlighted in green due to a status of ONE in the One-Shot's bit/box - even though the rung logic through the ONS instruction is actually FALSE – not TRUE ...

(c) "power-flow through the rung will cause an OTE to energize an output" ...

Click to expand...

that's a misconception – because an OTE works ONLY on a BIT (a "box" in the PLC's memory) ... specifically, an OTE instruction NEVER energizes (or de-energizes) a field device ... more specifically, the status of the bit/box for the field device must be "written" to the output module before the field device is actually turned ON or OFF ... and that's a separate step in the processor's scan cycle ...

actually you and I are on pretty much the same wavelength here ...

and my original point is that most of the "rules of thumb" that many PLC technicians believe in are flawed by so many inconsistencies that they often do more harm than good ... specifically, a lot of these misconceptions appear to work perfectly at the "beginner level" – but past that point a different way of looking at things is required ...

the biggest question then becomes: how long should we continue to water things down for the beginners – before we let them in on the secret handshake of what's REALLY going on in a PLC-controlled system? ...

and actually all of these concepts are covered in the eleven YouTube videos available from the "Sample Lessons" page of my website ... for those that are interested, please watch them all in NUMBER ORDER ... skipping around is NOT advised ...

Greetings Stationmaster ...

I apologize ... I'm afraid that somehow you've missed my point ... I was actually AGREEING with what you had said ... in fact, I myself often use EXACTLY the same approach that you mentioned ...

You DO realized don't you, that I was SIMPLIFYING on purpose for a rather inexperienced poster who obviously needed a very basic way to relate to TWO commands? I even STATED that in my post.

Click to expand...

yes, I did indeed realize that ... and again, I apologize since I obviously didn’t make myself clear ...

"Go look for a one" is at best an INCOMPLETE description of what the input is doing anyway. You THEN had to add "report as true" ... to complete the thought.

Click to expand...

here we have (exactly as you yourself said) yet another example of the results of SIMPLIFYING on purpose ... I cover ALL of the method that I've suggested (in great detail - and at a beginner level) in the series of videos that Mickey recommended back in post #2 ... naturally I don't expect everyone to watch those – but the material IS available there for free ...

How exactly does "if you have" NOT work on a round earth?

Click to expand...

it works fine for me – and obviously for you also ... but just to make a point:

actually it's the "if you DO-NOT have" part of your statement that many beginners have a problem with in certain situations ... here I'm thinking specifically of a limit switch which is wired Normally-Closed in the field ... consider the figure below and I think you'll see what I mean ...

using your terminology, the logic statement SEEMS to read "If I DO-NOT have the limit switch, then the head has been down" ... obviously (at least to you and to me) that statement of logic must be REVERSED due to the "fail-safe" wiring of the switch ... on the other hand, many students have a problem learning (and applying) that "reversed" line of reasoning ... I know this to be a fact because this is an actual example from the Monday afternoon "Grinder" exercise that I use in my classes ... and – I've been using it successfully for many years specifically to cover the types of concepts that I'm discussing here ...

and so ...

if instead, you use the method that I'm suggesting, then the rung logic below reads: "If I have a ZERO in the Limit Switch bit/box, then the head has been down" ... this "ONE-or-ZERO" line of reasoning works perfectly in ALL cases – regardless of how the switch might actually be wired in the field ...

and best of all, I can teach this simple yet reliable method as part of a systematic approach to analyzing ladder logic in just a matter of minutes – even to beginning students who have absolutely no previous experience in PLCs ... in fact, many of my beginning students have less problems with these concepts than some of my more experienced students - who often have years of misconceptions to overcome ...

JEEESH!

Click to expand...

once again, I apologize for not making myself more clear ... I'm sorry that I allowed myself to be misunderstood ... as I said earlier, I was actually AGREEING with what you had posted ...

but ...

at the same time, I still maintain that there is a more straightforward way for people to learn – and to fully understand - how PLCs really operate "under the hood" ...

in closing: one of the disadvantages of having my business pick up so dramatically lately is that I now have very little spare time to contribute to the forum ... I've just about decided to give it up completely since it's now becoming increasingly difficult to write "enough" to make myself fully understood – without unintentionally antagonizing other contributors ... provoking other members has never been my purpose for writing ...

You'll just have to trust me when I say: I didn't miss your point.

You'll just have to trust me when I say: I didn't miss your point.

Click to expand...

OK, I trust you ...

let's just move on ... we've devoted too much time to this already ... you've probably got more important things to do - and I know that I certainly have ...

peace ...

Ron Beaufort said:

in closing: one of the disadvantages of having my business pick up so dramatically lately is that I now have very little spare time to contribute to the forum ... I've just about decided to give it up completely since it's now becoming increasingly difficult to write "enough" to make myself fully understood – without unintentionally antagonizing other contributors ... provoking other members has never been my purpose for writing ...

Click to expand...

I think i speak for the vast majority of people on here when i say please don't do that if at all possible Ron.

Your Input here on this forum is like gold. I for one would not have the ability to be sucessfull in the job i currently have that pays for my college tuition and many other things without the vast knowledge that i have gained from your post's and many other primary contributors like you.

Please keep them coming as you can. I know many here feel the same way.

Thanks

I have to echo the plc kid being comparitively new to this forum I have learnt a lot from your postings

i know this is a plc forum but does any of you have any experience with wiring motor starters

maintman said:

i know this is a plc forum but does any of you have any experience with wiring motor starters

Click to expand...

what type of motors and starters

Ron Beaufort said:

how about:

--] [-- "go look for a ONE" ...

--]/[-- "go look for a ZERO" ...

Click to expand...

This is how i remember it

Sometimes when i see |/| and get a tad confused i say it out loud. Really helps. :thumbs: