Ron Beaufort
Lifetime Supporting Member
how deep shall we dig? ...
Greetings John ...
now I’m DEFINITELY NOT going to argue with that ... because I totally agree ... and there is ABSOLUTELY NOTHING WRONG with what you’ve posted ... but ... what I’m about to say “goes further” than the answer that you gave ...
in one of my classes the only “correct” answer to the question would be “by writing a ZERO into the bit/box” ... the subtle difference is that there are MANY things that can write a ZERO into a bit/box - other than the OTU (Unlatch) instruction that you mentioned ...
going even further down the “subtle detail” path ...
here I would tend to quibble ...
a little background first: many (most?) of my students come into the class believing that the OTE (Output Energize) instruction “works like a coil” ... in many (most?) cases they’ve been told this by well-intentioned co-workers (or even instructors) who are trying to “simplify” the subject of ladder logic so that the common maintenance technician can better understand it ...
in one of my five-day classes, the “works like a coil” idea will be TOTALLY and COMPLETELY annihilated before lunchtime on Monday morning - as will many other common misconceptions ...
now ... if we were in the lab, here’s the way I would demonstrate why I would prefer a different answer ...
first we would program in a VERY simple rung ... just one XIC - addressed to a real honest-to-goodness toggle switch ... and just one OTE - addressed to a real honest-to-goodness contactor coil ... naturally turning the switch ON and OFF in the field makes the contactor turn ON and OFF in the field ... the students can actually see and hear it clunking ON and OFF ...
at this point in the show, the “works like a coil” idea adequately explains the operation of the OTE ... but then - at THIS point in the show, the sun is shining, the birds are singing, and life is lovely ... and the maintenance technician is taking things easy ... (and THIS point in the show is where most instructors stop with the OTE - and move right on to another subject) ...
now let’s say that a “problem” occurs ... the problem is that “someone” has accidentally deleted the rung that controls our contactor ... now WHO would do such a thing? ... WHY would they do it? ... that’s not the point ... the point is that SOMETHING has gone wrong ... life is no longer quite as lovely as it was just a few short minutes ago - and the maintenance technician now needs to troubleshoot the system ...
now to the point ... at the precise moment that the rung was deleted from the program, the input switch was ON - so the OTE was being executed with TRUE logic ... that means that on each scan of the rung, the OTE was “writing a ONE into the bit/box” ... but now that is no longer happening ... (but keep in mind that the contactor in the field was ON at the instant that the rung was deleted) ...
and so ... the question is: WHAT happens to the real honest-to-goodness contactor coil out in the field? ... IF (that’s a BIG “IF”) the OTE really and truly DID “act like a coil” what would it do? ... to most people’s way of thinking, the “coil” would de-energize and “drop out” ... specifically, most people would assume (gosh, I hate that word) that the “disconnected” - “deleted” - “unused” OTE “coil-thing” would automatically TURN OFF ... but it doesn’t ... the real coil in the field is going to stay ON (electrically energized) until SOMETHING “writes a ZERO into its bit/box” ...
and THIS is why I tend to quibble with your earlier statement ...
my point is that the bit in the PLC is NOT really “off the rest of the time” ... specifically, to my way of thinking, that bit will still be ON years from now - unless and until SOMETHING tells it to turn OFF ... and (since our OTE has been deleted) WHO - or WHAT - is available to turn the bit OFF? ... in our simple one rung (well, actually NO rung) program, the answer is that NOTHING is available to make the bit turn OFF - and so it stays ON ... (and that’s my point) ...
to most people’s way of thinking, that “it just stays ON” idea seems to be “wrong” on some level or other ... seems like if nothing is telling the bit to be ON, then it ought to just politely “go off” ... at least it seems that way to most of the “experienced” guys in my classes ... actually the “green” guys usually have less trouble with the idea - because they have less to “unlearn” along the way ...
but the bit doesn’t automatically turn OFF ... and now we have to deal with the REALITY of how the PLC acts when the sun is NOT shining - when the birds are NOT singing - and when life is NOT as lovely as we’d like for it to be ... in other words, when the maintenance technicians have to do what they get paid to do ...
well, that’s as far as I’ve got time to type right now - but I’ve got LOTS more to say ... but the main idea behind all of this is that MOST instructors concentrate on how the PLC-controlled system “works” ... I’m weird ... I concentrate on what steps the technician needs to take when the PLC-controlled system does NOT work ... in order to nail down those steps in a SYSTEMATIC approach, my students need to understand EXACTLY what goes on under the hood of the PLC ... not just the common run-of-the-mill explanations such as “acts like a switch” - “acts like a coil” - “acts like a relay” - and all of the other analogies that have been passed around for years ...
now I’ll admit - freely - that those “simplifications” work well enough MOST of the time ... but what I’m talking about works EVERY single time ... and the amazing thing is that the “real stuff” explanations are actually easier to learn, easier to remember, and easier to apply than the “comes close enough” explanations that confuse a LOT of people when it comes time to troubleshoot a complicated problem ...
anyway ... (my wife just called me home to supper) ... PLEASE do not think that I’m arguing with you - or finding fault with what you’ve posted ... it’s just that I’m getting older (61 and counting) and it seems that revolutionizing the teaching of PLC skills is taking longer than I had planned ...
let me finish quickly with this line of reasoning that I always share with my students ...
suppose that the PLC-controlled system is broken down - and the plant maintenance technicians are unable to find the problem ... they’ve struggled in vain for hours - if not days ...
finally a “guru” is called in to help ... he goes online with the PLC and in MINUTES he has the problem pinpointed ...
now the question: what secret skills does the “guru” possess - skills that the technicians lack? ... what deeper understanding does he have? ... in many cases, the answer is that he simply looks at the operation of the PLC differently ... he doesn’t think in terms of “switches” and “coils” and other analogies along those lines ... instead he has learned what REALLY goes on under the hood as the PLC does its work ... and that knowledge isn’t a secret ... it’s just that most schools don’t even try to teach it at that level ...
supper time ... party on ...
Greetings John ...
now I’m DEFINITELY NOT going to argue with that ... because I totally agree ... and there is ABSOLUTELY NOTHING WRONG with what you’ve posted ... but ... what I’m about to say “goes further” than the answer that you gave ...
in one of my classes the only “correct” answer to the question would be “by writing a ZERO into the bit/box” ... the subtle difference is that there are MANY things that can write a ZERO into a bit/box - other than the OTU (Unlatch) instruction that you mentioned ...
going even further down the “subtle detail” path ...
here I would tend to quibble ...
a little background first: many (most?) of my students come into the class believing that the OTE (Output Energize) instruction “works like a coil” ... in many (most?) cases they’ve been told this by well-intentioned co-workers (or even instructors) who are trying to “simplify” the subject of ladder logic so that the common maintenance technician can better understand it ...
in one of my five-day classes, the “works like a coil” idea will be TOTALLY and COMPLETELY annihilated before lunchtime on Monday morning - as will many other common misconceptions ...
now ... if we were in the lab, here’s the way I would demonstrate why I would prefer a different answer ...
first we would program in a VERY simple rung ... just one XIC - addressed to a real honest-to-goodness toggle switch ... and just one OTE - addressed to a real honest-to-goodness contactor coil ... naturally turning the switch ON and OFF in the field makes the contactor turn ON and OFF in the field ... the students can actually see and hear it clunking ON and OFF ...
at this point in the show, the “works like a coil” idea adequately explains the operation of the OTE ... but then - at THIS point in the show, the sun is shining, the birds are singing, and life is lovely ... and the maintenance technician is taking things easy ... (and THIS point in the show is where most instructors stop with the OTE - and move right on to another subject) ...
now let’s say that a “problem” occurs ... the problem is that “someone” has accidentally deleted the rung that controls our contactor ... now WHO would do such a thing? ... WHY would they do it? ... that’s not the point ... the point is that SOMETHING has gone wrong ... life is no longer quite as lovely as it was just a few short minutes ago - and the maintenance technician now needs to troubleshoot the system ...
now to the point ... at the precise moment that the rung was deleted from the program, the input switch was ON - so the OTE was being executed with TRUE logic ... that means that on each scan of the rung, the OTE was “writing a ONE into the bit/box” ... but now that is no longer happening ... (but keep in mind that the contactor in the field was ON at the instant that the rung was deleted) ...
and so ... the question is: WHAT happens to the real honest-to-goodness contactor coil out in the field? ... IF (that’s a BIG “IF”) the OTE really and truly DID “act like a coil” what would it do? ... to most people’s way of thinking, the “coil” would de-energize and “drop out” ... specifically, most people would assume (gosh, I hate that word) that the “disconnected” - “deleted” - “unused” OTE “coil-thing” would automatically TURN OFF ... but it doesn’t ... the real coil in the field is going to stay ON (electrically energized) until SOMETHING “writes a ZERO into its bit/box” ...
and THIS is why I tend to quibble with your earlier statement ...
my point is that the bit in the PLC is NOT really “off the rest of the time” ... specifically, to my way of thinking, that bit will still be ON years from now - unless and until SOMETHING tells it to turn OFF ... and (since our OTE has been deleted) WHO - or WHAT - is available to turn the bit OFF? ... in our simple one rung (well, actually NO rung) program, the answer is that NOTHING is available to make the bit turn OFF - and so it stays ON ... (and that’s my point) ...
to most people’s way of thinking, that “it just stays ON” idea seems to be “wrong” on some level or other ... seems like if nothing is telling the bit to be ON, then it ought to just politely “go off” ... at least it seems that way to most of the “experienced” guys in my classes ... actually the “green” guys usually have less trouble with the idea - because they have less to “unlearn” along the way ...
but the bit doesn’t automatically turn OFF ... and now we have to deal with the REALITY of how the PLC acts when the sun is NOT shining - when the birds are NOT singing - and when life is NOT as lovely as we’d like for it to be ... in other words, when the maintenance technicians have to do what they get paid to do ...
well, that’s as far as I’ve got time to type right now - but I’ve got LOTS more to say ... but the main idea behind all of this is that MOST instructors concentrate on how the PLC-controlled system “works” ... I’m weird ... I concentrate on what steps the technician needs to take when the PLC-controlled system does NOT work ... in order to nail down those steps in a SYSTEMATIC approach, my students need to understand EXACTLY what goes on under the hood of the PLC ... not just the common run-of-the-mill explanations such as “acts like a switch” - “acts like a coil” - “acts like a relay” - and all of the other analogies that have been passed around for years ...
now I’ll admit - freely - that those “simplifications” work well enough MOST of the time ... but what I’m talking about works EVERY single time ... and the amazing thing is that the “real stuff” explanations are actually easier to learn, easier to remember, and easier to apply than the “comes close enough” explanations that confuse a LOT of people when it comes time to troubleshoot a complicated problem ...
anyway ... (my wife just called me home to supper) ... PLEASE do not think that I’m arguing with you - or finding fault with what you’ve posted ... it’s just that I’m getting older (61 and counting) and it seems that revolutionizing the teaching of PLC skills is taking longer than I had planned ...
let me finish quickly with this line of reasoning that I always share with my students ...
suppose that the PLC-controlled system is broken down - and the plant maintenance technicians are unable to find the problem ... they’ve struggled in vain for hours - if not days ...
finally a “guru” is called in to help ... he goes online with the PLC and in MINUTES he has the problem pinpointed ...
now the question: what secret skills does the “guru” possess - skills that the technicians lack? ... what deeper understanding does he have? ... in many cases, the answer is that he simply looks at the operation of the PLC differently ... he doesn’t think in terms of “switches” and “coils” and other analogies along those lines ... instead he has learned what REALLY goes on under the hood as the PLC does its work ... and that knowledge isn’t a secret ... it’s just that most schools don’t even try to teach it at that level ...
supper time ... party on ...
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