This was a great lesson to understand how ladder logic really works. Thanks for your lessons. I'm learning a lot with these tutorials. Regards from Guatemala
Just so I'm clear. My understanding is that newer PLCs using studio 5000 are asynchronous and that that means the light is updated during the scan of the program. Will that lead to issues such as flashing the light if one rung says it should be on and the next says it should be off.
SO, "last rung wins" is a generic statement - easy to say, but more complex when you evaluate it. "Winning" needs to be defined, perhaps as "last rung that acts on the bit address in the data table will over-ride any other actions prior to it." Insofar as the status of the physical output device, the thing that "wins" is the last output-type instruction that either sets the output address to either 0 or 1. The "rung" might contain edge-sensitive or non-retentive instructions {one shots, latches}, etc. In the case of the video example, you've created a special case - good for demonstration and discussion for sure. The point of the lesson, I think, is to make students THINK about what the 'outputs' on a rung actually DO...all they really DO is turn ON or turn OFF bits in the data table. The OUTPUT UPDATE task of the processor, which runs after the [END] is doing the work, insofar as the light is concerned. So, the OTE is acting on a DATA TABLE location, not a physical device. O:0/0 just-so-happens to be associated with an Output Card, i.e. an output device.
thanks for your great lessons in plc. but when i tried this lesson in emulator the output was unstable, it was flashing I don't know why it behaves like that ?
I don't know that I have ever tested this in Emulate but Emulate isn't a substitute for a PLC. Many programs don't get the same result in Emulate as they do in the PLC. The main thing for this one is to understand that the "Last Rung Wins" is an incorrect statement. Thanks for watching!
Hi, Nice video to understand clear view of the 2OTE with same address. So how many OTE woth same address/ Tag we can use in a program ? If we use 'n' number then as per last rung of OTE will win the race right ?
No watch this video again carefully. The last rung did NOT win. That is the point of this exercise is to show that the methodical scan cycle will always have a predictable outcome. Typically you will only use one OTE per address in a PLC program.
Is the data table updated (input status read) at the beginning of the logic execution or are the inputs physically read at every rung? Is there a rare possibility(theoretically of not practically) that in a long program the status of input might have physically changed while a single scan is completed (i.e. on at one rung and off at some other rung) ?
Best practice would thus be using the inputs and keeping up flags at the beginning of the program and further using just these flags instead of the inputs would make a huge difference IMO. Opinion?
In the end it's safe to say don't use duplicate outputs. Also avoid using ladder that's difficult to read if possible. I see a lot of rungs that work fine but are horrible to figure out logically.
Yes it is safe to say you shouldn't, but it is important to understand it doesn't make the PLC go "crazy". Then and the other PLC scan videos that went along with it are also good exercises in understanding how a PLC scans and executes code. Thanks for watching!
Great explanation. Do you know of any reason to purposely use this functionality? I assume there aren't many cases where it would be more beneficial than other code, but I can't be sure.
Same as an output data file and even an input data file...immediately. In the case of a Micrologix or SLC 500, the physical inputs are written to the input data table at the beginning of the scan and the physical outputs are updated at the end of the scan.
Yes you could do that, but the point is that it isn't a reliable saying since it doesn't always work. The last rung may have the last influence on the outcome, but it doesn't "win". Thanks for watching!
The most logical way to look at this question is to focus on the path before the output. If you have a true path the output becomes true regardless of where the rung resides. A true path is a true path is a true path.
This is a little low level example to what people are meaning when they say this. If I try to control multiple solenoids in different sequences, regardless of the software it locks up on the lower rung in the program every time I find.
Nope...ladder logic doesn't "lock up" unless the PLC's run light is out. I hear this from my students all the time when we do the classroom version of this. Ladder logic will always have a predictable and repeatable result.
@Tim Wilborne sorry, but totally disagree. I'll give the you the situation. 9 solenoids trying to use them in 5 conditions. 9 controlled by 1 button 5 controlled by another etc. If I toggle with an input simulator it will call some in the group. But not all because some are locked in from the previous switch toggle. maybe where talking about 2 different things but that seems locked in to me. Trying to get around it for 2 weeks. Watched all of ron Beauforts videos over the years, so totally get the go write a 0 go write a 1 explanation.
@@TJ-vk5qk There's nothing to disagree with, you're just not following how program scan works. Go back to the basic scan videos and work your way to this one. Also the simulators are not a perfect replication of a PLC.
This was a great lesson to understand how ladder logic really works. Thanks for your lessons. I'm learning a lot with these tutorials. Regards from Guatemala
Glad you are learning a lot Joseph. Thanks for watching!
Just so I'm clear. My understanding is that newer PLCs using studio 5000 are asynchronous and that that means the light is updated during the scan of the program. Will that lead to issues such as flashing the light if one rung says it should be on and the next says it should be off.
That is correct, I should do this video in Studio 5000 to show the difference.
SO, "last rung wins" is a generic statement - easy to say, but more complex when you evaluate it. "Winning" needs to be defined, perhaps as "last rung that acts on the bit address in the data table will over-ride any other actions prior to it." Insofar as the status of the physical output device, the thing that "wins" is the last output-type instruction that either sets the output address to either 0 or 1. The "rung" might contain edge-sensitive or non-retentive instructions {one shots, latches}, etc. In the case of the video example, you've created a special case - good for demonstration and discussion for sure. The point of the lesson, I think, is to make students THINK about what the 'outputs' on a rung actually DO...all they really DO is turn ON or turn OFF bits in the data table. The OUTPUT UPDATE task of the processor, which runs after the [END] is doing the work, insofar as the light is concerned. So, the OTE is acting on a DATA TABLE location, not a physical device. O:0/0 just-so-happens to be associated with an Output Card, i.e. an output device.
thanks for your great lessons in plc. but when i tried this lesson in emulator the output was unstable, it was flashing I don't know why it behaves like that ?
I don't know that I have ever tested this in Emulate but Emulate isn't a substitute for a PLC. Many programs don't get the same result in Emulate as they do in the PLC. The main thing for this one is to understand that the "Last Rung Wins" is an incorrect statement. Thanks for watching!
Hi, Nice video to understand clear view of the 2OTE with same address. So how many OTE woth same address/ Tag we can use in a program ? If we use 'n' number then as per last rung of OTE will win the race right ?
No watch this video again carefully. The last rung did NOT win. That is the point of this exercise is to show that the methodical scan cycle will always have a predictable outcome.
Typically you will only use one OTE per address in a PLC program.
Best explanation anywhere on this subject. Thank you.
Glad it was helpful!
Is the data table updated (input status read) at the beginning of the logic execution or are the inputs physically read at every rung?
Is there a rare possibility(theoretically of not practically) that in a long program the status of input might have physically changed while a single scan is completed (i.e. on at one rung and off at some other rung) ?
See if this video helps. th-cam.com/video/_FSvOVCyk1s/w-d-xo.html
Best practice would thus be using the inputs and keeping up flags at the beginning of the program and further using just these flags instead of the inputs would make a huge difference IMO. Opinion?
@@ankitbothra8205 No, I would call that the worst practice possible. It is on the list to cover.
I really liked the explanation, I recelntly watch the Bookcamp videos, It sounds like that, I need more practice to the step y step scanning.
Thank you! Keep practicing.
In the end it's safe to say don't use duplicate outputs. Also avoid using ladder that's difficult to read if possible. I see a lot of rungs that work fine but are horrible to figure out logically.
Yes it is safe to say you shouldn't, but it is important to understand it doesn't make the PLC go "crazy". Then and the other PLC scan videos that went along with it are also good exercises in understanding how a PLC scans and executes code. Thanks for watching!
Great explanation.
Do you know of any reason to purposely use this functionality? I assume there aren't many cases where it would be more beneficial than other code, but I can't be sure.
I've seen a few functional machines that have done this. I'll put it in the list to make a video on.
pretty crafty. :)
nice to know about plc scan !! thanks for sharing.
You're welcome
what is the o t e is an internal b b not a real world output when does it get updated
Same as an output data file and even an input data file...immediately. In the case of a Micrologix or SLC 500, the physical inputs are written to the input data table at the beginning of the scan and the physical outputs are updated at the end of the scan.
@@TimWilborne THANK YOU !
Swap Rungs 0 and 1 and then tell me the last rung doesn't win.
Yes you could do that, but the point is that it isn't a reliable saying since it doesn't always work. The last rung may have the last influence on the outcome, but it doesn't "win". Thanks for watching!
Oh you are a good teacher. ..you need more views and subscriptions...
Thank you!
Sir make the videos on LabVIEW SCADA
We're going to do a small HMI SCADA series but it'll probably be based off of the Panelview 800. Thanks for watching!
At 2:51 you refer to a red light when you meant green light
Yep
The most logical way to look at this question is to focus on the path before the output. If you have a true path the output becomes true regardless of where the rung resides.
A true path is a true path is a true path.
Thanks for watching Fred! Rung 0 and 1 having the same output address is a mistake but it makes a great lesson.
This is a little low level example to what people are meaning when they say this. If I try to control multiple solenoids in different sequences, regardless of the software it locks up on the lower rung in the program every time I find.
Nope...ladder logic doesn't "lock up" unless the PLC's run light is out. I hear this from my students all the time when we do the classroom version of this. Ladder logic will always have a predictable and repeatable result.
@Tim Wilborne sorry, but totally disagree. I'll give the you the situation. 9 solenoids trying to use them in 5 conditions. 9 controlled by 1 button 5 controlled by another etc. If I toggle with an input simulator it will call some in the group. But not all because some are locked in from the previous switch toggle. maybe where talking about 2 different things but that seems locked in to me. Trying to get around it for 2 weeks. Watched all of ron Beauforts videos over the years, so totally get the go write a 0 go write a 1 explanation.
@@TJ-vk5qk There's nothing to disagree with, you're just not following how program scan works. Go back to the basic scan videos and work your way to this one. Also the simulators are not a perfect replication of a PLC.