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Old June 11th, 2019, 04:21 PM   #16
TWControls
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I think the issue I take with normally or usually is what defines their normal or usual position? Between that and you would be surprised the people, including instructors, that I have to convince that you don't have to use a "normally open" PLC instruction with an input that has a normally open electrical contact connected to it.
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Old June 11th, 2019, 04:21 PM   #17
Operaghost
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Yeah I really dislike "contact" too.

Over the years I taught a lot of people that were familiar with relay logic, and I always tried my best to separate relay logic from ladder logic.

But, everyone sees those symbols and refers to them as contacts. I would avoid that term as well as "normally".

But you just want to slap the people that decided on those symbols and names.

OG
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Old June 11th, 2019, 05:21 PM   #18
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I like "AND" for a NO contact and "AND NOT" for a NC contact.
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Old June 11th, 2019, 05:40 PM   #19
jaden
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Normally Open and Normally closed define the state of relay contacts when the coil is not energized or sitting out on a table top. PLC "ladder" logic was meant to represent Relay Ladder symbology. If it is followed through, it is all consistent.
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Old June 12th, 2019, 01:01 AM   #20
ivo.maenen
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Quote:
Originally Posted by jaden View Post
Normally Open and Normally closed define the state of relay contacts when the coil is not energized or sitting out on a table top. PLC "ladder" logic was meant to represent Relay Ladder symbology. If it is followed through, it is all consistent.

So if I wire a NC-stop-pushbutton to an input and a NO-start pushbutton


being consistent means you would program something like (I do not use the seal-contact!):


STOP START motor

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


Thats not logical to me.....




I prefere:


-| |- : OK if status '1'



-|/|- : OK if status '0'
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Old June 12th, 2019, 04:48 AM   #21
Gene Bond
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The voices in my head say, "If On" or, "If NOT On", when I'm reading a ladder rung...
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Old June 12th, 2019, 05:13 AM   #22
Rson
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Quote:
Originally Posted by ivo.maenen View Post
So if I wire a NC-stop-pushbutton to an input and a NO-start pushbutton


being consistent means you would program something like (I do not use the seal-contact!):


STOP START motor

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


Thats not logical to me.....




I prefere:


-| |- : OK if status '1'



-|/|- : OK if status '0'
If you use relay logic terms or are used to it, it is still consistent.

For example, if I wanted to take 1 contact and make it many in relay logic, I would wire (for instance) a NC contact to a relay, and then all of my logic would be reversed on the relay’s contacts (since the relay would be ‘on’ in the normal condition)

I think preferred terminology all comes down to who has used relay logic (which PLCs were made to replace, and thus use NO, NC, and COIL contacts in ladder) or if you are approaching it as PLC-centric (on/off from the PLC’s point of view)
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Old June 12th, 2019, 05:15 AM   #23
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Jaden > Normally Open and Normally closed define the state of relay contacts when the coil is not energized or sitting out on a table top. PLC "ladder" logic was meant to represent Relay Ladder symbology. If it is followed through, it is all consistent.

>>>

I am in agreement with Jaden on this.
What little PLC programming I have done is in ladder. And NC or NO contacts make
perfect sense. If the 'contact' is an input, the NC type means if there are no
volts on that input give me 'continuity' through the contact on the rung and if
it is the NO type then one must have volts on the input to get continuity through
the rung.

When monitoring a running PLC program in ladder logic everything 'works'
properly if looked at as energized coils close NO contacts . . At least in Allen
Bradley the closed contacts (allowing continuity) are obvious.
Poet.

Last edited by Mad_Poet; June 12th, 2019 at 05:17 AM. Reason: Made a NO NC. Corrected.
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Old June 12th, 2019, 05:39 AM   #24
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The whole reason for the existence of PLCs, as opposed to other computer systems, was the concept of representing Boolean logic via elements of electrical circuits: contacts, relays, timers and counters. This is what made the PLCs so ubiquitous: making programming understood by electricians. Structured text and other more advanced methods appeared later, with the goal of overcoming the obvious shortcomings of Ladder approach.

Therefore, when explaining PLCs, one cannot skip the ladder representation even if it is not meant to be used in the future. And in the electrical world terms "normally open" and "normally closed", NO and NC, have existed for more than hundred years and have become de-facto standard understood by everyone. I think any deviation from them even when describing a bit in memory rather than a physical contact will just muddy the waters. Do not overthink it.
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Old June 12th, 2019, 06:13 AM   #25
saultgeorge
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+1 for LadderLogic.
I agree with those statements 100%. I was an industrial electrician for years before doing all programming, and of course I started out in hard-wired relay logic. And A-B ladder is easy for me to program in, easy to understand and also easy to explain to others. I am working on ST and a lot of other platforms like CodeSys, but A-B will always be my comfort level.
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Last edited by saultgeorge; June 12th, 2019 at 06:14 AM. Reason: typo
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Old June 12th, 2019, 06:24 AM   #26
Ron Beaufort
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rankhornjp nailed it back in Post #7

Quote:
That symbol means "Go look for a 1"
I've been teaching this "better mousetrap" explanation for about twenty years now - and the world (from even as far away as Australia) has truly beaten a path to my door ...

this approach allows you to teach the concepts to ANYONE (who's willing to learn) whether they have any previous electrical or PLC experience or not ...

the following YouTube is the first in a series of eleven ... these should all be watched in number order - since they all make up one 90-minute lesson ...

https://www.youtube.com/watch?v=T3tnXu-Eywc

I've lost count of how many schools have asked me for permission to use this series of videos as part of their curriculum - and I know for a fact that many others around the world are using them without bothering to ask for permission (and that's OK with me) ...

now that I'm finally retired, I welcome anyone to use them free of charge - as long as they are shown in their entirety - including the copyright notices and credits ...
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Last edited by Ron Beaufort; June 12th, 2019 at 06:52 AM.
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Old June 12th, 2019, 06:41 AM   #27
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I'm Old school (well at my age it's got to be). PLC's were considered a replacement of all those hard wired relays (and logic blocks) so I still use normally open & normally closed that is in their non energised state. IEC is just a continuation of Boolean logic symbols so like AND x AND Not Y OR etc.
examine if open seems a bit alien to me examine if open suggests check if it is open so as I see it a normally open contact (not energised) then it is not true so will not perform the function (sounds a bit backward to me).
Relays with multi contacts are normally described as 2 NO 2 NC so normally open = ----! !---- so in my book this is normally open (can of worms here I'm sure and I suspect the AB guru's out there will have their penny's worth).
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Old June 12th, 2019, 08:27 AM   #28
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Tim, I have been doing electrical now for a few days... I have never heard anyone use "Usually" and I am from the South and they will make up their own words down here.

Side note: Siemens STL is (was) the best AND, NOT, AND NOT.... never confusing
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Old June 12th, 2019, 09:19 AM   #29
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Here's the video, let me know what you think.
PLC Instructions are not the same as Electrical Symbols | Normally Open and Normally Closed?
https://youtu.be/ZuXUo8PfcDw
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Old June 12th, 2019, 01:05 PM   #30
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Quote:
Originally Posted by geniusintraining View Post
Tim, I have been doing electrical now for a few days... I have never heard anyone use "Usually" and I am from the South and they will make up their own words down here.

Side note: Siemens STL is (was) the best AND, NOT, AND NOT.... never confusing
So do you use the terms "cut it on" and "cut it off" instead of turn on and turn off?
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