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Old June 11th, 2019, 12:04 PM   #1
TWControls
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The symbol shown in the ladder diagram can be thought of as a contact that is...

I'm working on a video for this week and am finishing a response to a question that come up from this video. https://www.youtube.com/watch?v=KOxLh5oQEsU

What do you think about the statement ""The symbol shown below in the ladder diagram can be thought of as a contact that is usually opened."?
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Old June 11th, 2019, 12:13 PM   #2
jstolaruk
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I like "Normally Open" partly because I've always heard it used that way and that when the device (coil) that is represents is un-energized, the "N.O." contacts do not make a connection. I haven't seen it labelled "U.O."
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Old June 11th, 2019, 01:05 PM   #3
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My first thought is that statement is incorrect. It is not a contact that is usually 'opened'. That would be a NC contact (when it energizes, it is usually opened)

I would say it is a normally-open contact. If you want to go IEC, it could be a 'make' contact.

On a side note, I've always found it interesting that PLC software all uses the old JIC NO/NC/Coil symbols and that none of the IEC software companies (like siemens) use the IEC symbols.
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Old June 11th, 2019, 01:27 PM   #4
janner_10
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Usually Opened sounds like something a class room tutor would say.

Normally Opened is what someone working with PLCs would say.
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Old June 11th, 2019, 01:33 PM   #5
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Since this is your training video, you can kind of do what you want. The only reason "normally" anything makes sense to me is that I've been doing this 30 years and I don't have to think about what it means. Why does "normally" equate with "off" even if the "normal" state isn't off??

I would try to move the discussion away from the physical world with this. It isn't about normal. It is about "off" and "on" or "active" and "inactive" or "1" and "0". Personally, I like active and inactive.

The contact you have shown is "true if active". A "normally closed" contact would be "true if inactive", assuming of course you consider "normal" to be off.

Keith
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Old June 11th, 2019, 02:01 PM   #6
Rson
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You could also go the route "Examine if Open", "Examine if Closed" (ie XIO, XIC)
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Old June 11th, 2019, 02:14 PM   #7
rankhornjp
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That symbol means "Go look for a 1"

The problem with NC NO language, to me, is that it can get confusing when talking about PLCs and field equipment at the same time. Ex: Stop button wired NC would have a NO (Go look for a 1) symbol in the PLC Logic.

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Old June 11th, 2019, 02:44 PM   #8
Mickey
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Here is a related discussion. A very good read.

http://www.plctalk.net/qanda/showthread.php?t=12334
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Old June 11th, 2019, 02:46 PM   #9
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Looks like we're all on the same page, I just wanted to make sure before I posted this video. That statement is right off of a highly recognized test.

I don't like saying these symbols are normally open and normally closed and someone said I needed to make a video clarifying that point. Just wondering how much backlash I'll get :-)
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Old June 11th, 2019, 02:53 PM   #10
Mickey
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Quote:
Originally Posted by TWControls View Post

I don't like saying these symbols are normally open and normally closed and someone said I needed to make a video clarifying that point. Just wondering how much backlash I'll get :-)
I prefer "Normally Open" "Normally Close" ( I started before PLC's with "Relay Logic"), but it does need to be "clarified" for PLC logic. (As you pointed out).
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Old June 11th, 2019, 03:36 PM   #11
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When I am reading logic like this, I tend to say to myself "If true" and "if false". Or "If" and "If not".
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Old June 11th, 2019, 03:43 PM   #12
James Mcquade
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I like normally open and normally closed.
the terms examine if open or examine if closed was confusing.

I also did relay logic before my introduction to plc's

james
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Old June 11th, 2019, 03:52 PM   #13
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Quote:
Originally Posted by g.mccormick View Post
or "if" and "if not".
+1
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Old June 11th, 2019, 03:58 PM   #14
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I always described them as "If Open" and "If Closed" and relate that to the contacts for the device in the field

I hate the term "normally" in ladder logic. I would push it a step further sometimes and explain that the PLC has no concept of normally open or normally closed. All the PLC knows is what is it now.

The ladder logic is not showing what you have, it is showing how it works. If this limit switch is closed, turn on this output.

OG
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Old June 11th, 2019, 04:11 PM   #15
Ken Moore
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When I am explaining ladder logic to a non-plc person, I use "IF" and "NOT". On another note, I have been teaching myself CodeSys in my spare time, many of the online tutorials (mostly from Asia) use terms like "Open Contact" and "Closed Contact". "Add an open contact to the network". May be a language thing.
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