The symbol shown in the ladder diagram can be thought of as a contact that is...

TimWilborne

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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."?
attachment.php


no.png
 
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."
 
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.
 
Usually Opened sounds like something a class room tutor would say.

Normally Opened is what someone working with PLCs would say.
 
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
 
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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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 :)
 
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).
 
When I am reading logic like this, I tend to say to myself "If true" and "if false". Or "If" and "If not".
 
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
 
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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