NC vs NO

I've always had a question about something - but then I asked a couple of students from Chile (they said that they used the metric system down there) ...

when you go to Lowes or Home Depot (or whatever happens to be the overseas version of a lumber yard) do you ask for a two-by-four (in inches) or maybe a 5.08-by-10.16 (in centimeters) ? ...

according to Hector and Pedro - in Chile you ask for a two-by-four ...

and incidentally - a two-by-four isn't really a full two inches by a full four inches anyway ...
 
Depending on your age, you can ask for 2" x 2" or 50 x 50, most places work in both, but predominantly metric now. Bizarrely, the length is always metric.

The old imperial system hasn't been taught since the early 70's, so it's an age thing. My Dad is 70 and just can't shake imperial measures, I'm 45 and really have to think when he describes things in feet and pounds. My nephew is 25, hasn't got a clue what he's waffling about!

All of Europe has gone full on metric, the UK is stuck at 80%, why, I really couldn't say.
 
I remember about 40 years ago when I had to make a presentation to a group of Japanese engineers. I converted all my graphs to official SI units. About halfway through the presentation one of the Japanese engineers slowly raised his hand and asked, "Please sir, what is a Pascal?".

I pulled out my US Customary Unit transparencies. Smiles and nods all around.

I've had similar experiences in Canada and Europe. The moral: go with what the end user is comfortable with, and SI be d$%@#$ed.
 
Back on topic, I think that XIC and XIO are better terms than N.O. and N.C.
The "normal" is open for interpretation, and "closed/open" for power can flow/cannot flow is counterintuitive.
So if the XIC and XIO are prevalent in north America, then I have to hand it to you that you have us beat.
But who is to BLAME for the "N.O." and "N.C." ? That is the important question that needs answered.
Since N.O. and N.C. are english termns, and if they are not from north America, then it must be the british or the australians !!
In danish and german the N.O. and N.C. are not used. We call them "closing-contacts" and "opening-contacts" respectively.

edit: It just dawned on me that the "closing" and "opening" in german is exactly the the same as problem as in the N.O. and N.C.
So it must be the germans who did it ! Cant be the danes because the exact translation from the danish terms are "closing-contacts" and "breaking-contacts".

edit again.: Hang on hang on. "eXamine If Closed". With closed meaning power can flow. It is the same again. Now I believe it is the US who did it.
 
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1968 General Motors -Bedford MA US...The brand is still around: MODICON (Modular Digital Controller)...It is currently owned by (French) Schneider Electric...Go figures...:D...
 
Here's the thing, and I think someone else already expressed this: NO and NC actually mean different things when referring to a physical switch and an instruction on a PLC ladder rung (or ST or FBD or whatever).

The former has been beat to death: O(pen) or C(losed) is the switch state when the coil that drives it is de-energized (I hope I got that right or else maybe I should be beat to death;))

On a PLC, O(pen) or C(losed) refers to the switch (bit, really) state that causes the instruction to evaluate to False; the opposite state evaluates to True.

In most cases they are the same, but I could take an input driven by a NO physical switch and apply a NC ("XIO") instruction to that input, and that rung would evaluate True when the physical switch's coil was in it's "normal," de-energized state.

And what about a manual knife switch? There is no "normal" for that, but I could use one to drive an input, and evaluate it with either NO or NC instruction.

I don't think this is exceptionally confusing in principle, but it does make a slight case favoring non-NO/NC phrases, such as XIO/XIC, for clarity in the limited domain of PLC programming, because ultimately it is the actual switch/input state that is of interest to the programmer, and not so much the design choice made by the designer of the physical circuit about what the switch does when its coil is de-energized.

Am I way off base here?

I really hope I didn't flip anything in this boolean rant, but I guess either I did or I didn't.
 
for anyone who STILL hasn't seen the movie ...

here's a quick preview of the YouTube videos that I recommended way back in Post #2 ...

an XIC instruction ALWAYS does just one simple thing ... it ALWAYS tells the PLC processor to "Go Look for a 1" in a BIT/BOX ...

if the processor finds the 1 that it's looking for - then the processor will nod its little electronic head (yes) - and the XIC instruction will be evaluated as TRUE ...

if the processor does NOT find what it's looking for - then the processor will shake its little electronic head (no) - and the instruction will be evaluated as FALSE ...

on the other hand ...

an XIO instruction ALWAYS does just one simple thing ... it ALWAYS tells the PLC processor to "Go Look for a 0" in a BIT/BOX ...

if the processor finds the 0 that it's looking for - then the processor will nod its little electronic head (yes) - and the XI0 instruction will be evaluated as TRUE ...

if the processor does NOT find what it's looking for - then the processor will shake its little electronic head (no) - and the instruction will be evaluated as FALSE ...

it's all covered in the YouTubes ...
 
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an XIC instruction...
an XIO instruction...
Agreed! This is how I think of them.
] [ Means "if true"
]/[ Means "if false"
Thinking back on my post, any relation to actual relays is purely academic.
The hardest part is remembering their names.
I guess with IEC61131-3 it's easy: the open one looks more open.
With XIC/XIO just remember their names. They're 3 letters each.
 
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