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Old February 12th, 2020, 03:41 PM   #16
AustralIan
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Originally Posted by theColonel26 View Post
In Allen Bradley PLCs you mean. In most other PLCs they call them NO and NC.
"In most other PLCs" and also the IEC 61131-3 standard.
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Old February 12th, 2020, 04:34 PM   #17
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I felt bad about there being no nice explation for rockwell's naming convention:  

Variable1
------------------
| |
------- |
| | o---------
| ( ) \ |
| | o-----------
------- |
| |
|________________|

So Variable 1 is now just a SPST Normally Open Relay.

Now, "Examine" is a difficult word too, because it has so many meanings.
If you replace in your mind the words "Examine if" with "test to find out if" it all works

Variable1
In ladder, this symbol: --( )-- means "OuTput: Energize (the coil of Variable1)"

Variable1
In ladder, this symbol: --] [-- means "Examine If (the contact of Variable1 is) Closed"

Variable1
In ladder, this symbol: --]/[-- means "Examine If (the contact of Variable1 is) Open"

So the symbols I guess make sense if you ask, "How will I know if it is Closed or Open?"
Well, you would probably apply power to it, ...
... and use another relay, with either a Normally closed or normally open contact

"How to Test if a contact of Variable1 is closed"

Variable1
------------------
| |
------- | XIC
| | o------- 110V ------------------
| ( ) \ | | |
| | o------------------------- |
------- | | | o-------
| | | ( ) \ |
|________________| | | o---------
Neutral ------- |
| |
|________________|

"How to Test if a contact of Variable1 is open"

Variable1
------------------
| |
------- | XIO
| | o------- 110V ------------------
| ( ) \ | | |
| | o------------------------- |
------- | | | o-------
| | | ( ) / |
|________________| | | o---------
Neutral ------- |
| |
|________________|

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Old February 12th, 2020, 05:21 PM   #18
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AustralIan, good art work
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Old February 12th, 2020, 08:26 PM   #19
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Quote:
Originally Posted by Tom Jenkins View Post
To me NO or NC status on a switch identifies the state of the contact in the device when it is sitting on a table unpowered.

NO and NC in PLC ladder logic is easiest to identify if you consider the input on the PLC is functionally the same as a relay coil. (That's actually how it started back in the day.) Then an NO contact in the logic is open if the input (coil) is not energized. The NC contact is closed if the input is not energized.

This thinking upsets many purists, I know, but I don't care. It is consistent with the development history of ladder logic, which was intended to emulate JIC relay ladder diagrams. It has the merit if being ligical and easy to work with.

Speaking of old school hardwired systems, one situation that I've seen confuse a lot of electricians are conveyor belt pullcord safety switches. With a relay, "normal" always equals de-energized, but with passive switches like pushbutton contacts or limit switches, "normal" can have different meanings. A pullcord switch, for instance, is considered to be in it's "normal" state when it is set, and activated when tripped. You want to use a NC contact for a pullcord circuit, in order to make it failsafe, yet when you wire the microswitch inside the pullcord switch assembly, you have to connect the circuit to the COM and NO terminals on the microswitch to get a NC contact when the pullcord switch is in it's "normal", set position, because the switch is set by a cam that activates the microswitch when the pullcord switch is set.

The confusing thing about NO vs. NC contacts sometimes comes down to figuring out what's "normal".

As for ladder logic, I prefer to use "AND" for the NO function and "AND NOT" for the NC function. That's probably a vestige of having worked with boolean keypad programming interfaces on ancient PLCs back in the day.
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Old February 13th, 2020, 12:50 AM   #20
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In chemical engineering the units roughly analogous to switches are valves, and the wiring terms NC and NO correspond to the chemeng terms Fail Closed/FC and Fail/Open., respectively. So in this grammar normal=fail. The concept was the same: what does the valve do when pneumatic pressure is lost; what does a coil do when power is lost i.e. de-energized.

It was mainly a safety issue e.g. if pneumatic pressure was lost, then the process should fail to a safe configuration. Generally but not universally that meant relieving pressure (FO on outlets; FC on inlets), shutting down fire (FC on furnace fuel), etc.
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Old February 13th, 2020, 04:29 AM   #21
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One can go on, what is "normal" .... ?
And what is "closed" and "open" .... ? A closed electrical contact leads the electrical energy through, whereas a closed water or air valve blocks the flow.

One has to know what is meant and just accept it without thinking about it.

Quote:
Originally Posted by drbitboy View Post
In chemical engineering the units roughly analogous to switches are valves, and the wiring terms NC and NO correspond to the chemeng terms Fail Closed/FC and Fail/Open., respectively.
Nope, not the same. Normal = failure mode, yes. But a closed valve works the opposite of a "closed" electrical contact.
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Old February 13th, 2020, 07:40 AM   #22
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Quote:
Originally Posted by JesperMP View Post
One can go on, what is "normal" .... ?
And what is "closed" and "open" .... ? A closed electrical contact leads the electrical energy through, whereas a closed water or air valve blocks the flow.

One has to know what is meant and just accept it without thinking about it.

Nope, not the same. Normal = failure mode, yes. But a closed valve works the opposite of a "closed" electrical contact.
Right. FO~NC; FC~NO. At least a far as taking the "electrical current is analogous to fluid flow" approach (vacuum tubes were called valves in some countries).

Wow, what was I thinking? d'Oh!

Last edited by drbitboy; February 13th, 2020 at 07:43 AM. Reason: correct and apologize.
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Old February 13th, 2020, 08:54 AM   #23
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Originally Posted by AustralIan View Post
"In most other PLCs" and also the IEC 61131-3 standard.
You Europeans and your standards like NO, NC, and meter.
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Old February 13th, 2020, 11:40 AM   #24
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Quote:
Originally Posted by Helliana View Post
You Europeans and your standards like NO, NC, and meter.
To be fair, I think XIC predates IEC-61131. And I think US units predate the metric system.
So: You Americans and your traditions.
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Old February 13th, 2020, 11:53 AM   #25
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Quote:
Originally Posted by AustralIan View Post
And I think US units predate the metric system.
You know all my years on this great earth.... I never knew that, I always thought we came from that side of the pond so our Imperial System (American) was just the good old boys being hard headed and wanted nothing to do with the old land, learn something new everyday

https://www.interexchange.org/articl...metric-system/

PS, I still wish we used the metric system
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Old February 13th, 2020, 01:56 PM   #26
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blah blah blah metric blah blah blah imperial blah blah US
Oh great, my favorite religious debate.

Show me a reasonable quantity of useful physical constants (Planck, G, density of water, whatever) in either system that is a power of 10 and maybe there is a case to be made one way or the other.

Until then, anyone who could possibly some reason to care either already understands proportions, ratios and unit conversions so they don't care, or they should find another line of work.
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Old February 13th, 2020, 02:10 PM   #27
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I do believe NASA blew up a rocket or 2, because they collaborated (presumably with an EU space agency) due to an oversight between US gallons and metric-gallons.

I saw it on YouTube (vSauce channel), so it must be true.
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Old February 13th, 2020, 02:18 PM   #28
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I do believe NASA blew up a rocket
SpaceX did it Jan 19th it was really cool, if you want to see scroll to 20:05

https://www.youtube.com/watch?v=ARIZnaMXTEU

It was intentional but really neat how they had the escape capsule abort
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Old February 13th, 2020, 02:19 PM   #29
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I do believe NASA blew up a rocket or 2, because they collaborated (presumably with an EU space agency) due to an oversight between US gallons and metric-gallons.

I saw it on YouTube (vSauce channel), so it must be true.
They really messed up if they were working in metric gallons!
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Old February 13th, 2020, 02:51 PM   #30
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They really messed up if they were working in metric gallons!
I am not sure about the standards mixup with liquids but they definitely had a mixup btw inch-pounds and foot-pounds. This was for a male/female plug disconnect.

It was supposed to be inch-pounds but the supplier used a foot-pound rated plug. Evidently on launch the side of the rocket was pulled off because too much force was required to disconnect things.
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