AustralIan
Member
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.
In Allen Bradley PLCs you mean. In most other PLCs they call them NO and NC.
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 ------- |
| |
|________________|
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.
Nope, not the same. Normal = failure mode, yes. But a closed valve works the opposite of a "closed" electrical contact.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.
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.
"In most other PLCs" and also the IEC 61131-3 standard.
To be fair, I think XIC predates IEC-61131. And I think US units predate the metric system.You Europeans and your standards like NO, NC, and meter.
And I think US units predate the metric system.
blah blah blah metric blah blah blah imperial blah blah US
I do believe NASA blew up a rocket
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!