XIO/XIC vs. NO/NC

XIC (or the corresponding symbol, regardless of brand) is an INSTRUCTION, not a contact. It instructs the processor to check the state of a bit in memory - looking for a 'one'. When I was learning PLC, I actually taught myself to refer to the XIC as the "look for a one" instruction.
XIO is therefor the "look for a zero" instruction.

For folks that INSIST on referring to these 2 instructions as "normally open" and "normally closed", have them draw the ladder logic to "seal in" an output instruction, with a normally open start button and a normally closed stop button.

That single example should break them of the habit and provide the ultimate "aha!" moment.
 
For folks that INSIST on referring to these 2 instructions as "normally open" and "normally closed", have them draw the ladder logic to "seal in" an output instruction, with a normally open start button and a normally closed stop button.

That single example should break them of the habit and provide the ultimate "aha!" moment.

This is funny, but I spoke today with a guy who just completed a PLC course. When I asked if instructors emphasized that NO ≠ XIC, and NC ≠ XIO, he said no. It's pretty sad, as this is when problems start. If people are aware that NO ≠ XIC, then they will eventually figure it out, but if they aren't even told this, they almost certainly will have a lot of confusion later on.
 
This is funny, but I spoke today with a guy who just completed a PLC course. When I asked if instructors emphasized that NO ≠ XIC, and NC ≠ XIO, he said no. It's pretty sad, as this is when problems start. If people are aware that NO ≠ XIC, then they will eventually figure it out, but if they aren't even told this, they almost certainly will have a lot of confusion later on.

It IS sad, and one of my pet peeves when it comes to typical plc instructors.
 
For folks that INSIST on referring to these 2 instructions as "normally open" and "normally closed", have them draw the ladder logic to "seal in" an output instruction, with a normally open start button and a normally closed stop button.

As far as I know all the standards (IEC, etc) call them normally open and normally closed.

I'm not sure how the latching example proves your point? The normally open / normally closed naming doesn't have anything to do with the physical wiring. If you want to mirror the input you use NO, if you want to invert it, you use NC. Exactly like an actual relay.
 
iLuh2c2wUai8x4y62


As far as I know all the standards (IEC, etc) call them normally open and normally closed.

I'm not sure how the latching example proves your point? The normally open / normally closed naming doesn't have anything to do with the physical wiring. If you want to mirror the input you use NO, if you want to invert it, you use NC. Exactly like an actual relay.

The NO/NC thought process is exactly why 80% of my students (experienced electricians) get this exercise wrong (until I teach ‘look for a 1 / look for a zero’.

(See linked image below)
 
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For me it's NO = current/logic passing (in its default state) and NC = not current/logic passing (in its default state), whether in the real or virtual world.

Well you got that 100% wrong...

NO = Normally Open, in its default (normal) state, it is Open, so no current can "pass".

NC = Normally Closed, in its default (normal) state, it is Closed, so current can "pass".

I believe A-B did exactly the right thing calling their PLC instructions XIC and XIO, so as to alleviate the confusion that can be caused by NO and NC.

And it's not just physical inputs we are talking about, it works the same for internal bits.....

XIC = evaluate as "true" when the bit is TRUE
XIO = evaluate as "true" when the bit is FALSE

These instructions just pass rung logic continuity (true or false) to the next instruction in the logic....
 
Well you got that 100% wrong...

NO = Normally Open, in its default (normal) state, it is Open, so no current can "pass".

NC = Normally Closed, in its default (normal) state, it is Closed, so current can "pass".

I believe A-B did exactly the right thing calling their PLC instructions XIC and XIO, so as to alleviate the confusion that can be caused by NO and NC.

And it's not just physical inputs we are talking about, it works the same for internal bits.....

XIC = evaluate as "true" when the bit is TRUE
XIO = evaluate as "true" when the bit is FALSE

These instructions just pass rung logic continuity (true or false) to the next instruction in the logic....

Exactly!!!

There is no “power” or “current” in rings of ladder logic. Ladder logic is a programming language, checking 1’s and 0’s or changing 1’s and 0’s.

Just like Basic, or C++, or Fortran. Except ladder logic uses pictures to represent ‘if this, then that’.
 
Well you got that 100% wrong...

NO = Normally Open, in its default (normal) state, it is Open, so no current can "pass".

NC = Normally Closed, in its default (normal) state, it is Closed, so current can "pass".

I believe A-B did exactly the right thing calling their PLC instructions XIC and XIO, so as to alleviate the confusion that can be caused by NO and NC.

And it's not just physical inputs we are talking about, it works the same for internal bits.....

XIC = evaluate as "true" when the bit is TRUE
XIO = evaluate as "true" when the bit is FALSE

These instructions just pass rung logic continuity (true or false) to the next instruction in the logic....

Necropost alert
 
iLuh2c2wUai8x4y62




The NO/NC thought process is exactly why 80% of my students (experienced electricians) get this exercise wrong (until I teach ‘look for a 1 / look for a zero’.

(See linked image below)

For me, thinking of the IO cards as interposing relays makes everything fall into place:

kx6RADC.png



EDIT
I just remembered another PLC brand that has inverted output coils, just to confuse everyone even more xD

k6n1suQ.png
 
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