normally closed and normally open

[leitmotif]

Yes I understand fully. This is the reason that when I tried to learn at home I decided I had an incompetent for an instructor and replaced him (me) with a qualified instructor. Was much better off.

Here is what I did to understand (in class) what is going on here.

Wire a NC to input O configure as NC The output is 0
------ NC to input 1 ------------ NO ------------- 1
-------NO -------- 2 ------------ NO ------------- 2
-------NO -------- 3 ------------ NC ------------- 3

Wire one light to each of the outputs 0, 1, 2, 3.

Now predict what will happen when you
have it deenergized will any lite
energize will any lites lite?
What will the corresponding light do when you press the button (ie if switch is NC on input 0) what will lite on output 0 do?
Etc etc

Do this on paper and then go ahead and energize and test your predictions? Dan Bentler

Johnny

I think you are trying to learn it all at once. We learned it one step at a time and lots of us started when PLCs did not exist. So we really learned hard wired relay logic.

Mechanical switches contact positions are described for their -- oh heck I forget the real term -- call it rest state. In case of a "start switch) contacts are open until you press it ie NO.

GET ACCESS TO A PLC
you are not going to learn this until you are able to make a paper design "put it in the PLC" so to speak and test it.

PCL is commonly done with simulated relay logic to make it easy for us old geezers to understand. That is why the logic states are represented with contacts as you will see on a computer screen.


This is the third time I posted for you the setup I used to figure all this out. Lancie posted a truth table for you. It summarizes what I posted for you.
GET A PLC -- DO IT

What I found out from my effort is summarized by what Bernie said
- the input only cares about if there IS voltage input to it or there is NO voltage input to it.

DO THE HOMEWORK
We can only take you so far by teaching and
you then have to do the learning part
-- DO THE PRACTICUM --
we cannot do the learning for you
- you are the only one that can do that.

Dan Bentler

 

[Lancie1]

I think my earlier post needs a correction. The Truth Table would be more accurate as follows. It is more intuitive to represent the XIO as 1 and XIC as 0

 

   PLC Input Truth Table 

===========================

  Physical Switch : 0 1 0 1

 *PLC Instruction : 1 1 0 0

Rung Logic Results: 1 0 0 1

 

(* Examine If Open = 1, Examine If Closed = 0,) 

The Examine if Open ("NC") PLC instruction essentially performs a Boolean NOT operation on the physical input. It says "output the OPPOSITE of the physical input". On the other hand, the Examine if Closed ("NO") instruction performs the Boolean OR operation. It says "output the SAME as the physical input".

They are really two different types of instruction, with different operations. Attempts by the unwary often create errors, when used as if they were interchangable.