normally closed and normally open

[semilogical]

In my world a normally open contact in a PLC is called an XIO. A normally closed contact is called an XIC.

Now, if you use a normally closed contact on a push button you would normally use a normally open (XIC) in the program to make your system work.

i.e.

xic motor_stop bst xic motor_start_ nxb xic motor_aux bnd ote motor_coil

or in this case:

NO Start NC Stop Output

]---+---| |-------+--------| |----------------( )

 

| |

 

| Output |

 

+---| |-------+

it would be bst xic NO_Start nxb xic Output bnd xic NC_Stop ote Output

Here you are using the coil for the seal, in most cases you would use an auxiliary contact (especially in the motor start arena) to prove your output has actually turned on the motor coil.

BTW, I suck at trying to format keystrokes here to make it look like ladder logic. If I could insert a picture I'd show you that way.

 

[silva.foxx]

 

  start      stop             o/l

---] [---+---] [------( M )---]/[---

         |

         |

    M    |

---] [---+

 

    M                 Lamp1

---]/[----------------(   )

 

    M                 Lamp2

---] [----------------(   )



 

 

Is this what you have?

Why do you have o/l after the ( M )instead of before it?

If stop contact doesn't change... the signal must be missing.
Is the stop switch working correctly?
Is it wired to the correct input?
Is the input led on?

Also, what hardwired physical state is the o/l contact? NO or NC?

 

[Brijm]

Well lets confirm, that we are talking about the same thing.

The inputs/output registers may be different numbers, but is this representation of what you are trying to hook up.

 

[jhonnygaddar]

silvia foxx...the stop switch is a NC...
so when thw signal is passing it should turn open.so if this opens the circuit is not complete,i.e signal cannot flow further,but that doesnot happen,
i am complicating this simple quesn a lot,i apologize to everybody who've been answering it all the time

 

[jhonnygaddar]

can we use a normally open conatct for a stop switch?

 

[plcsoftvsd]

silvia foxx...the stop switch is a NC...
so when thw signal is passing it should turn open.so if this opens the circuit is not complete,i.e signal cannot flow further,but that doesnot happen,
i am complicating this simple quesn a lot,i apologize to everybody who've been answering it all the time

Jhonny,

I think you are confusing between a hardwire NC contact and and a PLC ( Ladder diagram) NC contact.

In hardwire it is simple,...a NC contact (say stop push button) will remain NC till it is pressed.

If you want to use that contact in ladder language say as under

----- [ ]-------[/]------------Output--

and let us say the NC contact has address % I 1.0

This address has to be high for output to be stopped.

Use animation mode in your program and you can see which contacts are in what state...(high or low).

a NO contact has to be high (ON) to pass and give output in above mentioned rung

a NC contact has to be low (OFF)to pass and give output in above mentioned rung.

Hope this helps...

 

[Brijm]

silvia foxx...the stop switch is a NC...
so when thw signal is passing it should turn open.so if this opens the circuit is not complete,i.e signal cannot flow further,but that doesnot happen,
i am complicating this simple quesn a lot,i apologize to everybody who've been answering it all the time

The input doesn't change, unless you push the button.... It is either a 1 or a 0 until one of the pushbuttons change.

     X          Y           M

|---| |--------| |---------( )-|

|    M   |

|---| |--+

What is the output in these two situations:

X=0 Y=0
X=1 Y=0
X=0 Y=1
X=1 Y=1
​

After the last situation if X turns off what will happen?

 

[nehe bhimaji]

Sorry for disturbing

I think
Electrical NO --- Normally Open
NC-----Normally close

But when you right the logic in PLC consider

---||----- Input Require(bit/flag require)
---|/|---- Input not require(bit/flag not require)

Pls write logic considering this you will get correct result

Nehe

 

[jhonnygaddar]

yeah,now i get what all u ppl were tryng to say
i was confused taking the plc logic as a hardwired circuit..
thanks for the help,i realyy appreciate thatt

 

[Lancie1]

--||-- NO Switch (which closes), feeding a NO rung instruction > results in Logic 1 Input: What you see--is what you get!

--|/|-- NC Switch (which stays closed)feeding a NC rung instruction > results in Logic 0 Input: What you "C" (for closed) is OPPOSITE of what your get!

--|/|-- NC Switch (which stays closed)feeding a NO rung instruction > results in Logic 1 Input: What you "C" (for closed) is what your get!

 

Truth Table (Examine If Closed=1, Examine If Open=0) 

    Physical Switch : 0 1 0 1

    PLC Instruction : 0 0 1 1

Rung Logic Condition: 1 0 0 1

So you see there are FOUR possible combinations, not TWO as many beginners believe at first.

 

[bernie_carlton]

To jhonnygaddar:

I really hope these various attempts to help were worthwhile. I have found, including in my own case, having a background in electronics or pure electrical (relay etc) control can present a hurdle to anyone attempting to learn PLC control. (On my own behalf, when first confronted with a PLC ladder listing I remarked, 'what are all the capacitors doing?' reflecting my electronics background).

There are certain concepts which are at the core of PLC control. One being the relationship of NC and NO contacts within a program to the state presented at input pins.

The other is the notion of the 'scan'. In the 'real world electrical' all parallel lines of electrical control are 'scanned' simultaneously. Converting to a 'this rung then the next rung' mentality may be hard to do. Actually, for me, coming from a microprocessor control environment, the idea of 'this is done THEN the next thing is done' was no problem at all.

Once you have those two concept firmly in your mind you are ready to tackle almost any PLC.

 

[jhonnygaddar]

In the figure below, will the power for the output on the first rung normally be on or off? Would
the output on the second rung normally be on or off?
I I
I---] [-----------( )-----I
I I
I---]/[-----------( )-----I
the answers are
yes,power would be off and output would be on,
if power is on the output would be off....is this correct
sorry for starting this again!!

 

[bernie_carlton]

In the figure below, will the power for the output on the first rung normally be on or off? Would
the output on the second rung normally be on or off?

I                         I

I---] [-----------( )-----I

I                         I

I---]/[-----------( )-----I

the answers are
yes,power would be off and output would be on,
if power is on the output would be off....is this correct
sorry for starting this again!!

Again it is important that the 'outputs' be DIFFERENT targets. If not (that is - if they are the SAME location) then the final status of the output at the end of the scan (when the 'logical' outputs are transferred to the 'real world' outputs), in most PLCs, would be dependent on the status set by the second rung only.

 

[jhonnygaddar]

what if they are diffrent otputs,
if power is ON on,what are the outputs on rung 1 and 2

 

[bernie_carlton]

What do you mean by 'normally'? It all depends on what is represented by the contacts at the start of the run and their state.

EDIT -
If the contacts represent digital inputs and those inputs are OFF in the 'normal' state then the first 'output' would be OFF and the second 'output' would be ON.
- END EDIT

Remember, the PLC knows nothing about the type of device which provides power to an input. It only reacts to 'power' - ON or 'no power' - OFF. So don't discuss about whether a NORMALLY OPEN or NORMALLY CLOSED switch is attached to the input.