Normally Open/Normally closed

[Supersteve]

Hello, my name is Steve and im completely new to programming and using plc's

Ive had LogixPro sim for 4 days now and sucessfully managed to program successful logic ladders for the Garage door and the silo.
Although i managed to program the simulators im still not totally understanding the terms "Normally Open" and "Normally Closed"

Probably a silly senseless question but im quite good at asking silly questions so :

Could someone please explain in 'Layman terms' what "Normally Open" and "Normally Closed" mean?


Thank you in advance to anyone that can help.

 

[eMpty]

NO & NC describe status when no energy exists.(like sensors).

 

[uptown47]

Hi Steve and welcome to the forums,

Normally Open just means that the contact will prevent 'power' from going through it, unless the address/input that it refers to is ON.

Power here      Normally Open      No Power here
-------------------| |--------------------------
 
In this example, if the Normally Open gate isn't energised then power can't get through to the other side.
 
For instance, in a real world example you may have....
 
Power here     Light Switch            My Light
-------------------| |--------------------( )

A Normally Closed contact is the opposite...

Power here    Normally Closed    So.. power here
-----------------|/|-----------------------------
 
As long as the address/input to which the Normally Closed switch refers ISN'T energised, then power can travel through it.

Hope that makes some sense. If it doesn't then please don't hesitate to ask for clarification.

Cheers

;-)


In other words.... if there is nothing else having an effect on that contact would it 'normally let power through' (ie Normally Closed) or would it 'stop power going through' (ie Normally Open).

 

[Mickey]

See link below, follow the lessons in sequence.

http://www.ronbeaufort.com/sample_lessons.htm

And when done with the above, here is another link.

http://www.patchn.com/index.php?opt...d=58:xioxic-explained&catid=20:plc&Itemid=118

 

[iant]

I'll put in my two cents
the terms are not allways correct - But 95%.
This refers to an electrical contact state.
With no power or no force applied to a switch or device, that device is said to be in it's 'NORMAL' State.
TEST WITH THE POWER OFF
If you use a multi meter (OHM Range) on that contact you will see the following.

OPEN - No connection across the contact or switch
THIS IS CALLED NORMALLY OPEN or NO
AND IN RSLOGIX (XIC - examine if closed)

CLOSED - the meter will display '0' Ohms
THIS IS NORMALLY CLOSED or NC
AND IN RSLOGIX (XIO - examine if open)

THIS IS FOR THE WIRING DRAWINGS OF MACHINERY

There are some special exceptions - but these will only confuse you.
USUALLY _ SOME ROTARY CAM SWITCHES are drawn opposite

 

[DamianInRochester]

Mickey's suggestion is a good starting point. It is very important to understand that there is a difference between these terms when used in the electrical circuit world, and in the ladder logic world.

In terms of ladder logic, NO and NC need to be viewed as instructions as opposed to devices. If you create an electrical circuit based on a ladder diagram, and operate both, it is very possible (and even likely) they will not operate the same. Ladder logic does not "simulate" an electrical circuit, it only provides an easy method of graphically representing logic operations. Therefore in order to fully understand a ladder program, you need to understand the rules that the controllers follows. You need to understand the scan, when physical and internal memory is refreshed, what order things are processed in at both a network and a subroutine level, and the definitions of the instructions you are using.

In the context of electrical circuits, NO and NC take on a whole new meaning. It is also very important to fully understand these as well, since most of your real world inputs will follow those laws. The type of device will play a roll in that interpretation. For instance, if you are considering a relay coil, then you can relate NO/NC to the energized state of the coil. However, not all contacts are driven by coils. They may be a reed switch that is activated by an actuator of some sort. They may be some sort of rotary CAM switch. Generally, you will relate NO/NC to what the "active" state of the device is. There are instances where this can become ambiguous.

To add to the confusion, in the electrical world you will also find normally open but held closed, and normally closed but held open, and other goofy combinations such as flip flops/toggles.

Then, in regards to safety devices like guard door switches, it may also get confusing. You have to take into account the relation between the actuator and its key.

Your question is neither silly nor senseless. It is actually a very good one. It underlies some of the fundamental stumbling blocks and misconceptions people have when trying to relate ladder programs and electrical circuits.

 

[daba]

In my work I've seen some people struggle with the understanding of this confusion, the root cause of which is usually because they still think of the PLC instructions -| |- and -|/|- as N/O and N/C contacts.

It is only when they can be persuaded to lose that analogy that the waters clear, and in a few cases I've seen grown men "jump up and shout Eureka" (not literally, just getting the point across).

For these perople, I get them to explain, in their own words, what is going on if a field N/C contact is wired to an input terminal on the PLC, and a "N/O contact" is on the ladder rung. Usually the response starts confidently, but then starts to waver, and has often broken down unanswered.

Perhaps Allen-Bradley engineers had a crystal ball when they started making their PLCs, because they deliberately didn't call them NO and NC contacts or instructions, instead adopting the descriptive XIC (eXamine If Closed), and XIO (eXamine If Open) instruction names.

I always correct people immediately when they use N/O or N/C when talking about logic instructions, I believe it will help them later on, and certainly when they have to interact with co-workers etc.

 

[iant]

the only hassle with not using N/o and N/c
is when you deal with hard wired relay logic.
some of us are that old.
the original 'Flip Flop' was a latching relay or latching button.
these still had 'N/O' or 'N/C' contacts.
Their state 'changed over' upon being latched.
Same electrical principle as I mentioned

 

[Lancie1]

In my work I've seen some people struggle with the understanding of this confusion, the root cause of which is usually because they still think of the PLC instructions -| |- and -|/|- as N/O and N/C contacts.

Yes, you have described the main problem in very concise language. The PLC input instructions are showing the state of input devices (which devices may be NO or NC). In Allen Bradley programs, the instuction that shows the state of a NC device (Stop Pushbutton) can be the SAME as the one that shows the state of an NO device (Start Pushbutton).

 

[jajalili79]

This thread has several great responses which answer the original question, but I wanted to provide input as N/O and N/C nomenclature is one of the most widely misunderstood topics in the industry and a topic I struggled with early in my career. As with many fundamentals, each individual needs to learn the topic in his own way.

My awakening to N/O and N/C in electrical circuits was for me as a young engineer performing a startup on a system with a substation and generator paralleling switchgear. I held an ohm meter and I stepped the PLC through the sequence of operation. Only then I 'saw' the change in state and grasped the concept.

Anyhow, my recommendation is to get out into the field for startup / troubleshooting and put your hands on equipment. This has assisted me numerous times and proven to be invaluable.

 

[dogleg43]

Rockwell calls their instructions XIC and XIO but what do the other manufacturers (Siemens, Automation Direct,GE, Modicon, Telemecanique, etc.) their instructions? Do they continue with normally open and normally closed?

 

[sthompson]

Rockwell calls their instructions XIC and XIO but what do the other manufacturers (Siemens, Automation Direct,GE, Modicon, Telemecanique, etc.) their instructions? Do they continue with normally open and normally closed?

Omron uses terms of AND, NOT AND, OR, NOT OR.

GE 90 Series uses NO/NC Contact and Coil.

Siemens uses NO/NC Contact, and Output Coil.

Modicon uses NO/NC Contact, and Coil.

Stu....

 

[iant]

Omron uses terms of AND, NOT AND, OR, NOT OR.

GE 90 Series uses NO/NC Contact and Coil.

Siemens uses NO/NC Contact, and Output Coil.

Modicon uses NO/NC Contact, and Coil.

Stu....

Omron is a graphical -] [- 0r -]/[- dwg
But if you go to Mnemonics it is a whole new ball game.

At least (And Not) (OR) (AND) etc. including LD instructions can be followed back to boolean algebra

And following on
Of course
STL etc....

 

[uptown47]

My awakening to N/O and N/C in electrical circuits was for me as a young engineer performing a startup on a system with a substation and generator paralleling switchgear. I held an ohm meter and I stepped the PLC through the sequence of operation. Only then I 'saw' the change in state and grasped the concept.

My own awakening was about 12 years ago when I was dropped in the brown stuff by a company I worked for. They sent me (with little to no experience) to commission a line. Before start up I checked the software and 'corrected' a normally-open Stop button by making it normally-closed.

Needless to say that the line wouldn't start. I soon figured out how to 'go online' and found my mistake. To my massive relief the line started up fine after that and there wasn't any major problems. "I'll not do that again!" - was the cry from the back of my brain!

;-)