Plc

[santhamoorthy]

sir actually i am in beginning stage op plc kindly give me exact
normally open & normally closed condition with ex

 

[Gil47]

The best explanation and examples I have seen out there is this series of You Tube videos by Ron Beaufort

http://www.youtube.com/user/RonBeaufort

 

[balash]

NO = 1 - on, 0 - off
NC = 0 - on, 1 - off

is that what you meant?

 

[GlennA]

Allen Bradley used to be 'Examine if On' and 'Examine if Off'. It can be confusing the first time you see a physical 'closed contact' programmed in a PLC as 'normally open'.

 

[daba]

It is best not to think of PLC instructions as NO or NC contacts.

Confusion to the beginner can be caused when a -||- PLC instruction is coming from a Normally-Closed contact in the field device, and vice-versa.

In PLC ladder language, we only want to know if the addressed memory location is on (use -| |-), or off (use -|/|-).

Once you get used to the concept that -| |- and -|/|- have absolutely nothing to do with Normally-Open and Normally-Closed field or internal memory locations it all becomes clear.

IMHO it was very wise that Allen-Bradley adopted XIC (eXamine If Closed) and XIO (eXamine If Open) as the names for their Boolean logic conditional instructions.

 

[GlennA]

I have worked with engineers that did not understand that emergency stop and limit switch circuits are programmed XIC but wired normally closed. Differentiating between internal logic and external wiring is important.

 

[Mickey]

IMHO it was very wise that Allen-Bradley adopted XIC (eXamine If Closed) and XIO (eXamine If Open) as the names for their Boolean logic conditional instructions.

That is not shared by all.


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

 

[santhamoorthy]

thank you boss

 

[santhamoorthy]

i understood wat ur telling but when i start to do program that time i get confused

 

[GlennA]

Can you give details of what you are trying to program, and which part is confusing you ?

 

[terryp84]

I agree, I like XIC, and XIO much better than NO and NC

 

[balash]

if you think as 24VDC as logic 1, which it is, then NO is closed when activated (high, 24VDC) which means when logic 1, and NC is closed when deactivated (low, no voltage) which means logic 0.
what's there to confuse?

 

[maxketcham]

if you think as 24VDC as logic 1, which it is, then NO is closed when activated (high, 24VDC) which means when logic 1, and NC is closed when deactivated (low, no voltage) which means logic 0.
what's there to confuse?

becuase the way you use the outside no and nc contacts for contol purposes without a plc differ than what you see on a plc when using the exact same contact, stop and estop circuits are a good example.

 

[balash]

yea that could be a problem if you think that hardware NO must match logic NO (which, IMO, is wrong in the beginning cause that's two separate systems, at least to me.)

 

[terryp84]

I understand it ok but I have been doing this foe a long time, during that time I have seen several new people to PLC programming get confused when using a NC contact in a field device but use a xic instruction in the program such as a stop button. Yes it is easy enough to get your head around but when communicating to other people it does save confusion if talking in XIC and XIO terms. but I also agree it can be just as simple to refer to it as high and low or on and off.