nc no whats the deal

can someone plz help me i have very very limited knowledge of plcs, i have the logix pro simulation software and i am very confused right now everything that i hear from the experienced plc guys at work tell me that if you have a momentary nc stop and no start button that you were using to control a light that you would have to use a no contact for the start and a nc contact for the stop and that you would also have to use the outpt as no contact for the seal in..this sounds right for me but when you lool at the silo sim the start stop are labled no nc but the only way i can get them to work properly is to make the stop a no. Can some one plz tell me why

It depends on the instruction you used with your switches "XIC or XIO".

See links below for some light reading to help you understand.

Can you post a screen shot of your logic ?

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

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

If you connect a NC contact to the PLC input, the input is always high (ON) so in your logic you need to use a NO operand which will be true until you press the stop botton or the contact breaks. In PLC's you can connect either a NO or a NC contact to the inputs and later manipuate the input with logic operands(XIO or XIC) but you also need to see it from the safety side of the load connected to the output. This is the reason we use a NC contact for stop buttons which means if the cable breaks or contact fails, the load will safely turn OFF whereas if you use a NO contact for such situations, the load wont switch off until you apply a voltage input to the specific input terminal or shut it down in Logic. Hope that helps.

The same happened to me when i first used the simulator, I put a NC contact and it wouldnt work, then I changed it and it worked fine. As previously stated, in the PLC logic you can use either XIC or XIO contact and it will work(as long as your logic is right) but if you are wiring an emergency or stop button you should always use a NC contact.

the thing is i am trying to prepare for a test on plcs, if i can pass i will most likely get a promotion at my job, all I have to do is write a simple rung of logic and make a lamp come on we deal mostly with fanuc and toyopuc at work i am still trying to learn allen bradley interface

maintman said:

the thing is i am trying to prepare for a test on plcs, if i can pass i will most likely get a promotion at my job, all I have to do is write a simple rung of logic and make a lamp come on we deal mostly with fanuc and toyopuc at work i am still trying to learn allen bradley interface

Click to expand...

Here are a couple more resources to help you lean PLCs. And they are right here on this site :site:

http://www.plcs.net/contents.shtml

http://www.plcsimulator.net/

For anyone who is confused over this NO/NC issue...

The very best thing you can do is COMPLETELY FORGET that these are physical contacts, and disassociate the names NO & NC from these PLC INSTRUCTIONS.

The -| |- instruction asks the processor to evaluate as TRUE, when the data-bit is ON.

The -|/|- instruction asks the processor to evaluate as TRUE, when the data-bit is OFF.

Of course, either of these instructions can be addressing a physical I/O point that is wired to a NO or NC contact in the field.

Allen-Bradley have never called these instructions NO or NC, preferring instead to give them names that describe their functions.....

-| |- is XIC - Examine Input Closed (On) - evaluates true if data-point is ON
-|/|- is XIO - Examine Input Open (Off) - evaluates true if data-point is OFF

daba said:

For anyone who is confused over this NO/NC issue...
.
.
.
Allen-Bradley have never called these instructions NO or NC, preferring instead to give them names that describe their functions.....

-| |- is XIC - Examine Input Closed (On) - evaluates true if data-point is ON
-|/|- is XIO - Examine Input Open (Off) - evaluates true if data-point is OFF

Click to expand...

I would these instruction names confused because the XIC symbol looks like the NO symbol and the XIO symbol looks like the NC.

I wish that AB had named the instructions

XIC: Examined Input Closed
XIO: Examined Input Open

because that is the 'question' that these instructions ask, i.e.

Is the examined input closed ? --> Is the XIC?
Is the examined input open ? --> Is the XIO?

daba said:

-| |- is XIC - Examine Input Closed (On) - evaluates true if data-point is ON
-|/|- is XIO - Examine Input Open (Off) - evaluates true if data-point is OFF

Click to expand...

Just a minor nit pick, but it is "if" not "input":
XIC - eXamine If Closed
XIO - eXamine If Open

how about:

--] [-- "go look for a ONE" ...

--]/[-- "go look for a ZERO" ...

COMPLETELY FORGET that these are physical contacts

Click to expand...

For me, if the input documentation is in terms of 'what is the state when the input circuitry is turned ON' then reading or constructing a rung of ladder logic, if kept reasonably short, documents almost completely the purpose of the rung.

I also name outputs with 'what happens when this output turns ON'.

I completely agree with Bernie. "Think positive" when you describe the I-O.

The nc most likely is the emergency stop buttons

Tharon said:

Just a minor nit pick, but it is "if" not "input":
XIC - eXamine If Closed
XIO - eXamine If Open

Click to expand...

Yes, Tharon, you are correct, eXamine If Closed, etc.,

Although I do vaguely recall that in one of the "families", they used to be called as I said.... PLC2 ?, PLC3 ?, PLC5/250 ? ... I really can't remember which, and of course it may have been that way with the old DOS 6200 softwares.

But that is irrelevant now, anyway, much like adel1961's comment, which IMHO may just confuse people. It is not clear whether adel1961 is talking about the field contact, or the PLC instruction which looks at it.

It is universally accepted that an Emergency Stop button has NC contacts that open when the stop is operated, (fail-safe), and these contacts should remove power from devices, either directly, or via a safety relay (Pilz etc.), and not via the PLC code.

It is also possible that the PLC has an input from the Emergency Stop button, so that the process can be shut-down in a controlled fashion, because typically the PLC will have lost control of its outputs.

Since this is not specifically safety related, it is just as acceptable to use a NO auxiliary contact on the E-Stop button, to inform the PLC when the E-top is operated.

And then, of course, the PLC code may want to know if the E-Stop is Pushed-In (E-Stop), or Pulled-Out (Healthy), so the code may be using XIC or XIO on that E-Stop input.

The contacts in the RSLogix never really confused me. They are the contacts from a coil while in the de-energized state. In the case of an input, the coil always exists and is energized by the state of the input. Bits and outputs, etc, are coils you must put in your program.

-| |- is a normally open contact. It is open (false), until the coil associated with it is energized (set to 1) and the contacts close (become true). XIC. The rung is examined if the coil is closed (true)

-|/|- is a normally closed contact. It is closed (true), until the coil associated with it is energized (set to 1) and the contacts open (become false). XIO. The rung is examined if the coil is open (false)