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Old May 14th, 2003, 04:43 PM   #1
GregPLC
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XIO/XIC vs. NO/NC

Ok, I know in my own words the differences between XIO/XIC and NO/NC. The problem comes up when I try to convey to someone in a good way so it makes sense to them. People generally understand NO/NC......the problem lies with XIO/XIC. What is a good way to look at this???

Thanks,

Greg
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Old May 14th, 2003, 04:56 PM   #2
Eric Nelson
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We've 'pondered' this subject a few times around here...

Other will probably have better thread links, but HERE'S ONE to get you started...

beerchug

-Eric
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Old May 14th, 2003, 04:59 PM   #3
Steve Etter
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I like "Active High" and "Active Low". This states the what the individual contact does with respect to it logical state.

Steve
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Old May 14th, 2003, 06:04 PM   #4
bernie_carlton
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I tend to see NO/NC (Normally Open/ Normally Closed) as properties of the external sensing device. It defines under which conditions it sends and ON condition to the PLC input card. I see XIC/XIO (Examine If Closed / Examine If Open) as tests internal to the PLC itself, that is, whether the PLC input ON condition is there or not. As long as I keep them seperate in my mind I'm less confused. I always name an input by the condition that is sensed when the input is ON.

For example, if a Normally Closed/Held Open switch falls off the end of a moving slide to close and signal that the slide is forward, the PLC input is named 'Slide Is Forward' without any reference to the wiring of the sensing device. I let the schematic diagram take care of the external wiring stuff.
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Old May 14th, 2003, 08:10 PM   #5
Tom Jenkins
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Forget XIO and XIC. That is specificaly and only Allen Bradley nomenclature. Any other ladder logic I can think of uses NO and NC.

XIO is A-B speak for NC and XIC is NO.
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Old May 14th, 2003, 09:12 PM   #6
rsdoran
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I kind of disagree, I always thought of XIC as EXAMINE IF CLOSED and XIO as EXAMINE IF "ON" . For that matter Phil's book teaches it that way....ie if you use a N.C. device like a limit switch as an input and it is in its natural state then use an XIO in the ladder it will show in a false condition. The oppposite happens when you use an XIC with the N.C contact, it will be true.

When you use a device like a Pushbutton with NO contacts as an XIC then it will be true when the Pushbutton is in its natural state..ie no signal to the plc. When you push the Pushbutton then it will be false in the ladder. When you use the same NO Pushbutton with an XIO device it will be false in the ladder UNTIL you PUSH the button...ie send a signal to the plc.

N.O. & N.C. are terms relative to hardware devices..ie the device is either OPEN (no power flow) or CLOSED (and power flows)

XIO & XIC are terms relative to the way an input is treated within the ladder...ie how software can act upon the fact of the input being on or off with latitude in how you use the features. It allows more versatility then hardwiring, you get 2 choices of actions per device or internal bit.

I am sure I have misstated some part of this in some way, the more I look at it the more I KNOW I have.
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Last edited by rsdoran; May 14th, 2003 at 09:21 PM.
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Old May 14th, 2003, 09:28 PM   #7
rdrast
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Quote:
Originally posted by rsdoran
I kind of disagree, I always thought of XIC as EXAMINE IF CLOSED and XIO as EXAMINE IF "ON" .
[snip part]
N.O. & N.C. are terms relative to hardware devices..ie the device is either OPEN (no power flow) or CLOSED (and power flows)

XIO & XIC are terms relative to the way an input is treated within the ladder...
I have a pet peeve about the 'Examine' if closed and 'Examine' if on (open).

The instruction is examined in either case. Better mneumonics for those two would be TIC and TIO, indicating 'True if Closed' or 'True if Open'.


In your example too, the explanations for N.O. and N.C. are missing a very important qualifier...Both NO and NC contacts (depending on who you talk to, but most generally) refer to a device in it's inactive or rest state.

'Normally', motor thermals are 'Closed', so they are NC, and allow power flow. 'Normally', Start Pushbuttons are 'Open', so they are NO, and only allow power to flow on an exceptional event (they are operated).

In other words, the 'NO' pushbutton contact will let power flow (when operated) just as well as the motor thermal will when nothing special is going on.


/shrug. What do I know, I think in 1's and 0's.
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Old May 14th, 2003, 09:59 PM   #8
RBergmann
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Ah ... this brings back memories of almost twenty years ago when my partner (now ex) and I had a shouting match in the parking lot about TRUE and FALSE as it related to NO/NC. We couldn't have a conversation about the project because we talked different languages. He had just completed an Allen-Bradley PLC course and was instructed in A-B speak (I liked that). We had genuine confusion until I figured out what A-B was "really" teaching.

I think that A-B relented somewhat with re-defining their True-False as XIO - XIC, which makes sense (kinda, but is none the less still confusing when dealing with neo- and non-programmers) and still allows them to ignore NO and NC. But this also permits them to illustrate their forward thinking (yeah, right). They see things not as they are but as how they will be.

For me it's NO = current/logic passing (in its default state) and NC = not current/logic passing (in its default state), whether in the real or virtual world.

Now ... talk to a fluid power (pneumatics, hydraulics) designer where NC = Not fluid passing and NO = fluid passing.

Now ... head up a project where you switch conversations between the two.
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Old May 14th, 2003, 10:21 PM   #9
93lt1
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Quote:
Originally posted by rdrast
In your example too, the explanations for N.O. and N.C. are missing a very important qualifier...Both NO and NC contacts (depending on who you talk to, but most generally) refer to a device in it's inactive or rest state.
This gets really confusing when you have a Photoeye that has a NC and a NO contact as well as a LO (light on) and DO (dark on) output configuration switch.
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Old May 14th, 2003, 10:44 PM   #10
Jay Anthony
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Where's Terry?
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Old May 14th, 2003, 11:26 PM   #11
rogerhollingsworth
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XIO/XIC

The way I see it when relating XIC/XIO to NC/NO is like comparing apples to oranges.
I relate XIC/XIO to the state of a relay and NO/NC to contacts on that relay.

The state of the NO/NC contacts are always drawn and referred to with the relay coil or activating force being at rest.

If one is going to use relay logic to understand ladder logic then you canít have a XIC as a stand alone contact, it must be tied to a relay coil or activating force, contacts canít energize themselves.

Inputs do in fact have a relay, electronic relay or Triac involved itís just not visible in the ladder, thatís where the confusion comes in.

XIC = Examine if the relay is (closed/energized or a one.) Therefore its NO contacts would be closed and the NC contacts would be open.

XIO = Examine if the relay is (open/de-energized or a zero.) Therefore its NO contacts would be open and the NC contacts would be closed. (Relay at Rest).

Maybe one of you guys can better state what I have attempted to convey.

Roger
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Old May 15th, 2003, 09:48 AM   #12
rdrast
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Re: XIO/XIC

Quote:
Originally posted by rogerhollingsworth

Maybe one of you guys can better state what I have attempted to convey.

Roger
It seems that as with so many day to day 'simple' things, the actual explanations are really tough.

In a sense, we probably all pretty much take it for granted how XIO/XIC/NO/NC/True/False/1/0 relate to each other, but that's because sometime in the past we've had the 'Lightbulb' moment where it all clicked. I've never been able to explain a Lightbulb moment to anyone else, even after spending a long time looking for the switch.
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Old May 15th, 2003, 07:39 PM   #13
Bob Jones
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A matter of perspective

Most folks understand NO and NC because it relates to the deenergized contacts in the physical world.

This is how I look at XIC and XIO...
(Using "true" to mean power passes to the next rung function)

From the data table perspective:
XIC: 1 is true; XIO: 0 is true;
It's simple for my little brain.

From the ladder logic perspective:
XIC: energized coil is true;
XIO: denergized coil is true:
This is simple for "real" technicians

From the I/O perspective:
XIC: high for sourcing input = true
(at least for the PLC's I've known)
low for sinking input = true
(or false for negative logic sinking inputs)
XIO: low for sourcing input = true
(there's bound to be an exeption out there)
high for sinking input = true
(or true for negative logic sinking inputs)
This is simple for somebody, I'm sure... just not me.

Bob
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Old May 15th, 2003, 08:41 PM   #14
93lt1
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Re: A matter of perspective

Quote:
Originally posted by Bob Jones
From the data table perspective:
XIC: 1 is true; XIO: 0 is true;
It's simple for my little brain.

From the ladder logic perspective:
XIC: energized coil is true;
XIO: denergized coil is true:
This is simple for "real" technicians
Luckily, I think that is the way I saw it from the beginning. I didn't have major problems with keeping my understanding of "NO" and "NC" real world contacts with "XIC" and "XIO" seperate.

But, I also hadn't spent years looking at hardwired relay logic before the introduction of the "magic" box. PLC's were going strong when I came into the field.
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Old May 15th, 2003, 08:44 PM   #15
thunda
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Wow now i'm confused too I thought N.O. and N.C. had very little to do with xio xic. I mean xio, xic is the condition of the N.O. or N.C. contact isn't it. Just as if you asked your kid if the door is open or closed, the kid really has nothing to do with the door just check it. Normally open means (to me) in it's shelf state, right out of the box the contact is open. Normally closed is the oposite. Xio means check the address is it open, Doesn't really matter if it's in "shelf state" or not does it.
"From the ladder logic perspective:
XIC: energized coil is true;
XIO: denergized coil is true:
This is simple for "real" technicians"
now this part of your question is easier for me to think if the coil is on then the circuit that feeds that coil is closed XIC if the circuit is open XIO the coil is off.
banghead OHH if I could find the light bulb switch
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