Newbie Question

Sometimes I'm confuse when I see a normally closed contact
like a Stop buton being represented as a XIC normally open
in a PLC diagram. Sometimes I see a normally closed
contact represented as a XIC too in a ladder diagram.
Can somebody explain to me why ?

Think about what is the state of input at input card when stop is not pressed. It is "true" or "1". That instruction in plc really is not "normally open" it is "examine if true". So you detect if stop is not pressed by using "examine if true". That is safer way to do it. Allow program to execute only when "examine if true" on stop is true.

You should watch these:

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

They are very good videos made by Ron Beaufort

Thank You, I think I found the answer here too by googling

http://www.plcs.net/dcforum/DCForumID1/2032.html#1

Now I need to learn how it is hardwire ?

After watching the suggested videos it should become clear that the instructions in the ladder logic refer not to the state of a switch but only to the presence on non-presence of a signal at the PLC input pin.

I find it easiest in my mind to assign to the input pin the name of the state when there is power to the input pin. Leave out any reference to the method of wiring the external device (Normally Open, Normally Closed.) Then the logic itself becomes more readable.

So in the ladder diagram XIC representing the normally closed STOP BUTTON the value is 0. So when I hardwire this STOP Button to the PLC
then my value will turn 1 and the XIC diagram will turn green, indicating current flow, Is that what you are saying ?

Please watch the videos first, then it will be appropriate to discuss this in terms of what is shown there. The teacher who developed these has spent years making clear concepts just as you have expressed here. It won't take long to watch them.

Here is another good read.

http://www.patchn.com/index.php?option=com_content&task=view&id=58&Itemid=74

I think I am getting the idea , that the PLC is a different kind of machine, it cannot conform and follow the normal ladder diagram but
a computer that only understand what the input is, and the program
that have been created inside the computer or PLC is the one
conforming to the circuit. It is the job of the processor to
run this circuit with all the input fed throught it.

sounds like you're almost there ...

basic idea: the PLC is a "computer" - pure and simple ... the PLC's program tells the "computer" what to do ... an XIC does NOT represent any sort of switch – "open" or "closed" or otherwise ... instead, an XIC is an "instruction" – FROM the programmer – TO the processor ... the XIC "instruction" tells the processor what to do ...

RULE: whenever the processor reads an XIC instruction, the processor is commanded to "GO LOOK FOR A ONE" in a bit/box ...

Click to expand...

there are NO exceptions to this particular rule ... any attempt to include a "switch" in this otherwise simple rule, does NOTHING except confuse the issue ... specifically, an XIC does NOT "examine a switch" in any way ...

now then ...

when the processor looks in the bit/box and it DOES find the ONE that it's looking for, then the XIC will be evaluated as "TRUE" ... on the other hand, when the processor looks in the bit/box and it does NOT find a ONE, then the XIC will be evaluated as "FALSE" ...

going further, whether the bit/box contains a ONE – or whether it contains a ZERO - is another lesson ... and that lesson is fully covered in the series of videos that my distinguished colleagues have been so graciously recommending ...

the full series of eleven videos takes about 90 minutes to watch – and they should be watched IN NUMBER ORDER ... the material that they cover has helped MANY people clear up EXACTLY the same confusion that is vexing you ... if you still need to post any specific questions AFTER you have watched all of the videos (IN ORDER) then I'm sure that the forum members will be able to offer enough detailed help to make it all understandable ...

trust me: you are not alone ... many people have been confused by this same problem – some of them for many years ... I sincerely hope that this helps you understand ...

(and thank you all for the kind compliments) ...

To everyone thank you for the explanation, I m just not used
to it, Im been away for so many years in the control field,
but I have background on power system protection and have
shifted to doing electrician construction job.
But because of the present econoomy I'm trying to get
a job in maintainance and PLC is one of the requirement
that I have to learn and also VFD and SCADA.
Thank You.

Is there any rule when you will use the XIO symbol in a
PLC or just ignore this symbol.

rene6985 said:

Is there any rule when you will use the XIO symbol in a
PLC or just ignore this symbol.

Click to expand...

It will depend on the result you want. If you want something to happen when the Bit is 1 then you will use an XIC (Examin if Closed). If you want something to happen when the Bit is 0, then you will use and XIO (Examin if Open).

Lets say you have a Normally Open Pushbutton wired to Input I:000/00. You are going to control a Hydraulic Press with it. When the Pushbutton is pushed, you want to start a Hydraulic Pump, and a Hydraulic Solenoid Valve routing the Hydraulic Fluid to the Cylinder on the Press. When the Pushbutton is released you want to route the Hydraulic Fluid from the Press Cylinder back to Tank until the Press activates a Press Retracted Limit Switch on another Input.

You would us an XIC from the Pushbutton Input for controlling the Hydraulic Pump Starter, and the Press Up Solenoid. When the Button is released, you would use an XIO from the Pushbutton Input to control the Press Retract Solenoid.

Obviously over simplified, but I hope it gives you the idea.

Stu.....

Ok I think I got it XIC instruction are use for combination
of switches , when you want one of the two combination
true and the other false.

rene6985 said:

Ok I think I got it XIC instruction are use for combination
of switches , when you want one of the two combination
true and the other false.

Click to expand...

Sort of. It is what happens when the Memory Location associated with the Address is one (1), and when it is zero (0). If you use an XIC it will be evaluated true if it 1, and false if it is 0. If you use an XIO, it will be evluated false if it is a 1, and evaluated true if it is a 0.

Go to the Ron Beaufort video lessons listed in post #2, and watch the entire series. They are very well done.

Stu.....

if it bleeds, we can kill it ...

suppose that you have TWO momentary pushbuttons wired up to your PLC as inputs ...

Button-A is physically/electrically wired Normally-Open ...
Button-B is physically/electrically wired Normally-Closed ...

suppose that you have FOUR lamps wired up to your PLC as outputs ...

Lamp-E ...
Lamp-F ...
Lamp-G ...
Lamp-H ...

sketch all of those components out on a piece of paper ... do NOT wire/connect the buttons to the lamps ... just connect the buttons to the PLC input terminals ... connect the lamps to the PLC output terminals ...

now then ...

write FOUR simple rungs to control the four lamps (one rung per lamp) ... you should use ONLY eight "instructions" (total) in your program ... you'll need TWO XICs and TWO XIOs and FOUR OTEs ... there will be only TWO instructions on each of your rungs ...

write your program to meet the following specifications ...

(1) Lamp-E must be ON when Button-A is pressed ... it must be OFF when Button-A is released ...

(2) Lamp-F must be ON when Button-A is NOT pressed ... it must be OFF when Button-A is pressed ...

(3) Lamp-G must be ON when Button-B is pressed ... it must be OFF when Button-B is released ...

(4) Lamp-H must be ON when Button-B is NOT pressed ... it must be OFF when Button-B is pressed ...

ask the forum members for help (if you need it) – and post your answer so they can check your work ... I'm teaching non-stop for the next two weeks or I'd offer more help myself ...

BIG PICTURE IDEA COMING UP ...

with the PLC's ladder logic, you can make "something" happen:

(A) when you DO have an electrical signal coming in from the field ...

OR ...

(B) when you do NOT have an electrical signal coming in from the field ...

it's dealer's choice

Click to expand...

once you get this little experiment done, you should have a better grip on the basic concepts that are troubling you ...