Examine on and off concept

I would like to when to use the on or off when programming

I think you might want to get more experienced help if this is the sort of questions your asking. Although being in Jamaica, that might be impossible.

Harry we'd love to help you, if ONly we knew what you are really asking. So when you get back ONline shoot OFF a new post so we can OFFer you some advice. Your question about ON and OFF is a little vague; I wONder if you mean ONline programming. Grab some cOFFee and write back.

Just guessing here!

I would like to when to use the on or off when programming

Click to expand...

and you definitely have to learn to do that. As the others have stated, your post is a lot vague. From the title line in the post you mentioned the Examine On and Examine Off definitions. These two terms are uniquely Allen-Bradley and have been almost since the dawn of PLCs on the industrial scene.
Examine On

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is an instruction to the PLC processor and the instruction will be true if there is some device connected to an input module that is placing a voltage on that input. Of course the field device can only place a voltage on the input if it is indeed a closed switch of some type. There is no attempt here on the part of the instruction to determine the state of the field device, just to ask is there something in the field that is closed. That actual device could be Normally Open or Normally Closed but that fact is only important to we programmers so we know when to ask that question in our program.
Examine Off

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is an instruction to the PLC processor that asks the opposite question from the one above. Does the input have no voltage on it? Then the instruction will evaluate true.
This XIC XIO definition stuff has been going on a long time. I figure it will cease to be an issue when we stop programming PLCs with ladder logic languages. You think?

Greetings, Harry,

I think that what you're asking about is "when to use Examine On and when to use Examine Off" ... if I'm right, then maybe this will help ... I'll assume (gosh I hate that word) that you're working with an Allen-Bradley processor ...

here is the basic idea using a 120 volt input module as an example ...

before we begin, you need to know that the word “bit” refers to a “box” in the processor’s memory ... the bit/box can hold either a “1” or it can hold a “0” ...

now let’s consider a screw on the input module ... assume that the address of the screw is I:000/0 ...

first suppose that there IS a 120 volt signal present at that screw ... the PLC processor will see that signal and put a “1” in the bit/box assigned to that screw ... and so I:000/0 will now have a “1” stored in it ...

now suppose that there is NOT a 120 volt signal present at that screw ... the PLC processor will see that there is no signal and will put a “0” in the bit/box assigned to that screw ... and so I:000/0 will now have a “0” stored in it ...

it is IMPORTANT that you realize that the on/off status of the bit/box will be handled automatically by the processor EVEN WITHOUT a program ... specifically, even before you have programmed any rungs into the processor, you can still see the “1” and “0” status of the inputs being recorded on the input table ...

now suppose that you enter a rung and that the only input condition on the rung is an XIC (Examine If Closed) instruction ...

let’s stop for a second and think about that word “instruction” ... the INSTRUCTIONS in the ladder program are called “instructions” for a reason ... they literally are instructions FROM (you) the programmer TO the processor ... in the case of an XIC, the instruction tells the processor to “examine a bit to see if it contains a 1” ...

now suppose that at the time that the processor executes the XIC instruction, there just happens to be a “1” in the bit/box at I:000/0 ... the processor will look for a “1” ... and it will see a “1” and it will be happy ... that will make the XIC “true” ... it will highlight green on your computer screen ...

but now suppose that at the time that the processor executes the XIC instruction, there just happens to be a “0” in the bit/box at I:000/0 ... the processor will still be looking for a “1” ... but it will not see a “1” and it will be sad ... that will make the XIC “false” ... it will not be highlighted on your computer screen ...

so that’s it for an XIC ... now for the flip side of the coin ... the XIO instruction ...

now suppose that you enter a rung and that the only input condition on the rung is an XIO (Examine If Open) instruction ...

again, think about that word “instruction” ... the INSTRUCTIONS in the ladder program are literally instructions FROM (you) the programmer TO the processor ... in the case of an XIO, the instruction tells the processor to “examine a bit to see if it contains a 0” ...

now suppose that at the time that the processor executes the XIO instruction, there just happens to be a “0” in the bit/box at I:000/0 ... the processor will look for a “0” ... and it will see a “0” and it will be happy ... that will make the XIO “true” ... it will highlight green on your computer screen ...

but now suppose that at the time that the processor executes the XIO instruction, there just happens to be a “1” in the bit/box at I:000/0 ... the processor will still be looking for a “0” ... but it will not see a “0” and it will be sad ... that will make the XIO “false” ... it will not be highlighted on your computer screen ...

to summarize:

many people make the mistake of trying to think of the XIC and XIO instructions as representing actual “switches” in the field ... in fact, some well-meaning instructors will use the “switches” analogy to try to make the concepts of XIC and XIO “easier” for beginners to understand ... this is unfortunate ... and it often just leads to more confusion down the road ...

it is much better to consider the XIC and the XIO to be instructions to the processor ...

the XIC instruction tells the processor to go look in a bit/box for a “1” ... the processor will either find a “1” (and be true) or it will not find a “1” (and be false) ...

the XIO instruction tells the processor to go look in a bit/box for a “0” ... the processor will either find a “0” (and be true) or it will not find a “0” (and be false) ...

finally ... this might help too:
old post about processor scan

friendly advice ... put some more detail in your questions from now on ... you'll get much better answers ... and welcome aboard ...

PS ... I see that randylud has already done a good job of answering what we both assume (gosh I hate that word) to be your question while I was typing this up ... sorry if I've overkilled again ...

Just wanted to emphasize one point that Ron made:

XIC and XIO are instructions that evaluate a memory location for a "1" or a "0".

It does not have to be a memory location associated with a real world input that you are evaluating, but can be any memory location.


Marc

Thanks Randy &Ron for your reply i have a beter understanding
jut as you said it can be confusing with the real world
devices ladder logic,thanks a million again and have agood
day
Regards
Harry