Output Coil Instruction in LAD

what is the Output Coil Instruction function in a typical Ladder Diagram Program?

Depends!

Most PLCs have an instruction that looks like:

----( )-----

and means turn on or off a bit at the address associated with the instruction symbol. The on/off status of the bit is determined by the logic of the rung before the output. If all the logic before it is true, the output will be true and the bit turns on. If this bit is addressed to an output module, then some real device on a machine or process turns on as well. If it is just an internal bit used for logic purposes, the same thing is true, the bit turns on when the logic before it is true. In each case, when the logic of the rung goes false, the bit turns off.

Why do we always write the PAPER before doing the OUTLINE?

Guest,

I was pleased to see that you started off asking about OUTPUTS. Most questions on here start off by giving detailed descriptions of all the Input conditions, with nary a thought about the RESULTS, the Outputs. It reminds me of my high school English class. Mrs. Brewer would say, "now class, before you write your paper, first make an outline summary of your subject." Of course we always wrote down a bunch of unorganized sentences, then went back and tried to make an outline fit what we had written. Doing the outline first would have made the writing SO much easier, had I only known how.

Why do programmers always start on the left side with a bunch of INPUTS? We then try to make them fit an OUTPUT at the end of the rung, usually finding that by the time we get over that far, our output doesn't exactly work with the hodge-podge of inputs that have been strewn about, so we go back and re-do them. Is it because we were taught to read from right to left, so we think we have to program in the same direction? Wouldn't it be easier to put in all the outputs on the RIGHT side of blanks rungs, labeling and documenting each one, then go back and do the Stop Inputs, and then the Start Inputs for each rung? Is there something scary about blank rungs? Will someone please explain this to me? Surely I am not alone in remembering how it was to build relay panels. With relays, you had to put in your "outputs" (coils) first, before you could connect any inputs. This seems the natural way to do it to me, but I have never found anyone else that seems to think it matters.

Have never worked that way Lancie. The inputs appear to me to be what matters or there is no output.

You know what your output must be but the inputs/conditions are what affects the output value. The inputs are the easiest thing to foul up quite frankly.

I agree, the inputs are more difficult, there are usually about 5 times more of them, and it is less clear at the beginnig exaclty how the inputs need to be used with any specific output. So is it not like writing a theme paper: you need a plan, then work the plan? Why not start with the least difficult problem first (the outputs) and then as your thoughts become more organized, finish up with the difficult part, the inputs? Bob, would you agree that the outputs are where the "action" is, where the rubber meets the road?

Bob, would you agree that the outputs are where the "action" is, where the rubber meets the road?

Click to expand...

Yes, but I still start at the other end. I am perhaps strange in that I only make a few necessary notes on what is required. A specification is always supplied for my work. I read the spec, design the system, do the control and PLC I/O drawings, make some notes and start writing software. No nonsense basically. Usually build the panels also.

The design is always done in a way that simplifies what I have to do in software. If an extra input or two will make my life easier, they go in.

I absolutely hate working with a design done by someone else as they do not think this way. In fact, I quite often knock back work if I do not do the design. A lot less aggravation that way. Do not need it at my time of life. Get someone else basically.

What Lance is saying is: Outputs determine Inputs.
Know what you need to control (outputs), then determine what you need (inputs) to control it. You can't determine what your inputs are unless you know your outputs. This is a fundamental principle of program and design.

I guess I never thought about before, but suppose I generally use Lancie's method. I decide what I want to happen (outputs?) and then figure out the conditions that need to be satisfied before it does happen (inputs?).

Interesting!

Whew! Tom, I thought maybe it was just me, and that I was dsylexic, backward-thinking, or something. It seems to me that it makes the organization and planning a lot easier if you start on the Output side. Another thing I have noticed is that most rungs have, besides the Output, two other things in common. They usually have a way to energize (Start) the Output, and a way to de-energize (Stop) the Output. In many but not all cases the Start & Stop are the same Input.

I agree

It makes since to me, here's an analog:

REQUIRED: Start truck engine

--|IGN SWI|---|NEUT GEAR|---|Clutch depressed|---(MO1)

ETC.

Yep! you have to start with the requirements otherwise you will not know what inputs are needed. I like it.

Thanks,

Rod (The CNC dude)

Rod,
Good analogy. Now, can we convince the PLC manufacturers to move the Outputs to the left side of the rung? Wouldn't that be a hoot!

I guess I think the same way but dont get why outputs would need to be on left side of ladder. Relay logic was drawn with coils on right side. We are conditioned to read left to right, so makes sense as is to me.....the IF THEN thing.

I do alot with pencil and paper first. Obtain I/O count. Develop sequence of action....what happens when. Assign names addresses to I/O, assign names to interal bits, setup hardware, setup any HSC or analog.

Most of the time when I get done with the pencil and hardware setup the program is virtually done.

I also look at what needs to happen (Outputs) and then work to make it so deciding what I need to know to do it (Inputs).

This is also how I troubleshoot. What was supposed to happen. What makes it happen. Why didn't it do what it should have. I have seen many people look at everything they can find leading up to the action to diagnose a problem but I find it simpler to start ate the problem (a single point) and work back.

I dont know of any way to troubleshoot except START at the problem THEN goto the source....ie light isnt on goto switch to turn on light, computer isnt on then goto switch....still doesnt come on then see if plugged in.

Output doesnt come on you goto inputs to see what didnt "make" so it would.

The left side part I dont understand but the rest makes sense whether tshooting or designing..."Whats it do"...ie conveyor, motor turns belt. How to turn on motor? How to turn off motor? What to do in an emergency? What to do in case of power outage? What, What if, How, When....probably more but its late.

Only took 3 or 4 years but I think y'alls smart is rubbing off on me.

Lancie1 said:

Rod,
Good analogy. Now, can we convince the PLC manufacturers to move the Outputs to the left side of the rung? Wouldn't that be a hoot!

Click to expand...

Try programming a control logix...You can put the coils at the left side....Dont know how the logig is solved or what would happen but it will let you do it..Hmmm.. i might just try that and see what happens!