Why, oh why, Do Beginners Think PLC Inputs Are More Important Than PLC Outputs?

Someone please tell me so that I can die with a satisfied mind.

Why, oh why, Do Beginners Think PLC Inputs Are More Important Than PLC Outputs?

I have asked this to students and others here a hundred times (most recently in this thread, post #27
http://www.plctalk.net/qanda/showthread.php?t=77217&page=3

I have never got any good answers. Is someone out there teaching this concept? Why do they think that? Where did the idea start? What keeps it going? Is it because that is the first thing they see, or read, or are told about?

I can't figure it out. Please help me.

Here's my $0.02.

At some point in our life, we are taught by another human to perform a task. Think of learning how to use a toaster.
The responsible adult is going to say
"First, take the bread out of the package"
then
"Put one slice a piece in each of these two slots"
(this is the "input")

"Pull the lever down"
(also an "input")

"Wait a minute"
and finally,
"The bread will pop out as toast"

Now, think of trying to explain the same process to a child starting with "The toast comes out up here"

Yes, but if the goal is to program a machine (similar to say, solving a word puzzle), you need to start with the answer (similar to the PLC outputs) and work back to what you have to get there, the given clues or letters - (the PLC inputs).

In solving a puzzle, if you start the backward way, there are thousands or millions of dead ends that will not work (****, you really don't even know where you are trying to get because you started without looking at the solution). That is exactly what happens to the students who show up here, confused and frustrated. They don't know where they are going, so are unlikely to KNOW if or when they get there.

Heck, it is no wonder that this problem keeps coming up again and again. Nobody seems to know the method of solving puzzles anymore.

Now, think of trying to explain the same process to a child starting with "The toast comes out up here"

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Hmm... if I said to some children that I know, "the toast will come out here. What could you do to make that happen?", then it would not be long before they figured out what that big hole is for. The rest is details.

Because with no inputs PLC would not work ???

Bit too simplistic I think??

Dan Bentler

Because with no inputs PLC would not work ???

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For which one would a PLC work the best if left off, inputs or outputs?

If you had no legs, you could go through all the training, eat the right foods, get plenty of sleep, follow your trainers orders, in short, complete all the known "inputs" to win a foot race, but would it be likely that you would win?

Bit too simplistic I think??

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It is a simple question, and I know I have asked it of some complaining beginners about 100 times here on this site, and never have I received an answer that explained anything.

Outputs are evaluated according to expectations.

"I put the quarters in the slot, press the button for the product selection of my choice, and the product (a Coke) drops out the bottom. If it doesn't, I'm gonna start kicking something!"

Input are in MY control. No quarters? No expectations of a coke dropping into my hand.

Simple that way.....

Your mind may work differently, I hope so. ;-)

Cal

Lancie1 said:

Someone please tell me so that I can die with a satisfied mind.

Why, oh why, Do Beginners Think PLC Inputs Are More Important Than PLC Outputs?

I have asked this to students and others here a hundred times (most recently in this thread, post #27
http://www.plctalk.net/qanda/showthread.php?t=77217&page=3

I have never got any good answers. Is someone out there teaching this concept? Why do they think that? Where did the idea start? What keeps it going? Is it because that is the first thing they see, or read, or are told about?

I can't figure it out. Please help me.

Click to expand...

Because they have been taught things like "garbage in, garbage out" which give the perception that "input" controls "output". Plus, those in "control" are more important than those who do the work, or "output". The logical conclusion is that "inputs" are more important than "outputs" take heart in that they are thinking "logical".

Because they have been taught things like "garbage in, garbage out" which give the perception that "input" controls "output". Plus, those in "control" are more important than those who do the work, or "output".

Click to expand...

Yeah, they have been taught backwards. The focus is all on the available parameters, not on the tool to get the job done.

Turn on a light? Student response: "Well I have some switch inputs, a timer, a relay, blah, blah, blah". WRONG! What you have to turn on a light is an Output wired to a light bulb. Now go from there.

Only picture...

Temporary flags are most important!

Vartile, that is a very fine example that shows the point. It is much easier to start on the Output side and work back to the correct Input, than to do it the other way. Exactly the way I think it should be in solving a PLC programming problem. I have not found a beginner yet that does it correctly. Look at those dead ends! Look at all that confusion, as the dog chases it's tail around and around! Think of the hundreds of new questions that will be pop up here!

Vartile, You will go far, my friend!

Has anyone figured out the correct input yet?

Perhaps the difference lies in perception of what is important.

If I can control the inputs to "light the light"

Why then can I not use those same inputs to control the world?

After all, I have a system which controls an output with inputs that are in my control.

A light, an engine, an idea, a philosophy, a people.

Wraping ones thinking around a single result is limiting possibility.

Perspective...

Now I have another.. I think that this is something that every student should do..

If you have biggish program let say 60 outputs and 150 inputs and you are asked to solve the logic between I/O what is the way you should start to look at it, when there is no comments and/or documentation and you cant test the functionality..let say $/min. is too expensive to test.
You know physical devices (N.C. button1, N.O. motor relay1 etc.) in I/O and their addresses.

I don't know the right solution..

If you have biggish program let say 60 outputs and 150 inputs and you are asked to solve the logic between I/O what is the way you should start to look at it?...You know physical devices in I/O and their addresses.

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1. Start with the first Output. What should it do, what action should it perform? You NEED to know this part!
2. Which one or more of the 150 Inputs do I need to make Output 1 go ON? Add those to the logic.
3. Do I need any other functions, such as timers, couters, whatever, to make Output 1 work. If so, add those.
4. Go to Output 2 and repeat steps 1, 2, and 3 until finished.

this seems like a chicken vs the egg argument.

Lancie1 said:

1. Start with the first Output. What should it do, what action should it perform? You NEED to know this part!
2. Which one or more of the 150 Inputs do I need to make Output 1 go ON? Add those to the logic.
3. Do I need any other functions, such as timers, couters, whatever, to make Output 1 work. If so, add those.
4. Go to Output 2 and repeat steps 1, 2, and 3 until finished.

Click to expand...

I clarify a bit.. You don't know what it should do and you have this biggish and fairly complex program already made (let say in apple juice factory). Your task is to write the functionality inside plc program to paper in words.

Anyway part 1, 3 and 4 (excluding 2) still apply I think.

Yes egg vs chicken, but It's still good depate..
You need to have first the chicken to know what kind of an egg you need.