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

[ebalarde]

I was wondering if anybody was reading.

I taught a class once, and, when covering the elements of the system, I mentioned that there are INPUT modules, and OUTPUT modules; digital and analog. I also said (then..not now) that all systems have some combination of INPUT and OUTPUT modules.
Safe, right?
Not this class.
S: "Jeff, we have a system at work that only has input modules."
Me: "That's hard to believe. It's like a lawn mower that won't cut your foot off....it doesn't cut grass either. What do you use it for?"
S: "Troubleshooting."

[It turns out that it was connected to a relay-controlled machine, and all that it did was to use internal memory bits to track the status of various inputs from relay contacts and monitoring of relay coil energization status. It was on a machine that the OEM refused to use programmable controllers on....an amusement park ride; in this case, a roller coaster.]

So, as I teach, just because _you_ haven't seen one, doesn't mean that they're not out there, somewhere.

But they were still dealing with OUTPUTS, though not physical ones. The troubleshooting data is the OUTPUT.
They needed to figure out what data they needed (OUTPUTS) before deciding which signals to bring to the INPUT modules.

 

[Mickey]

Oh, but the "readings" and "status" are outputs also.

 

[ebalarde]

To Lancie's original question though, I have to say that from my experience, I've often had to look at a list of inputs to give me a hint as to how the outputs were expected to be controlled. It happens more on spec'd projects where the control sequence is incomplete. You're given a list of inputs and outputs, and an incomplete set of control sequences and you somehow have to figure out what to make this thing do.

For example, I'm working on a steam plant and there are two normally connected feedwater headers each with their own pressure transmitter. I know I'm supposed to control the pump speed to maintain the desired pressure in the feedwater header(s), but the spec's didn't mention a thing about how to deal with the two pressure transmitters. From experience, I'm assuming that I need to program a selector switch which allows the operator to choose which transmitter acts as the process value for both headers.

Using the switch and bulb example mentioned before. Knowing there's a switch (input) helps you deduce that the bulb (output) is meant to be controlled by that switch as opposed to act as a "PLC OK" indicator. I understand that you need to know the expected output function before you know how to control it using whatever input is available, but I wonder if it's often taught or explained by starting with the inputs for the very reasons I'm mentioning above.

 

[Lancie1]

What if those inputs (or switches) are simply spares, and you really need a dedicated pressure transmitter for each header? It seems sort of dangerous to base your program on the number of provided inputs.

 

[ebalarde]

I'm with you on that.

I'm not saying it's a good practice to start with the inputs (I guess there might be exceptions to this out there??). My example was meant to provide a possible reason as to why it's being taught / explained that way.

Ideally, a control system would be engineered and the control sequence written in such a way that all a programmer needs to do is translate English into "PLC Code".... I've yet to see that.

The only option I have left is to use all the information I have, including the list of inputs and output, combined with my experience to fill in the holes in the control sequence. Mind you, I'm usually working on relatively simple processes where I can make those assumptions without introducing any additional risk. Not everybody has that luxury...

But, to your point, I still need to look at the output, first figure out what it is supposed to do, then use the inputs and internal logic to accomplish that.

 

[Demendia]

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.

Exactly.

If you don't know what your output want to do then you can't do anything at all.

 

[jaywillnot]

My original question was about why students focus on Inputs instead of Outputs when they are trying to develop logic to solve a problem.

Think of it this way. The internal I/O bits mentioned in post #57 were and are used to monitor inputs and outputs for safety (I'm guessing). Think of the internal I/O as your 15 year old son (internal) sitting on the couch monitoring you(physical output) while you're cleaning up after him.

I realize this really has nothing to do with the conversation but I just thought it was a funny analogy

 

[Lancie1]

Think of the internal I/O as your 15 year old son (internal) sitting on the couch monitoring you (physical output) while you're cleaning up after him.

That could explain a focus on outputs. Apparently between 15 and 18 when he enrolls in tech college, his focus changes to inputs.

 

[jaywillnot]

Good one Lancie!

 

[shazad5444]

Just simple, When you hear then you speak, So hearing is input and speak is output.

 

[dploof23]

That could explain a focus on outputs. Apparently between 15 and 18 when he enrolls in tech college, his focus changes to inputs.

Too funny!:nodi:

 

[Vartile]

So we didn't ever get the definitive answer to this dilemma.

 

[RussB]

So we didn't ever get the definitive answer to this dilemma.

Sure we did, close to fifty reasons as I see the thread.

 

[Vartile]

I need to point out that output method is correct in the sense that if you start with the middle or at the input you need to give a thought at the output. Iterative process.

 

[RussB]

I need to point out that output method is correct in the sense that if you start with the middle or at the input you need to give a thought at the output. Iterative process.

I don't see how this answers the question of the OP?