Ladder Logic diagram help

coldplay2010

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Hello, I am a beginner with PLCs, so I am new to the ladder logic diagrams. I need help with the following:
1:When switch one (SW1) IS CLOSED, THE GREEN PILOT LIGHT TURNS ON
2:When switch two is closed, the white pilot light turns on.
3: When both switch one and switch two are closed, the Green and white pilot lights are off, and the red pilot light turns on.
I have completed simple ones, but this one confuses me. If you someone can show me the ladder logic diagram, it would greatly help me.
 
coldplay2010 said:
Hello, I am a beginner with PLCs, so I am new to the ladder logic diagrams. I need help with the following:
1:When switch one (SW1) IS CLOSED, THE GREEN PILOT LIGHT TURNS ON
2:When switch two is closed, the white pilot light turns on.
3: When both switch one and switch two are closed, the Green and white pilot lights are off, and the red pilot light turns on.
I have completed simple ones, but this one confuses me. If you someone can show me the ladder logic diagram, it would greatly help me.
Click to expand...

This sounds like a homework assignment.... Think about how you represent digital equivalent of 1, 2, and 3, using two digits. Zero would be 00, One would be... ?
 
Are you studying Truth Tables by any chance? Try to simplify the conditions and put it into pseudocode. From there you can write your ladder. For example:

If SW1 and not SW2 then Green
If SW2 and not SW1 then White
If SW1 and SW2 then Red
Click to expand...
Now you need to turn that into ladder.

TT.png
 
Jeev said:
Are you studying Truth Tables by any chance? Try to simplify the conditions and put it into pseudocode. From there you can write your ladder. For example:


Now you need to turn that into ladder.
Click to expand...

No, we currently just started doing these. The only thing I know how it works is the examine if closed (in the simulation program). So it is all trial and error. I will find out eventually. Also:
f SW1 and not SW2 then Green
If SW2 and not SW1 then White
If SW1 and SW2 then Red
This has simplified my reading. It is more clear. Thank you.
 
Once you have the answer, ask if there's anything you still don't understand. Answers are good. Knowing why is better (y)
 
coldplay2010 said:
1:When switch one (SW1) IS CLOSED, THE GREEN PILOT LIGHT TURNS ON
3: When both switch one and switch two are closed, the Green and white pilot lights are off, and the red pilot light turns on.
Click to expand...

Rule 3 contradicts Rule 1. However, the intent is easy to understand. Modify the rule for number 1 so it does not contradict rule 3, and the ladder becomes obvious.
 
proof said:
Rule 3 contradicts Rule 1. However, the intent is easy to understand. Modify the rule for number 1 so it does not contradict rule 3, and the ladder becomes obvious.
Click to expand...

Doesn't seem contradictory to me... what's implied in rule 1 is that SW2 is open. So the green light is on ONLY if SW1 is closed AND SW2 is open.

Rule 3 outlines the situation when BOTH SW1 AND SW2 are closed.

Why does this seem contradictory?
 
fyi,

while we are more than happy to help anyone with their homework, all we ask is that the individual put forth some effort first and show what they have done.

it makes you think about the problem and we can point you in the right direction / offer suggestions. you learn as you go and you will also be a better programmer in the real world.

james
 
James Mcquade said:
fyi,

while we are more than happy to help anyone with their homework, all we ask is that the individual put forth some effort first and show what they have done.

it makes you think about the problem and we can point you in the right direction / offer suggestions. you learn as you go and you will also be a better programmer in the real world.

james
Click to expand...

This. Mostly because we don't want to have to keep supplying you answers when you come and land a job at our factories :ROFLMAO: Offering guidance instead of answers is a good compromise. It shows us that you're willing to put in the work along with us.
 
Why does this seem contradictory?
Click to expand...
Because it forces you to interpret the true meaning of rule 1.
I could interpret rule 1 as, "No matter what else happens, when SW1 is closed, turn on the green pilot light". Or I could interpret it as "When SW1 is closed turn on the green pilot light except when SW2 is also closed". Given that rule 3 covers the case when both SW1 and SW2 are closed simultaneously and it contradicts the first interpretation of rule 1, it is probable that the second interpretation of rule 1 is the correct one.
So the OP needs to ask if this is an exercise in writing ladder logic or an exercise in interpreting ambiguous job specs. Both of those are important skills for a controls engineer to have.
 
Steve Bailey said:
Because it forces you to interpret the true meaning of rule 1.
I could interpret rule 1 as, "No matter what else happens, when SW1 is closed, turn on the green pilot light". Or I could interpret it as "When SW1 is closed turn on the green pilot light except when SW2 is also closed". Given that rule 3 covers the case when both SW1 and SW2 are closed simultaneously and it contradicts the first interpretation of rule 1, it is probable that the second interpretation of rule 1 is the correct one.
So the OP needs to ask if this is an exercise in writing ladder logic or an exercise in interpreting ambiguous job specs. Both of those are important skills for a controls engineer to have.
Click to expand...

I'll give you ambiguous, especially for a student just starting out. Usually, in programming languages (in my experience), NOTs always take precedence over ISs, so it seemed straight forward to me.

Edit: I'm not attempting to communicate that NOTs usually/always take precedence over ISs, just explaining my earlier comment. I don't know more than a couple programming languages as of now, so I'm admittedly coming from a place of narrow experience.
 
Last edited:
Steve Bailey said:
Because it forces you to interpret the true meaning of rule 1.
I could interpret rule 1 as, "No matter what else happens, when SW1 is closed, turn on the green pilot light". Or I could interpret it as "When SW1 is closed turn on the green pilot light except when SW2 is also closed". Given that rule 3 covers the case when both SW1 and SW2 are closed simultaneously and it contradicts the first interpretation of rule 1, it is probable that the second interpretation of rule 1 is the correct one.
So the OP needs to ask if this is an exercise in writing ladder logic or an exercise in interpreting ambiguous job specs. Both of those are important skills for a controls engineer to have.
Click to expand...

Ambiguous specs... Is there another kind?
 

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