PLC - Ladder Logic Help

Thread Starter


Joined Mar 28, 2021

I am struggling and been working on this for a while, its like one of those classes where now matter how much effort you put in, there is still no output.

I'm working on PLCs and ladder logic, and I'm trying to figure this out and learn it.

The Scenario:
A PLC is used to control a flying knife application. The PLC must provide simple on/off control to the applications motion controller, track the total number of parts produced, the number of bad parts produced, and use math instructions to determine the number of good parts produced.

In this application, PVC tubing is continuously fed into the rotary knife machine where it is cut into 10 foot lengths. The parts are then transported to a quality technician where they are examined and then measure. If specifications are met, the operator places the part back onto the conveyor line. If a part does not meet specifications, the operator places the part into a scrap bin and presses a pushbutton, indication that the part is bad.

General Sequence:
- Pressing PB1 momentarily causes the process to begin operation, causing the reversing contactor motor starter to move the motor in the forward direction. The machine begins to continuously feed and cut PVC couplings.
- Inductive proximity sensor IP1 senses the knife as it passes through the PVC, and can be used to determine the total number of parts produced. This value should be stored in MW2
- Pressing PB3 momentarily causes the bad parts count to increment, or increase by 1. The bad parts count should be stored in MW4.
- The PLC keeps track of the number of good parts produced by subtracting the reject parts from the total number produced and storing this number in word MW6.
- Pressing PB2 momentarily causes the application to stop, but not reset.
- Turning SS1 on causes the application to reset (total parts, bad parts and good parts)

PB1 - START - Address I0.0
PB2 - STOP - Address I0.1
PB3 - BAD PART COUNT - Address I0.2
SS1 - RESET - Address I0.4
IP1 - NEW PART - Address I1.0


If anything else is needed. please message me. Don't pay attention to the addressing too much, I think that doesn't matter with setting up the logic atleast. Appreciate it!


Joined Oct 5, 2017
If you've been working on this for a while, where have you got to? If you show what you've put together, there'll be something to critique.


Joined Jul 18, 2013
What PLC device is the problem based on?
Any particular make?

I realize it is an imaginary scenario, but one that most likely would not exist in real life, the motion controller for the flying knife would generally handle the conditions listed for a PLC.! ;)


Joined Jun 7, 2009
you have two basic functions;
a start/stop function, and a count function.
Show us what you have come up with for these
You’ve stated that we are to message you for information. Be advised that you are seeking support on a community forum, and should contribute accordingly.
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