PLC Input Not Responding

[madchad]

Hello,
I am new at this so please excuse the question if it is something simple.

I have a PLC Input that is not responding. Let me explain. The machine has a basic limit switch with a mechanical contact. It is attached to a swinging door that is pressed down with the weight of the product which is in boxes. (The product doesn't matter here but I thought I would mention how simple the situation is.)

The limit switch is closed. I have tested it with an ohmmeter. I have also verified with a voltmeter that there is 24V at the input of the PLC, but the PLC input is not responding.

The input card on this PLC also has LED indicators. The indicator lights when we close the switch and goes out when we open the switch so I am convinced the circuit is working properly.

Is it possible that the PLC input has an electronic failure. I have heard that a PLC input not responding to voltage is rare.  Thanks for any help with this matter.

MadChad

[Cheller]

I have a PLC Input that is not responding.

If you have voltage at the input and the input LED is working, then it sounds like there is an internal failure in the input card, or input side of the PLC. If it is a card, you can always replace the input card. If it is a smaller PLC without removable cards, you can probably move the limit switch wire to another input and then modify the program accordingly.

[madchad]

I decided to try changing the wire to another input that wasn't being used. I was able to change the address in the ladder to reflect the wiring move on the inputs. It worked. I should have just tried this on my own but I wasn't positive that I was on the right track.

Next questions:

What causes this?

Can I just leave this alone now or do you think that more of the inputs will start to go bad?

Also, are there internal fuses or anything I can repair, or should I replace the card?

Thank You!

[tomtester]

What causes this?

Can I just leave this alone now or do you think that more of the inputs will start to go bad?

Also, are there internal fuses or anything I can repair, or should I replace the card?

You can leave the card where it is, just because one input went bad doesn't mean the entire PLC is going to fail. Depending on your specific case, you may want to order a replacement and set it on the shelf, or just let it go for now.

As for the cause, inputs can burn out due to voltage spikes or maybe an improper voltage being applied to it. I don't think there are any PLC manufacturers that advertise fused PLC inputs.