PLC backboard error?

[theripley]

Hi,

This is in relation to my previous post (title: PLC trouble?).
I was thinking maybe something is wrong with the cpu or the modular-type PLC backboard because when limit switch A is activated, on the program it is limit switch B that reacted. I have confirmed that program has no error, as sequence is running correctly. Trouble arises due to the "unexplainable" limit switch issue.

But this problem does not happen always, but it happens on the same location persistently. Limit switch has been confirmed good as per manual testing.

What do gurus think about this?

 

[JeffKiper]

What type of PLC are you using? Can you post your program?

 

[Gil47]

In my post in your previous posting I suggested that you had a miss wire ,what you are describing in this post still has me thinking that way.

It now appears to me that the live for switch B is fed from the wire that becomes live only after switch A has operated.

Are you able to answer these questions for us.
When switch A is operated does Input light A come on ?.
When switch B is operated does switch B input light come on ?.
When switch A and B switch's are both operated together do both input A and Input B Lights come on together ?.

The PLC type she stated is a FUJI

 

[theripley]

In my post in your previous posting I suggested that you had a miss wire ,what you are describing in this post still has me thinking that way.

If Im having a miss wire, why the trouble does not appear always?

Are you able to answer these questions for us.
When switch A is operated does Input light A come on ?.

Yes, I confirmed this by manual checking.

When switch B is operated does switch B input light come on ?.

Yes, I confirmed this by manual checking.

When switch A and B switch's are both operated together do both input A and Input B Lights come on together ?.

Yes sir.

 

[theripley]

Just last week, another carrier has the same problem as carrier 2..It seems that each time, problem worsens.

 

[Joel Mancilla]

I had same problem with our machine and even worst sometimes wrong output activates, I just changed the CPU card problems are solved.

 

[Gil47]

Hi,


when limit switch A is activated, on the program it is limit switch B that reacted. I have confirmed that program has no error, as sequence is running correctly. Trouble arises due to the "unexplainable" limit switch issue.

But this problem does not happen always, but it happens on the same location persistently. Limit switch has been confirmed good as per manual testing.

You now have me wondering about my suggestion so could you elaborate on the section in quotes from your first post.

Another suggestion check that the common on the PLC for those 2 inputs is always present maybe by taking a direct lead to the power supply

I still have that feeling your problem is with a wiring issue in your panel or in the field.

 

[theripley]

I had same problem with our machine and even worst sometimes wrong output activates, I just changed the CPU card problems are solved.

But as per client advise, they had changed the cpu. They changed all the modules except for the backboard. But I am thinking, if problem is backboard shouldn't the problem arise always & not sometimes?

 

[OkiePC]

I have never seen a PLC input card, output card, backplane, or CPU cause the type of problem you are seeing.

Just keep throwing parts at it, and eventually someone will accidentally fix the stray strand of wire that is connected the two inputs together.

 

[theripley]

I have never seen a PLC input card, output card, backplane, or CPU cause the type of problem you are seeing.

Just keep throwing parts at it, and eventually someone will accidentally fix the stray strand of wire that is connected the two inputs together.

If miswiring, shouldn't the problem be occurring always?