PLC 5 o/ps turning on and off rapidly

Hey guys,

We have experienced a few weird issues with some PLC 5 racks recently. On some cards, the outputs will turn on and off real rapidly, without being told to. If you look at the o/p lights on the front of the card (Regular 120VAC o/p card), you can see a faint glow on the o/ps that are "chattering".

On one of the previous cases, we changed out the backplane itself, and that seemed to resolve the issue. It happened another time where we had to change out several cards to correct the issue.

Now it has happened again. Changed the card in question, same result. I am fixing to get online and throw the processor into Program mode to see if that stops the behavior. But does anyone have any other ideas as to what could be causing it? It could be another backplane, but boy those don't fail very often at all. Now we have had 3 cases like this in the space of 2 weeks, in completely different parts of the plant.

Any ideas would be appreciated!

Changing the processor to Program Mode, made all the outputs turn off, including the "chattering" ones. Have also tried a new processor, and a new power supply. I guess the next step is going to be the back plane again!. Awfully strange how this has happened again.

We have approx 60 PLC 5 installations in the plant, and this is maybe the 4th time in 20+ years that we have had to change a back plane?

mutabi said:

We have approx 60 PLC 5 installations in the plant, and this is maybe the 4th time in 20+ years that we have had to change a back plane?

Click to expand...

I would recommend contacting Smoke and Mirrors Inc. for a complete investigation of this issue.

http://www.plctalk.net/qanda/printthread.php?t=20167&pp=15&page=8

Not quite sure what you are alluding to...but here are my guesses

1. They have been in service 20+ years with few failures...so failures are bound to happen.....true. Thats a long time. However it seems awfully coincidental that we have suddenly had 3 failures in the space of 2 weeks all with identical symptoms.

2. Weird sh#t happens...deal with it.

3. Quit asking dumba$$ questions.

4. All of the above.

mutabi said:

3. Quit asking dumba$$ questions.

Click to expand...

I would never tell anyone to do that.

My comment was about, if you have had only 4 failures in 20+ years count your blessing.

Whats the environment like, where these PLCs live?

I would never tell anyone to do that.

Click to expand...

Good on you!


The environment is climate controlled. By that I mean if its cold outside, its cold inside. If its hot outside its hot inside. Right now its about 100F inside the plant. Its dirty. Lots of vibration. I would call it a ....harsh environment!

We ended up changing the back plane and sure enough...the problem is fixed. Sigh.

Just a theory but, 20+ years of temperature cycling might have weakened some of the solder joints, which are everywhere, to the point they've loosened and become intermittent.

However it seems awfully coincidental that we have suddenly had 3 failures in the space of 2 weeks all with identical symptoms.

Click to expand...

No, that is not coincidental or unexpected. Because they are all about the same age, with similar operating conditions and running time, it is logical that the failures will occur at about the same time. Unfortunately, with 60 PLC5s, you should prepare to have more similar incidents. Perhaps you better order a bunch of spare backplanes!

what is the environment inside the control panel?

clean sealed enclosure, or dirty - dusty?

if its dirty, is the contaminents conductive?

are the electrical conduits going into the control panel located directly over the plc rack?

could be contaminents are getting onto the backplane and causing the problem.

we have had 5 failures in the last year due to contaminents getting into the electronics and causing shorts. mostly from piping directly over the plc and no seal offs are used (before my time). others are from poor cabinet sealing.

also, moisture has dripped onto the plc rack.

regards,
james

Is this the 4th time you have changed the SAME backplane or was it 4 different ones that failed?