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Old October 16th, 2020, 11:14 AM   #1
unsaint33
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SLC500 occasional CPU fault toggling

We bought a used SLC500 (1747-l40A) from a vendor. Comes with one year warranty. It's still on our test bench. I downloaded a program to it and verified every I/O terminal and comm with HMI. However, once in a while, its CPU fault LED comes on for a second. Then it goes away & PC RUN LED comes back on, and everything working again like nothing happened. I should return it, right?
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Old October 16th, 2020, 11:26 AM   #2
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Quote:
Originally Posted by unsaint33 View Post
We bought a used SLC500 (1747-l40A) from a vendor. Comes with one year warranty. It's still on our test bench. I downloaded a program to it and verified every I/O terminal and comm with HMI. However, once in a while, its CPU fault LED comes on for a second. Then it goes away & PC RUN LED comes back on, and everything working again like nothing happened. I should return it, right?
Yes, I think so. I don't know of any way a CPU fault can be cleared and the unit return to run mode by itself without using programming software or cycling the keyswitch. I don't remember if the fixed SLCs even have a key switch.

Maybe the PLC power is intermittent internally and it is configured to go to run mode on power up...
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Old October 16th, 2020, 11:27 AM   #3
parky
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I would, SLC's are renowned for faults after a number of years, generally rack or PSU faults, Rack is common for intermittent faults mainly because of either dry joints or bad connectors (I suspect heat over time or vibration on this one), aging components in the PSU are common.
One thing you can try is removing & replacing cards a few times to effectively "clean" the contacts, I have had success doing this & using contact cleaner, however, using cleaning solution would probably invalidate your warrantee.
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Old October 16th, 2020, 12:36 PM   #4
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I definitely would try to replace that shoebox of a PLC.

Get either a 5/03 or 5/04 at least, or 5/05 if you want Ethernet.

Better, but they still have trouble with racks or analog cards taking down the power supply, shorting out the battery circuit when off and wiping the porgram, or just bein PITA's.
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Old October 16th, 2020, 12:43 PM   #5
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That unit was a "brick" style so it usually didn't have I/O modules (although you could add on up to 2 with an expansion unit). There is no key on these. You must use the software to change the mode or clear a fault. I suspect that they did have a fault routine though which would allow the controller to reset a fault, but that should never allow the fault light to appear. Once that light comes on, the PLC shuts down and it cannot restart itself.

Either the fault light is just a glitch and the unit is never actually faulting, or you've got a bad controller. Either way, I would return/replace it.

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Old October 16th, 2020, 09:28 PM   #6
JaxGTO
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I thought if you have a Fault routine that you can programmatically reset some faults and the program will continue to run. Faults like the overflow that most people have a hot branched OTU of.
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Old October 17th, 2020, 08:11 AM   #7
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Yes you can, but the red fault light will not turn on if the program clears the fault. The fault light only turns on when the controller has shut down.

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Old October 17th, 2020, 09:26 AM   #8
geniusintraining
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Quote:
Originally Posted by unsaint33
I downloaded a program to it and verified every I/O terminal and comm with HMI.

I am with everyone else and send it back but... just out of curiosity, remove everything and the program then does it still do it?
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Old October 17th, 2020, 12:32 PM   #9
lee1968nh
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The SLCs will fault after awhile if there is no fault...it is just their thing. So long w/o a fault, it must be a fault! J/k of course.
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