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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
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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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Old October 22nd, 2020, 12:53 PM   #10
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thank you for replies. I am going to send it back. But just out of curiosity, I want to count how many times it faults in a day or so. So, I wrote one rung program with S:1/15 first pass XIC and CTU OTE. I downloaded it to the PLC and power cycled few times. I figured that was simulating the CPU faulting since whenever I power up the CPU fault LED comes on first for a second for so before it goes off. I went on online and the counter acc was still 0. Can you tell me a program that can count the CPU faults? Thanks.
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Old October 22nd, 2020, 01:53 PM   #11
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It might be that during the first scan counters and timers are automatically reset, so if you are using First Scan to count a counter it can't.

At the end of ladder 2 put XIC S:1/15 (FirstPass) OTE B3:0/15 (SecondPass) (or any not-used bit)

Then at the start of ladder 2 XIC SecondPass and CTU after the first scan is done.

EDIT: It might also be that the CPU is not actually faulting and stops scanning, so there won't be a FirstPass.
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Old October 22nd, 2020, 04:57 PM   #12
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There is such a thing as a Fault Routine, the PLC will run the routine for one scan after the fault happens, I think you would need to use this and store it in a non volatile memory... I remember this from a long time ago but never researched how to program it

Quote:
When designating a subroutine file, the occurrence of recoverable or
non-recoverable user faults causes the designated subroutine to be executed
for one scan.
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Old October 22nd, 2020, 08:34 PM   #13
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Quote:
Originally Posted by geniusintraining View Post
There is such a thing as a Fault Routine, the PLC will run the routine for one scan after the fault happens, I think you would need to use this and store it in a non volatile memory... I remember this from a long time ago but never researched how to program it
Has anyone actually used the 'fault' routine? I admit I never have and am quite skeptical if it would even work.
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Old October 22nd, 2020, 08:41 PM   #14
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One thing that cranked me about AB is CLX would and may still will fault if timer ACC is negative? Why oh why the hell why? All that means it the timer has expired...maybe even long ago. Always felt like a big-brother, we know better kind of thing. I disdain that feature. Dammit if the ACC is less than Present value, it is what I wanted it to be. Shutting down a process because of that(fault) is a software engineer's(AB) heavy-handed misconception. Yea, AB has bitten me in the but with "major faults" several times. I do not respect the "major fault" so-called feature. Really really don't.
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Old October 22nd, 2020, 08:49 PM   #15
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Let me add a rant about the processor faulting if a pointer goes above an array dimension.



just flag a minor error that it can't do that and go on running the machine for the customer so they can make some money off their capital investment that could be more than their house or Lamborghini cost.
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