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Old July 11th, 2018, 10:18 PM   #1
kalabdel
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SLC500 CUP power flicker

Hello everyone,

I have a SLC 5/03 with two racks, the first with 10 I/Os and the second with 12. The CPU intermittently restarts looking like it lost power . It's very quick but enough to reset all the outputs. Sometimes it happens a few times in a row and then it could be another half hour or more with nothing.

Replaced the power supply and CPU with spares (used), problem persisted. Downloaded blank program with inputs active and outputs off, the problem disappeared or so it seemed.
Downloaded the original program and the problem reappeared.


Any ideas?
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Old July 12th, 2018, 01:39 AM   #2
GaryS
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The fact that it restarts is puzzling
I have seen this happen when the math overflow bit is set when the rolls over from the last line to first line. That will usually cause the processor to fault and just stop not restart.
I that case the answer is simple on the last line of program put an unconditional unlatch of the math overflow bit.
The problem is definitely in the program someplace maybe an indirect address overflow or pointing to an invalid address. The 500's can only access a file size of 250 elements 0 to 249
I know that will cause then to crash maybe when it crashes the index resets to 0 on reload.
I hope this helps let us know what you find.
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Old July 12th, 2018, 05:56 AM   #3
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It could be an intermittent short in the rack, either the backplane itself or one of the cards shorting out. I have had problems with analog cards causing shorts.

I would start swapping one IO card at a time, if you can swap it with a card on the second rack that would be best - if it is that card then the CPU will fault because the second rack shuts down, and then it won't restart. If this happens replace the card you put in the second rack.

If none of the cards are doing it I would then try the backplane.

Another thought - is it a P1 or P2 power supply that may be too small for the rack? Try a P4 as a test.
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Old July 12th, 2018, 06:03 AM   #4
Christoff84
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Is it a 24 volt power supply? Could there be an issue with the circuit, where the 24V drops low on energising some output and then clears when the slc loses power?
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Old July 12th, 2018, 07:05 AM   #5
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Could be a backplane issue. Have you changed the backplane?
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Old July 12th, 2018, 10:03 AM   #6
kalabdel
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Thanks guys. I am heading back today for a second look. The error has to do with I/O but I forgot to take a screenshot or it and will do that later.
The power supply is P2 120VAC to 24VDV and the 24 is feeding an analog sensor of some sort as I see the wire going somewhere and coming back to an analog input. I will trace it today. Does loading a blank program and not having issues point somewhere?

Edit: it’s running well today and I don’t want to shut them down again. I will recommend replacing the backplane and take it from there.

Thanks

Last edited by kalabdel; July 12th, 2018 at 10:06 AM.
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Old July 12th, 2018, 12:27 PM   #7
OkiePC
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Quote:
Originally Posted by kalabdel View Post
...
The error has to do with I/O but I forgot to take a screenshot or it and will do that later.
The power supply is P2 120VAC to 24VDV and the 24 is feeding an analog sensor of some sort as I see the wire going somewhere and coming back to an analog input. I will trace it today. Does loading a blank program and not having issues point somewhere?
Probably not, but I suppose if there is a card that only acts up when called to do something by the program it could be a factor. Backplane failures are pretty rare, but hard to figure out and they do happen. I think it is more likely that one of the cards has a problem. Trapping that exact fault code is valuable. If there is an HMI, I try to display the error details and log them to an alarm history.
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Old July 12th, 2018, 01:52 PM   #8
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Quote:
Originally Posted by kalabdel View Post
... the 24 is feeding an analog sensor of some sort as I see the wire going somewhere and coming back to an analog .
I do not ever use the 24 volts coming off the power supply to power anything - even a single proxy. One short or wiring problem on the line and the PLC shuts down. Exactly as you are having happen.

Once the 24 volts leaves the control panel it could have something tapped into it that someone saw 24 volts and didn't know where it came from.

Put in a small 24 power supply to power that/those sensors.
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Old July 12th, 2018, 03:13 PM   #9
Ken Roach
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That +24V auxiliary power supply terminal on the 1746-P2 is only rated to supply 200 mA. I've definitely seen a short circuit on that cause the PSU to "crowbar" and shut down the PLC.

Replacing that source with a standalone 24V DC power supply would quickly exclude that circuit as a suspect. Add a little 1/2A fuse for extra diagnostic evidence.
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Old July 12th, 2018, 03:44 PM   #10
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Hi, let me see if I can help you, your CPU seems to be restarting every few minutes, then you must try without I/O modules to discard that there any problem with them, if the problem persist then you must check the code because maybe you are making an overflow, I hope be helpful to you and keep me in touch,
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Old July 12th, 2018, 04:35 PM   #11
Aabeck
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Quote:
Originally Posted by kalabdel View Post
Does loading a blank program and not having issues point somewhere?
If the analog sensor is sensing something that would be off, or 0, when the right PLC output is off [say a pressure transducer that gives 0-20mA that is giving 0mA when the pump or solenoid valve is off] then when you energize the pump or valve the transducer starts putting out a signal and overloads the -P2. Same thing if it's a 0-10V signal.

Or the output lead from the transducer is shorted to ground somewhere in the run or inside the transducer and that would bring down the power supply when it tries to output a signal to a grounded lead.
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