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Old November 14th, 2017, 10:32 AM   #1

JohnCalderwood is offline
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1756-L55 Clearing Memory

Some information for the folks on here.

We had a 1756-L55 clear its memory whilst running. The guys managed to power it down and reload an old backup to get it going.

The PLC was checked out by Rockwell - here is their response :

The 1756-L55 debug.out diagnostic file has been analyzed, here is the report:
The Logix controller in your system experienced a Major Non-Recoverable Fault (MNRF). This specific MNRF was caused by a soft memory error. After careful review of the data provided to Rockwell Automation, it has been determined that this occurrence of a soft memory error was not caused by a firmware or hardware anomaly. The controller is working as expected. Furthermore, we have not seen soft memory errors caused by power, grounding, or electrical noise.

Logix controllers run a continuous series of background diagnostic tests including monitoring for soft memory errors. Diagnostic tests are performed to ensure the safe operation of the controller. If a soft memory error is encountered, a Major Non-Recoverable Fault (MNRF) will be declared by the controller. A MNRF is a controlled action taken by the controller when it is determined that the controller could no longer continue safe operation. When a Logix controller determines that a MNRF is the right course of action, the controller is designed to fault, taking it out of run mode, logging diagnostic data, and then invalidating and deleting the controllerís memory. This action requires an application program reload to guarantee the controller has a valid program to continue safe operation.
Never seen this one before, but we have a few ageing L55 PLCs, already changed one previously due to memory leak from InTouch 2014R2.
One to watch with the older processors I think....
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Old November 14th, 2017, 01:17 PM   #2
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Well, that response said nothing that I can comprehend. Basically I took that as "**** happens".

We had a similar fault on a electronic trip relay (basically a controller) last year. It came back from the manufacturer as a "one-time abnormality"

I guess the good thing in all that the controller took itself out of commission before any bad stuff happens.
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Old November 14th, 2017, 07:34 PM   #3
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That's a very typical response you will get from RA.

Something went wrong....
The diagnostics did not tell us what....
The diagnostics did not tell us why, or where....
But the controller is working normally... That's what it does when it detects an error we can't tell you about, because we don't know, but rest assured, it is perfectly healthy !!
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Old November 15th, 2017, 03:22 AM   #4

JohnCalderwood is offline
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Aye, well....we could not take the chance.....the fault happened at a weekend.
We ran with the dodgy processor for less than a week, took the chance at the next shut and replaced the processor...
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Old November 15th, 2017, 07:03 AM   #5
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Maybe a high energy cosmic ray just happened to zap a particular bit in memory.

I've had that happen though as well, but I don't bother wasting time with Rockwell on it. Typically, I've always seen it with a power off, or sag, with the battery backup not holding. Especially on L55's, the backup there was measured in minutes if lucky, unless the external BATM module was used.
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Old November 15th, 2017, 10:10 AM   #6
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I've also had this in the past with a L55 on one machine on multiple occasions. It could run months without issue. Firmware & battery change made no difference. Replaced the processor in the end and the issue went away.
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Old November 19th, 2017, 08:37 AM   #7
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We had two machines with L55s that would randomly do the same. New processors, replaced batteries, firmware updates, etc...always the same thing.
We finally upgraded the processors to L61s for firmware support reasons (when we upgraded our PCs to Win7 64-bit) and we haven't had the problem since.
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