Micrologix 1500 -- Strange Behaviour

[antsrealm]

Hi guys this is my first post on this forum but I have been a long time browser.

I have this issue with a micrologix 1500, it's a strange one. The other day I'm looking at the code, I see a bit is on however it's unlatch condition had just been driven but it didn't unlatch. The latch condition was not being driven yet it wouldn't unlatch. The ladder was being called and I couldn't see any reason why it wouldn't turn the bit off. I started to think there was a problem with the PLC.

I re-downloaded the whole program and it was fine again. Now a week later the PLC has just wiped itself clean no faults and no program. Re-download the program and it's fine again. Thinking about replacing the PLC. Has anyone ever seen anything like this ???


** Edit :- Also no battery light and apparently power was not cycled to the PLC either.


Thanks,
Tony.

 

[Mickey]

I can't address the latched bit issue without seeing the code when the problem occurs.

As far as the lose of program issue, check and double check grounding for the controller.
Also make sure all inductive loads have surge protection.

 

[antsrealm]

Thanks Mickey.

Yeah I reprogrammed that part of the ladder just to be sure but it just seemed off. I saw on a separate occasion a similar symptom in another section of the program not unlatching bits that were high. However that's just suspicious but not a deal breaker at this stage.

As far as the power goes it is a pretty crude setup, no power filter. Im sure there is a few motor contactors around 4kw that dont have supression on the coils and A small VSD in close proximity. So plenty of suspects, it's run for years with minimal trouble just lately it's been playing up.

 

[antsrealm]

The guys in the factory tell me that this has happened two weeks in a row now both on a Sunday morning after the plant has been idle for 2 days. The machine is never turned off just left idle and when they come back the program is gone with no faults.

 

[Anirban Hazra]

I have this issue with a micrologix 1500, it's a strange one. The other day I'm looking at the code, I see a bit is on however it's unlatch condition had just been driven but it didn't unlatch. The latch condition was not being driven yet it wouldn't unlatch. The ladder was being called and I couldn't see any reason why it wouldn't turn the bit off. I started to think there was a problem with the PLC.

You should post your program here for better assistance on that front, but taking a shot in the dark, it seems that there is more than one rung latching the same output.In other words, though the rung you are checking is not latching that bit there might be another or more rungs doing so.Do a cross-reference for that bit to ascertain if that is so.