1756-L72 Control Logix loss and return of power?

cockrell960

Member
Join Date
Aug 2014
Location
Texas
Posts
12
I have a system that uses remote IO with redundant L72 processors communicating with a fiber loop. My question is if i loose power to the processors on return of power will the logic remain in the same state as when it lost power. I know the ECM stores the data of the program but not sure if it saves the states of tags/bits. "for example a tag that is latched will it remain latched when power is returned." I do not have an on power up program or an on fault program so from what i can tell when power returns to the processor the program will be in a fault mode and i will have to manually clear the faults but after that will all the bits be latched that were latched when the processor failed/faulted/lost power or should i make a first scan/power up logic that will latch all that need to be latched. I should probably make the first scan/ power logic anyway but as of right now i don't have one.
I appreciate it.
Thank You
 
Latched bools, and all data types will remain as they were.

Any rung that has an OTE however is scanned once as if the rung were false, so seal in logic will unseal.
 
The way it actually operates:
All memory in a Rockwell PLC is retentive.
The prescan operation will change the state of some instruction operands on power up or a change from program to run mode. The most notable of these is the operand of all OTE instructions is set to zero. This gives the appearance that this data is "non-retentive".

Also, there is no reason to expect your processor to fault from a power cycle.
 
Last edited:
A good UPS might be nice.
Be sure you don't use one for white box computers, however.
You want a UPS that runs off the pure sine wave inverter and fails back to the line.
The white box UPS will work, but if the batteries are not right it will not come on after power fail and you don't want this in control.
Ask how I know this.........
 

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