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RsLogix Lesson - What happens to a OTE vs OTL/OTU during a power cycle?

This article is specifically about what happens during a power cycle with method A and D of our RsLogix 500 Lesson 5 - Start Stop Motor Programming in an Allen Bradley Micrologix PLC article.




Both of these instructions do the same thing with one exception.  People are shocked to learn when you cycle power and the motors were running that the motor in method A will not restart but the motor in method D will restart without any intervention when the power is turned back on.  Click here to see for yourself.

But no one clearly understands why.  I've heard all types of theories on why, the most bizarre being people concluding that a PLC is unstable or unpredictable during a power cycle.


The answer is actually written right below the description of the of the OTE instruction in the Reference Manual but you have to read it several times to understand what it is saying.  There is a note below the instruction that states that "OTE instructions are reset (turned OFF) when: You enter or return to the program or remote program mode or power is restored".  


Here is the documentation for the OTL/OTU.  While it has several warnings that need to be understood, there isn't one that says that either of these instructions will "reset" when power is restored.

The data files that store these outputs OFF/ON status are retentive which means that they remember what they were during a power cycle.  It is the instructions used that determine if anything changes states on a power cycle.  In this case, only the OTE writes to its corresponding data address on power up.  Also note that having an OTE "disabled" by an AFI or putting it in a subroutine that isn't executed in the program does not exclude it from the "reset" feature.  ALL OTEs will be reset.

Here is a link to the Micrologix 1100 Programmable Controllers Instruction Set Reference Manual that both of these excerpts came from. 

Prescan - Working a little backwards here, now we know what does it, but what is it called?  The answer according to the manual is nothing.  Yet I have always heard it called prescan, not to be confused with the first scan which S2:1/15 addresses.  I have never found reference to the term in any modern Allen Bradley PLC manual, but if you go back to the PLC5, there was a term called prescan that defined this power up event.  Why the term's use was abandoned is not known, but it left this undefined void of understanding of the very important feature of powering up a PLC.

Sometimes seeing is believing.  Here is a video that shows Motor A not restarting while Motor B restarting when power is resumed.