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From where the PLC begins scanning from on power on / off


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Hi Everyone

I am a bit of a newb and using a US57/ -B10-TR22 PLC with the current version of unilogic

The short version of my question is:

 when there is a power off/ on when the PLC reboots does it start the scan from the top of the ladder or does it continue from where it was when the power went off?

Is there a way to make it default back to the top of the ladder?


My Long explanation / issue is

I have a bit that activates a compressor - even though the bit is supposed to default to 0 on startup and i have a initial cycle set bit to 0 condition if i turn the power off when the compressor is running for about one second as the PLC is intialising the compressor will come on for a second 

I can only assume this is the time it takes the PLC to scan from where it was on power off to the condition that resets the compressor bit...

It's for an aerospace application 

Any help greatly appreciated 

Kind Regards

Tyler White

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  • MVP 2023

The PLC starts scanning at the top of the program on power cycle.

Which implies that all the outputs should be off initially based on your logic.  However, one of my T-shirt sayings is "Empirical Data is the Best Data", and I believe your observation.

It sounds like the output memory area is not getting cleared on a power cycle.  I would send an email to with the contents  of your post to support@unitronics.com.  Please let us know what they get back to you with.  We're mostly volunteers here.

Joe T.

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  • MVP 2023

I recall someone like Kikis or Orso doing a topic about how they observed all the outputs turning on for a moment on boot, and they correctly saw it as the large problem it was.  I couldn't find it with a quick search...maybe someone else or the original poster will chime in?  I think it might have been within the last year, but time does fly.

Perhaps the issue they observed is related to Tyler's one?

That said, Tyler, given scan times and you saying that the issue is a "second" long, the On/Off time involved for 2 scans doesn't get anywhere near 1 second.  It sounds like you have some other control working on the output, that you are missing.

cheers, Aus

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