Logic Race

Hi Everyone, I am new here. I have been working with PLC's for a few years now so I still have much to learn, but I have an issue that has been confusing me and wracking my mind for a few weeks now. All I know is that the issue is probably related to the scan sequence but no matter how I look at it, it has not made sense. I have even set up a test bench to try to analyze it further but with no luck. I have attached the section of logic that I am referring too.

Some things to note:
Working with a SLC 504 Processor communicating with an HMI through DH+

N16:60/15 is triggered by a momentary pushbutton on an HMI

B3:2/0, B3:2/2, B3:6/3 and B3:5/0 start condition is 0 before triggering N16:60/15

N10:68/0 and T4:23/DN start condition is 1 before triggering N16:60/15

So where the confusion sets in... when the logic is as seen in the attachement I can latch B3:2/0 but cannot unlatch it. However when as shown in red you move up rung 144 two lines you can latch and unlatch without a problem.

I have gone through this the way I understand scanning (as described by the Ron Beaufort Method) but I do not get the same results I do in real life.

What am I missing? Thanks in advance for the replies!

Greetings Jorge ...

any chance that you could attach the original RSS project file? ... the contrast in the PDF that you attached is too weak for my 72 years old eyes to read ...

(but maybe some of the younger members can make it out) ...

take a good look at the MVM instruction in Rung 144 of Ladder File 2 ... notice that its Destination is aimed at word B3:2 - which of course includes your "problem" bit B3:2/0 ...

so maybe when you move the MVM rung up ABOVE the OTL and the OTU rungs, then the Latch and Unlatch functions don't get overwritten by the MVM instruction ...

I'm working on some other projects right now - so I don't have time to go all the way through your problem - but SO FAR it looks like the MVM instruction could be overwriting the OTU (Unlatch) instruction ...

hope this helps ... I'll spend some more time on it later if necessary ...

Ron Beaufort said:

take a good look at the MVM instruction in Rung 144 of Ladder File 2 ... notice that its Destination is aimed at word B3:2 - which of course includes your "problem" bit B3:2/0 ...

so maybe when you move the MVM rung up ABOVE the OTL and the OTU rungs, then the Latch and Unlatch functions don't get overwritten by the MVM instruction ...

I'm working on some other projects right now - so I don't have time to go all the way through your problem - but SO FAR it looks like the MVM instruction could be overwriting the OTU (Unlatch) instruction ...

hope this helps ... I'll spend some more time on it later if necessary ...

Click to expand...

Thanks Ron!

This is exactly what is happening. I did notice that as a possibility before and tested a few different things to verify that but I must have missed something because I did it again and sure enough rung 144 is changing that bit. Thanks again!