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Old December 4th, 2018, 09:49 PM   #1
harleymc100
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Logix500 CTU question

Hi all

I have a simple timing sequence ive been playing with, question is, i cant get my /DN bits to cause a transition for the CTU.
Am i missing something here?

Ive attached a screenshot. below
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File Type: jpg Image 5.jpg (273.5 KB, 115 views)
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Old December 4th, 2018, 10:08 PM   #2
Mickey
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Move the C5:1 RES to below the C5:2 counter instruction.
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Old December 5th, 2018, 01:06 AM   #3
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To elaborate a little, you are resetting C5:1 before you are trying to count C5:2.

Mickey's solution will work. If you are trying to use C5:1/DN in later rungs that we cannot see in the picture, then it might make sense to move the C5:1 RES above the C5:1 counter.
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Old December 5th, 2018, 01:12 AM   #4
harleymc100
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Ah yep that makes sense. Thanks guys. It works perfectly now.
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Old December 5th, 2018, 07:31 AM   #5
hagos
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Ctu

harleymc100,


This is called ORDER OF EVALUATION


Means, where you place your rungs/networks , has great impact on your program behavior


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Old December 5th, 2018, 01:21 PM   #6
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Safe rule of thumb is that things read top down and left to right. There are some caveats, but you eliminate most problems if you arrange your program into steps that follow that rule.
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Old December 6th, 2018, 07:09 AM   #7
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Yes Logan,


For this reason you can't Jump to a label backwards, otherwise the CPU will fault.
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Old December 6th, 2018, 08:05 AM   #8
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Quote:
Originally Posted by hagos View Post
.....For this reason you can't Jump to a label backwards, otherwise the CPU will fault.

Yes you CAN jump "backwards", but you MUST NOT do it indefinitely - there must be a way to terminate this jumping, or the watchdog will time out and fault the processor.
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Old December 6th, 2018, 12:03 PM   #9
TL140
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Now, one kinda dumb question that I have is that would OP's Logic work on a ControlLogix CPU? I know scan cycle is a bit different. Just curious.
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Old December 6th, 2018, 12:42 PM   #10
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Quote:
Originally Posted by TL140 View Post
Now, one kinda dumb question that I have is that would OP's Logic work on a ControlLogix CPU? I know scan cycle is a bit different. Just curious.

No, the RES instructions would work exactly the same.
The only difference is that the physical IO scan is asynchronous, but updating if internal tags is not.
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Old December 7th, 2018, 05:30 AM   #11
harleymc100
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great info here. Thanks
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