Logix500 CTU question

harleymc100

Member
Join Date
Jan 2014
Location
london
Posts
21
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

Image 5.jpg
 
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.
 
Ctu

harleymc100,


This is called ORDER OF EVALUATION


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


Regards
 
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.
 
.....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.
 
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.
 
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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