# Tag update during PLC program scan

**URL:** <https://forum.inductiveautomation.com/t/tag-update-during-plc-program-scan/38568>\
**Category:** Ignition Early Access\
**Created:** [September 10, 2020, 6:23pm UTC](https://forum.inductiveautomation.com/t/tag-update-during-plc-program-scan/38568 "2020-09-10T18:23:54Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![rfedyk](https://avatars.discourse-cdn.com/v4/letter/r/45deac/32.png) [@rfedyk](https://forum.inductiveautomation.com/u/rfedyk)\
**Post date:** [September 10, 2020, 6:23pm UTC](https://forum.inductiveautomation.com/t/tag-update-during-plc-program-scan/38568/1 "2020-09-10T18:23:54Z")

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Quick question.  
Am I correct in thinking that Ignition reads directly from a PLC memory area, regardless of where the PLC is in a scan?  
i.e. If I were to turn a bit on at the top of a PLC program, then it turns off down the bottom, the tag could be read by Ignition at any point in time, including during the scan. Thus, it could be read as on, despite the fact it turns off during the program scan.

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**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\
**Post date:** [September 10, 2020, 6:48pm UTC](https://forum.inductiveautomation.com/t/tag-update-during-plc-program-scan/38568/2 "2020-09-10T18:48:28Z")

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That is an implementation detail that is likely unique to every PLC brand and model. I’m not sure I could answer that with confidence even if you did specify one.

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**Author:** ![pturmel](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pturmel/32/3295_2.png) [@pturmel](https://forum.inductiveautomation.com/u/pturmel)\
**Post date:** [September 10, 2020, 7:01pm UTC](https://forum.inductiveautomation.com/t/tag-update-during-plc-program-scan/38568/3 "2020-09-10T19:01:10Z")

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It is definitely true for all ControlLogix and CompactLogix processors. It is even true for those processors physical inputs and outputs.

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**Author:** ![rfedyk](https://avatars.discourse-cdn.com/v4/letter/r/45deac/32.png) [@rfedyk](https://forum.inductiveautomation.com/u/rfedyk)\
**Post date:** [September 10, 2020, 8:37pm UTC](https://forum.inductiveautomation.com/t/tag-update-during-plc-program-scan/38568/4 "2020-09-10T20:37:38Z")

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Thank you.  
I already planned to structure the PLC program accordingly.  
It will likely affect some Customers who have program structures in place as a standard, but I can at least explain why there may have to be accommodations for altering these.

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**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\
**Post date:** [September 10, 2020, 8:39pm UTC](https://forum.inductiveautomation.com/t/tag-update-during-plc-program-scan/38568/5 "2020-09-10T20:39:14Z")

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Did you mean to post that in the thread about AOIs?

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**Author:** ![rfedyk](https://avatars.discourse-cdn.com/v4/letter/r/45deac/32.png) [@rfedyk](https://forum.inductiveautomation.com/u/rfedyk)\
**Post date:** [September 11, 2020, 10:45am UTC](https://forum.inductiveautomation.com/t/tag-update-during-plc-program-scan/38568/6 "2020-09-11T10:45:31Z")

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No, I did mean this thread, but I guess it applies to both.  
Once you know both of these things it helps in planning tag and program structure.

Thanks again.  
People here are always quick to respond with relevant and helpful guidance.

It makes the transition to using Ignition so much easier.