# Tag scan class vs historical scan class

**URL:** <https://forum.inductiveautomation.com/t/tag-scan-class-vs-historical-scan-class/11877>\
**Category:** Ignition\
**Created:** [June 29, 2016, 5:08am UTC](https://forum.inductiveautomation.com/t/tag-scan-class-vs-historical-scan-class/11877 "2016-06-29T05:08:39Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![richardNZ15](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/richardnz15/32/33373_2.png) [@richardNZ15](https://forum.inductiveautomation.com/u/richardNZ15)\
**Post date:** [June 29, 2016, 5:08am UTC](https://forum.inductiveautomation.com/t/tag-scan-class-vs-historical-scan-class/11877/1 "2016-06-29T05:08:39Z")

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I’ve been trying to tweak scan classes to prevent comms issues and getting a bit confused as to the effects they have.

Can i get a bit of breakdown as to best practise.

If i have an analog tag reading a motor current, and its scan class is set to the following:

Mode: Leased  
Slow Rate: 10,000ms  
Fast Rate: 2,000ms

And i want to record history for the values with the following historical scan class:

Mode: Direct  
Slow Rate: 1,000ms

Because the scan class is slower than the historical scan class, I’d be missing out on real time data right? If the tag is using the fast rate, my true value of the tag gets updated every 2 seconds, and i’ve recorded two historic values of the same value.

If my tag is using the slow rate, then the true value only updates every 10 seconds, and i’ve recorded 10 values in my historical table with the same value.

Is that correct? Or does the execution of a historical tag read the value from the OPC when its run?

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**Author:** ![AleInge](https://avatars.discourse-cdn.com/v4/letter/a/46a35a/32.png) [@AleInge](https://forum.inductiveautomation.com/u/AleInge)\
**Post date:** [July 25, 2016, 12:10pm UTC](https://forum.inductiveautomation.com/t/tag-scan-class-vs-historical-scan-class/11877/2 "2016-07-25T12:10:16Z")

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Hi,

as far as I know, the historical values are retrieved from the realtime ones, without a dedicated polling.  
Therefore, the realtime scan class should be at least as fast as the historical one.

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<div class="post-metadata">

**Author:** ![richardNZ15](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/richardnz15/32/33373_2.png) [@richardNZ15](https://forum.inductiveautomation.com/u/richardNZ15)\
**Post date:** [July 25, 2016, 8:05pm UTC](https://forum.inductiveautomation.com/t/tag-scan-class-vs-historical-scan-class/11877/3 "2016-07-25T20:05:26Z")

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Thanks,  
Yes I think this is the conclusion i have come to as well.