# Expression tag alters the scan rate of referred tag(-s)

**URL:** <https://forum.inductiveautomation.com/t/expression-tag-alters-the-scan-rate-of-referred-tag-s/32380>\
**Category:** Ignition\
**Created:** [February 18, 2020, 3:33pm UTC](https://forum.inductiveautomation.com/t/expression-tag-alters-the-scan-rate-of-referred-tag-s/32380 "2020-02-18T15:33:41Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![d.filos](https://avatars.discourse-cdn.com/v4/letter/d/258eb7/32.png) [@d.filos](https://forum.inductiveautomation.com/u/d.filos)\
**Post date:** [February 18, 2020, 3:33pm UTC](https://forum.inductiveautomation.com/t/expression-tag-alters-the-scan-rate-of-referred-tag-s/32380/1 "2020-02-18T15:33:41Z")

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Hi,  
I am using UDTs with expression and OPC tags (Ignition 8.0.7). The issue I have encounter is that eventhough all tags belong to a direct mode tag group with rate at 5sec those OPC tags that are used in expressions are scanned (subscribed) at 1 sec. The behaviour can be replicated even without udts by just adding an expression tag that uses a simulated tag from the internal opc server and then restarting the opc tag. It seems that when opc tags are involved in expressions their scan rate is not respected and I am wondering if this is normal or something crucial skipped my attention.

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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:** [February 18, 2020, 4:34pm UTC](https://forum.inductiveautomation.com/t/expression-tag-alters-the-scan-rate-of-referred-tag-s/32380/2 "2020-02-18T16:34:37Z")

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I’m pretty sure this is related to a bug that has been fixed and will be available in a nightly release later this week.

Same as this one: [Modbus TCP uncontrolled polling](http://forum.inductiveautomation.com/t/modbus-tcp-uncontrolled-polling/32176/6)

Confusingly, the fix is available in the current 8.0.9 nightly release, but there is a “hot fix” 8.0.9 release coming today that will not actually include it. The next nightly release that includes it will be for 8.0.10.

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**Author:** ![d.filos](https://avatars.discourse-cdn.com/v4/letter/d/258eb7/32.png) [@d.filos](https://forum.inductiveautomation.com/u/d.filos)\
**Post date:** [February 18, 2020, 4:46pm UTC](https://forum.inductiveautomation.com/t/expression-tag-alters-the-scan-rate-of-referred-tag-s/32380/3 "2020-02-18T16:46:47Z")

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Thanks Kevin I am pretty sure that lot of udt users will appreciate it as it is can drastically affect the performance on a system.