# Defining an expression tag expression from a UDT parameter

**URL:** <https://forum.inductiveautomation.com/t/defining-an-expression-tag-expression-from-a-udt-parameter/76595>\
**Category:** Ignition\
**Tags:** ignition81, vision\
**Created:** [July 19, 2023, 6:20pm UTC](https://forum.inductiveautomation.com/t/defining-an-expression-tag-expression-from-a-udt-parameter/76595 "2023-07-19T18:20:22Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![apmalone1983](https://avatars.discourse-cdn.com/v4/letter/a/3be4f8/32.png) [@apmalone1983](https://forum.inductiveautomation.com/u/apmalone1983)\
**Post date:** [July 19, 2023, 6:20pm UTC](https://forum.inductiveautomation.com/t/defining-an-expression-tag-expression-from-a-udt-parameter/76595/1 "2023-07-19T18:20:23Z")

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Hello,

I am using Ignition Vision 8.1.25 and was curious if anyone has ever set up a UDT where you define an expression in a parameter and then use that parameter to define the expression in an expression tag. simple example:

 ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/a/8/a8b9abbc56eb199938517e6acbdbedb18a122913.png)

I have to OPC tags with live data, I want to be able to write an expression in the parameter and then use that parameter to define my expression on the expression tag.

 ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/2/1/21adce5e3f282f0002e0cfb0e9902312fbd2af91.png)

Any input would be greatly appreciated.

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**Author:** ![Jasmine\_Cartner](https://avatars.discourse-cdn.com/v4/letter/j/8491ac/32.png) [@Jasmine\_Cartner](https://forum.inductiveautomation.com/u/Jasmine_Cartner)\
**Post date:** [November 14, 2023, 2:40am UTC](https://forum.inductiveautomation.com/t/defining-an-expression-tag-expression-from-a-udt-parameter/76595/2 "2023-11-14T02:40:28Z")

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Hi @apmalone1983 I am also working on something similar, but referencing the parameter within a opc tag for the OPC source tag path. I was wondering if you made any progress with this?

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**Author:** ![apmalone1983](https://avatars.discourse-cdn.com/v4/letter/a/3be4f8/32.png) [@apmalone1983](https://forum.inductiveautomation.com/u/apmalone1983)\
**Post date:** [November 14, 2023, 8:45pm UTC](https://forum.inductiveautomation.com/t/defining-an-expression-tag-expression-from-a-udt-parameter/76595/3 "2023-11-14T20:45:39Z")

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The progress I made was they determined it can't be done at this point. Because the way the parameter is read in it won't calculate.