# Write Expression values back to OPC Item

**URL:** <https://forum.inductiveautomation.com/t/write-expression-values-back-to-opc-item/59195>\
**Category:** Ignition\
**Tags:** ignition80\
**Created:** [May 4, 2022, 7:23am UTC](https://forum.inductiveautomation.com/t/write-expression-values-back-to-opc-item/59195 "2022-05-04T07:23:27Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Susanne\_Weidinger](https://avatars.discourse-cdn.com/v4/letter/s/839c29/32.png) [@Susanne\_Weidinger](https://forum.inductiveautomation.com/u/Susanne_Weidinger)\
**Post date:** [May 4, 2022, 7:23am UTC](https://forum.inductiveautomation.com/t/write-expression-values-back-to-opc-item/59195/1 "2022-05-04T07:23:27Z")

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Hi  
I would like to add an expression to a tag in Ignition, and to that I would like write the calculated value from the expression back to the OPC item. In the link below there is described on how to do. But I can’t find the check box. I think my Ignition is 8.0.  
Does anyone know how I can fix it?

[Creating Expression Tags - Ignition User Manual 7.8 - Ignition Documentation (inductiveautomation.com)](https://docs.inductiveautomation.com/display/DOC/Creating+Expression+Tags)

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**Author:** ![pascal.fragnoud](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pascal.fragnoud/32/24728_2.png) [@pascal.fragnoud](https://forum.inductiveautomation.com/u/pascal.fragnoud)\
**Post date:** [May 4, 2022, 7:32am UTC](https://forum.inductiveautomation.com/t/write-expression-values-back-to-opc-item/59195/2 "2022-05-04T07:32:39Z")

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You should upgrade, 8.0 is not a version you want to be using - I don’t think it’s supported anymore.  
And then, you should use the doc related to the version you’re using, there’s a huge gap between 7.x and 8.x.

I think you’ll have to use a derived tag, which can hold a read expression and a write expression. Those expressions are used to ‘transform’ a value from and to another tag.  
[https://docs.inductiveautomation.com/display/DOC81/Types+of+Tags](https://docs.inductiveautomation.com/display/DOC81/Types+of+Tags)

> **[Derived Tags - Other Standard Tag Types](https://inductiveuniversity.com/videos/derived-tags/8.0)**
>
> Derived Tags: Derived tags are similar to expression tags, in that they utilize Ignition's Expression Language. They differ in that they must be associated with another tags (known as the "source tag"), as well as support writing to the source tag...

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**Author:** ![Alex\_Verba](https://avatars.discourse-cdn.com/v4/letter/a/9d8465/32.png) [@Alex\_Verba](https://forum.inductiveautomation.com/u/Alex_Verba)\
**Post date:** [June 22, 2023, 9:13pm UTC](https://forum.inductiveautomation.com/t/write-expression-values-back-to-opc-item/59195/3 "2023-06-22T21:13:42Z")

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You can add custom properties in an expression tag:

1. OPCWriteBackItemPath for instance: ns=4;s=|var|PLC210 OPC-UA.Application.Analogue.PG24
2. OPCWriteBackServer for instance: PLC210  
(see the picture below)  
 ![OPC_](https://us1.discourse-cdn.com/inductiveautomation/original/3X/6/2/62a919cdf9cdf771dc467a28254d0a220ed6af4a.png)

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<div class="post-metadata">

**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:** [June 22, 2023, 9:31pm UTC](https://forum.inductiveautomation.com/t/write-expression-values-back-to-opc-item/59195/4 "2023-06-22T21:31:53Z")

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Are you sure that does anything? (The core functionality was removed in v8+, upgrades use a tag event to emulate the old behavior.)