# Question for data write of derived tag

**URL:** <https://forum.inductiveautomation.com/t/question-for-data-write-of-derived-tag/28912>\
**Category:** Ignition\
**Created:** [October 8, 2019, 12:25pm UTC](https://forum.inductiveautomation.com/t/question-for-data-write-of-derived-tag/28912 "2019-10-08T12:25:20Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![dawei.liu](https://avatars.discourse-cdn.com/v4/letter/d/76d3ee/32.png) [@dawei.liu](https://forum.inductiveautomation.com/u/dawei.liu)\
**Post date:** [October 8, 2019, 12:25pm UTC](https://forum.inductiveautomation.com/t/question-for-data-write-of-derived-tag/28912/1 "2019-10-08T12:25:20Z")

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Hello

I have a UDT derived tag as shown in below snapshot. I use the OPC content as DataSource and use jsonGet/jsonSet to read/write its member.  
The jsonGet function works fine and can reflect the values from OPC source as shown in snapshot 1, but it seems that jsonSet function doesn’t work when I update its value.  
For the writing expression: I use “jsonSet({source}, ‘X’, {value})”  
When I udpate the value of X in the tag direcotry or using the system.tag.write(’[VisionProvider]VectorTag/X’, value)  
, it pops below error info in the log and the writing of value wil fail.

On: [client]Window Titles  
20:23:03.149 [AWT-EventQueue-0] ERROR com.inductiveautomation.ignition.designer.tags.util.TagUtilities - Error writing to X.value: Error\_ExpressionEval

So could anyone give some suggestions?  
Thanks in advance.

 ![png](https://us1.discourse-cdn.com/inductiveautomation/original/2X/d/df63117b308df111cae19bd1b8e7dc414d4b27c1.png)

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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:** [October 8, 2019, 1:20pm UTC](https://forum.inductiveautomation.com/t/question-for-data-write-of-derived-tag/28912/2 "2019-10-08T13:20:39Z")

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What version of Ignition is this? I set up something similar and don’t have any problems with the writes.

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<div class="post-metadata">

**Author:** ![dawei.liu](https://avatars.discourse-cdn.com/v4/letter/d/76d3ee/32.png) [@dawei.liu](https://forum.inductiveautomation.com/u/dawei.liu)\
**Post date:** [October 8, 2019, 3:57pm UTC](https://forum.inductiveautomation.com/t/question-for-data-write-of-derived-tag/28912/3 "2019-10-08T15:57:24Z")

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@Kevin.Herron my version is 8.0.4nightly

It works on my Ignition now.  
The error was caused by the update of UDT type is not synchronized with the instanced tag.