# How to read a opc-tag value not use scan-class

**URL:** <https://forum.inductiveautomation.com/t/how-to-read-a-opc-tag-value-not-use-scan-class/3123>\
**Category:** Ignition\
**Created:** [July 22, 2011, 2:39pm UTC](https://forum.inductiveautomation.com/t/how-to-read-a-opc-tag-value-not-use-scan-class/3123 "2011-07-22T14:39:34Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![weng](https://avatars.discourse-cdn.com/v4/letter/w/bbce88/32.png) [@weng](https://forum.inductiveautomation.com/u/weng)\
**Post date:** [July 22, 2011, 2:39pm UTC](https://forum.inductiveautomation.com/t/how-to-read-a-opc-tag-value-not-use-scan-class/3123/1 "2011-07-22T14:39:34Z")

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I want to read a opc-tag value only once because when it was read a time,the devise state changed,when it was read again ,the devise state changed again,so I do not want it was read by scan-class because it read again and again.

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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:** [July 22, 2011, 3:30pm UTC](https://forum.inductiveautomation.com/t/how-to-read-a-opc-tag-value-not-use-scan-class/3123/2 "2011-07-22T15:30:43Z")

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[system.opc.readValue](http://inductiveautomation.com/support/usermanuals/ignition/system_opc_readvalue.htm) should do the trick.