# Modbus addressing clarification

**URL:** <https://forum.inductiveautomation.com/t/modbus-addressing-clarification/90248>\
**Category:** Ignition\
**Tags:** perspective, ignition81\
**Created:** [May 10, 2024, 5:59pm UTC](https://forum.inductiveautomation.com/t/modbus-addressing-clarification/90248 "2024-05-10T17:59:23Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![mjohnson0238](https://avatars.discourse-cdn.com/v4/letter/m/4bbf92/32.png) [@mjohnson0238](https://forum.inductiveautomation.com/u/mjohnson0238)\
**Post date:** [May 10, 2024, 5:59pm UTC](https://forum.inductiveautomation.com/t/modbus-addressing-clarification/90248/1 "2024-05-10T17:59:23Z")

</div>

SO, I have an address I need to read. The device is currently setup like this:

 ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/a/c/ac51fc4b4373d58a10d1bf7d60f262435a4bea28.png)

I only need to read 1 of the bits and it is one addressed "1.Reg0002.2"

Should I be able to delete all these addresses and only look for "1.Reg0002.2"?

Set an OPC tag like this:

ns=1;s=[UPS S]1.HR2.2

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<div class="post-metadata">

**Author:** ![mjohnson0238](https://avatars.discourse-cdn.com/v4/letter/m/4bbf92/32.png) [@mjohnson0238](https://forum.inductiveautomation.com/u/mjohnson0238)\
**Post date:** [May 10, 2024, 6:10pm UTC](https://forum.inductiveautomation.com/t/modbus-addressing-clarification/90248/2 "2024-05-10T18:10:56Z")

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Anytime I delete any of these listings the tag gives me an unknown value error.

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<div class="post-metadata">

**Author:** ![dkhayes117](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/dkhayes117/32/68344_2.png) [@dkhayes117](https://forum.inductiveautomation.com/u/dkhayes117)\
**Post date:** [May 10, 2024, 6:35pm UTC](https://forum.inductiveautomation.com/t/modbus-addressing-clarification/90248/3 "2024-05-10T18:35:30Z")

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I think the prevailing advice is to **NOT** use the modbus address mapping. Use the full OPC path in the tag.

[Modbus address mapping - Ignition - Inductive Automation Forum](https://forum.inductiveautomation.com/t/modbus-address-mapping/75768)

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<div class="post-metadata">

**Author:** ![mjohnson0238](https://avatars.discourse-cdn.com/v4/letter/m/4bbf92/32.png) [@mjohnson0238](https://forum.inductiveautomation.com/u/mjohnson0238)\
**Post date:** [May 10, 2024, 8:03pm UTC](https://forum.inductiveautomation.com/t/modbus-addressing-clarification/90248/4 "2024-05-10T20:03:35Z")

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Alright, so when I do use the modbus' addressing, that you set up in the device, I get this when using the OPC addressing method:

 ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/9/1/918230d638bf4c53e32ab9d620a5b595a0d4c910.png)

 ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/2/a/2aec3fd2df84314e9f68e1965b3e2d900bbada81.png)

A good return of value.

Then when I do not use the modbus' addressing, that you set up in the device, I get this when using the OPC addressing method:

 ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/5/f/5f56fdaec802f98f7548530ee0cdebee6f5ed599.png)

 ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/1/2/125afa3ab172beeeca891eac2527b7742e64f22b.png)

A "Unknown" return of value, stating that whatever was updating the tag doesnt exist.

Any ideas?