# Modbus Addressing TX/RX

**URL:** <https://forum.inductiveautomation.com/t/modbus-addressing-tx-rx/24487>\
**Category:** Ignition\
**Created:** [April 29, 2019, 11:48pm UTC](https://forum.inductiveautomation.com/t/modbus-addressing-tx-rx/24487 "2019-04-29T23:48:28Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![cclose](https://avatars.discourse-cdn.com/v4/letter/c/278dde/32.png) [@cclose](https://forum.inductiveautomation.com/u/cclose)\
**Post date:** [April 29, 2019, 11:48pm UTC](https://forum.inductiveautomation.com/t/modbus-addressing-tx-rx/24487/1 "2019-04-29T23:48:28Z")

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I am trying to differentiate between the RX/TX Holding registries on my RTU over TCP Modbus device. Since only values written to tags associated with he RX register can adjust values in the device, I need to distinguish read vs. write tags. Is there any special syntax when writing the OPC item path? [device]HR(n)

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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:** [April 29, 2019, 11:52pm UTC](https://forum.inductiveautomation.com/t/modbus-addressing-tx-rx/24487/2 "2019-04-29T23:52:45Z")

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In Modbus Holding Registers are Read/Write and Input Registers are Read-only.

Are you referring to something else?

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<div class="post-metadata">

**Author:** ![cclose](https://avatars.discourse-cdn.com/v4/letter/c/278dde/32.png) [@cclose](https://forum.inductiveautomation.com/u/cclose)\
**Post date:** [April 30, 2019, 12:00am UTC](https://forum.inductiveautomation.com/t/modbus-addressing-tx-rx/24487/3 "2019-04-30T00:00:16Z")

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Oh! So can I write my OPC item paths with some pointing to Holding Registers and others to Input Registers?

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<div class="post-metadata">

**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:** [April 30, 2019, 1:03am UTC](https://forum.inductiveautomation.com/t/modbus-addressing-tx-rx/24487/4 "2019-04-30T01:03:17Z")

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Sure, as long as the data is actually at those registers in your device…