# Polling Modbus

**URL:** <https://forum.inductiveautomation.com/t/polling-modbus/8996>\
**Category:** Ignition\
**Created:** [September 3, 2014, 8:11pm UTC](https://forum.inductiveautomation.com/t/polling-modbus/8996 "2014-09-03T20:11:34Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![mikehill100](https://avatars.discourse-cdn.com/v4/letter/m/bcef8e/32.png) [@mikehill100](https://forum.inductiveautomation.com/u/mikehill100)\
**Post date:** [September 3, 2014, 8:11pm UTC](https://forum.inductiveautomation.com/t/polling-modbus/8996/1 "2014-09-03T20:11:34Z")

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Hello,

I hope ya’ll are having a good day. I had a question about address requests from a Modicon.

As far as communication to the device is concerned, will I add network traffic if I poll bits from a register (A) rather than polling the register and parsing the bits with expressions (B)?

A  
Tag 1 = HR1000.0  
Tag 2 = HR1000.1  
Tag 3 = HR1000.2

B  
Tag = HR1000  
Tag 1 = getBit(Tag, 0)  
Tag 2 = getBit(Tag, 1)

I thought I would check because I know that sometimes when an OPC polls it just grabs a whole block of registers. The device we are connecting to is a little delicate, and as convenient as it is to grab lots of individual bits into their own little tags, I wanted to get your opinions before I started faulting things all over town.

Thanks!

- Mike

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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:** [September 3, 2014, 8:18pm UTC](https://forum.inductiveautomation.com/t/polling-modbus/8996/2 "2014-09-03T20:18:06Z")

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Both cases should just request HR1000.

The Modbus protocol doesn’t support reading registers at the bit level (if you want bits that’s what Coils and Discrete Inputs are), so in either case the breakdown into bits doesn’t affect the register(s) requested.

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<div class="post-metadata">

**Author:** ![mikehill100](https://avatars.discourse-cdn.com/v4/letter/m/bcef8e/32.png) [@mikehill100](https://forum.inductiveautomation.com/u/mikehill100)\
**Post date:** [September 15, 2014, 8:27pm UTC](https://forum.inductiveautomation.com/t/polling-modbus/8996/3 "2014-09-15T20:27:39Z")

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Thanks.

What happens if I request

Tag 1 = HR 1001.1  
Tag 2 = HR 1001.1

Will it request HR 1001 twice?

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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:** [September 15, 2014, 9:09pm UTC](https://forum.inductiveautomation.com/t/polling-modbus/8996/4 "2014-09-15T21:09:05Z")

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[quote=“mikehill100”]Thanks.

What happens if I request

Tag 1 = HR 1001.1  
Tag 2 = HR 1001.1

Will it request HR 1001 twice?[/quote]

As long as they’re being requested at the same rate (same scanlass, or scanclass at equal rate), I think just once.