# Sequential Polling of Devices

**URL:** <https://forum.inductiveautomation.com/t/sequential-polling-of-devices/7450>\
**Category:** Ignition\
**Created:** [October 22, 2013, 5:52pm UTC](https://forum.inductiveautomation.com/t/sequential-polling-of-devices/7450 "2013-10-22T17:52:30Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![jsham](https://avatars.discourse-cdn.com/v4/letter/j/ac91a4/32.png) [@jsham](https://forum.inductiveautomation.com/u/jsham)\
**Post date:** [October 22, 2013, 5:52pm UTC](https://forum.inductiveautomation.com/t/sequential-polling-of-devices/7450/1 "2013-10-22T17:52:30Z")

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Our systems usually consist of multiple remote sites, connected by low-bandwidth (~56k) radios or leased phone lines. Each site, when communicating, requires about 10 Kbps of bandwidth. What we would like to be able to do is sequentially poll each site on a regular interval.

I know we can do this using scan classes and the old prime number trick to reduce overlaps, but short of creating a new module that creates “device polling groups”, is there any other way to force ignition to only talk to one device at a time?

Maybe a better understanding of how a scan class executes would help as well… do multiple devices inside a scan class get queried in any particular order, or does the whole class execute (communicate) at the same time?

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**Author:** ![Pat](https://avatars.discourse-cdn.com/v4/letter/p/8491ac/32.png) [@Pat](https://forum.inductiveautomation.com/u/Pat)\
**Post date:** [October 23, 2013, 1:06pm UTC](https://forum.inductiveautomation.com/t/sequential-polling-of-devices/7450/2 "2013-10-23T13:06:59Z")

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When I have projects that have low bandwidth (radio, leased lines) I use a dedicated PLC to do the network communications. This gives you absolute control over network communications. Using a dedicated PLC for networking communications will also give you the fastest data throughput since you won’t have to worry about OPC polling collisions or retries.

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**Author:** ![Bullfrog3d](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Bullfrog3d](https://forum.inductiveautomation.com/u/Bullfrog3d)\
**Post date:** [October 23, 2013, 3:56pm UTC](https://forum.inductiveautomation.com/t/sequential-polling-of-devices/7450/3 "2013-10-23T15:56:06Z")

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I too have the same issue I am trying to work with a 30 station remote network over slooooow radio, I have been looking into installing a PLC just for the connectivity issue, can you recommend any particular brands. We are using Alan Bradley in our Hydro operations but the Hydro department is looking at a different solution.

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**Author:** ![Pat](https://avatars.discourse-cdn.com/v4/letter/p/8491ac/32.png) [@Pat](https://forum.inductiveautomation.com/u/Pat)\
**Post date:** [October 23, 2013, 5:35pm UTC](https://forum.inductiveautomation.com/t/sequential-polling-of-devices/7450/4 "2013-10-23T17:35:54Z")

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I don’t use Allen-Bradley much, but you’ll want a PLC that speaks the same protocol as your remote units and has enough memory to store all the remote read and write data required for your remote units.

Here’s a pdf by Kepware on how to optimize OPC communication using their OPC Server. Granted it’s about their OPC Server software, but there’s good general information about optimization in there.

[http://www.kepware.com/Support\_Center/SupportDocuments/Optimizing\_KEPServerEX\_V5\_Projects.pdf](http://www.kepware.com/Support_Center/SupportDocuments/Optimizing_KEPServerEX_V5_Projects.pdf)