# Direct scan class

**URL:** <https://forum.inductiveautomation.com/t/direct-scan-class/10382>\
**Category:** Ignition\
**Created:** [August 22, 2015, 8:23pm UTC](https://forum.inductiveautomation.com/t/direct-scan-class/10382 "2015-08-22T20:23:09Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![AlThePal](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/althepal/32/3603_2.png) [@AlThePal](https://forum.inductiveautomation.com/u/AlThePal)\
**Post date:** [August 22, 2015, 8:23pm UTC](https://forum.inductiveautomation.com/t/direct-scan-class/10382/1 "2015-08-22T20:23:09Z")

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I’m driving 2 tags from Simulator values with different scan classes, both set to Direct mode. I’ve found that if a scan class is set to longer than 5 seconds its setting is ignored - the tag continues to update at a 5 second rate on the screen.

Is there any reason for this behaviour?

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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:** [August 22, 2015, 11:57pm UTC](https://forum.inductiveautomation.com/t/direct-scan-class/10382/2 "2015-08-22T23:57:37Z")

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It’s a quirk of how dumb the simulator drivers are combined with an implementation detail regarding OPC-UA subscriptions.

- the simulator drivers run at 100ms regardless of the requested rate
- in the OPC-UA client, scan class rate translates to both the subscription publishing interval and the monitored item sampling rate, with one exception: a subscription publishing interval will never be slower than 5000ms regardless of the scan class. This is done so that keep-alive notifications happen at a reasonable rate and a subscription’s timeout isn’t calculated as some time too far ahead in the future.

So… these things combined result in tags from the simulator drivers coming in no slower than 5000ms.

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<div class="post-metadata">

**Author:** ![AlThePal](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/althepal/32/3603_2.png) [@AlThePal](https://forum.inductiveautomation.com/u/AlThePal)\
**Post date:** [August 24, 2015, 7:21am UTC](https://forum.inductiveautomation.com/t/direct-scan-class/10382/3 "2015-08-24T07:21:10Z")

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Thanks for clearing that up Kevin.