Direct scan class

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?

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.

Thanks for clearing that up Kevin.