[Solved] When are scan classes written to the PLC? As needed?

Scan classes/tag groups have nothing to do with writes [1]. When you press a momentary button, the control value will be written, as a direct write. Then some period of time later when you release, the released value will be written, again as a direct write.

  1. You can enable ‘Read After Write’ in the scan class settings, so that an out of band (non-subscription) read is triggered automatically for tags, but that’s a significant performance penalty. Also, you can enable ‘optimistic writes’, which basically make the Ignition tag change value immediately on a write (but doesn’t change anything about how values are actually written to the PLC).

Also, I’d be remiss if I didn’t mention that there’s some excellent threads on this forum about things you should consider when using momentary buttons, including whether to use momentary buttons at all; i.e.: Very Concerning Read/Write Issues. Important information for all users

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