# \[Solved\] When are scan classes written to the PLC? As needed? **URL:** \ **Category:** Ignition\ **Created:** [November 5, 2020, 7:00pm UTC](https://forum.inductiveautomation.com/t/solved-when-are-scan-classes-written-to-the-plc-as-needed/40362 "2020-11-05T19:00:50Z")\ **Posts on this page:** 1\ **Showing post:** 2
**Author:** ![paul-griffith](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/paul-griffith/32/37249_2.png) [@paul-griffith](https://forum.inductiveautomation.com/u/paul-griffith)\ **Post date:** [November 5, 2020, 7:31pm UTC](https://forum.inductiveautomation.com/t/solved-when-are-scan-classes-written-to-the-plc-as-needed/40362/2 "2020-11-05T19:31:03Z")
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](http://forum.inductiveautomation.com/t/very-concerning-read-write-issues-important-information-for-all-users/18583) --- _[View the full topic](https://forum.inductiveautomation.com/t/solved-when-are-scan-classes-written-to-the-plc-as-needed/40362)._