Scan Class clairification

Could someone please confirm/clarify the following:

If a sqlTag is assigned to a scan class with Mode direct, if it is not an alarm tag or logged historically then the PLC will only be polled for that data when the tag is being used on an active client window, correct?

I just want to make absolutely sure that I am not going to be hammer the PLC for a bunch of data that will only need to be looked at every couple of months. I know I could put my occasional use tags in a driven scan class but if they are not being polled for when the window is not open then it is just extra work.


What you are describing and likely want is a Leased Scan Class. Leased tags will poll at the fast rate when on an active window and then at the slow rate when not on an active window.

After rereading the manual on scan classes my initial post sounds silly, but for some reason I thought I had read that the sqlTags were only updated when the they were subscribed to :question:

One follow up question: If a tag is configured for alarm, does that change the behavior of the leased scan class?

All tags are subscribed, and the scan class determines the subscription rate (basically).

No, the behavior of the scan classes are not affected by configured alarms on tags (for the most part).