How is read/write determined in a Driven Tag Group?

Ignition
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1

I am working on a recipe editing page, I have a driven tag group which is set to one-shot on a bit.
This bit is called update, and updates all the recipe tags when this bit goes to true.

My question is this; How does ignition determine whether the tags should be read from the PLC or written to the PLC? Understanding this would help guide me on how to approach this recipe page

This question came up when working with Driven Tag groups, but I am sure this is also an issue with the other Tag Groups

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It doesn’t determine anything. Tag groups and direct/driven/leased are all about subscriptions, which are reads. Writes happen immediately when you send them. Writes are not part of subscriptions.

It seems you have conflated the ability of transaction groups to have traffic in both directions, on a trigger, with driven tag groups. Not the same kind of “group” at all.

3 Likes
3

Thanks for the reply, could you help me clarify this?

I went through the documentation on tag groups again. So, the tag groups define the subscription only, different rates and so forth for reading from the PLC.

But writing? What happens when I change a tag value in Ignition? If I understand you correctly then that value is immediately written to the PLC, In other words, if I poll a tag every hour, but change the value in ignition 10 minutes after a read, that value will be written immediately on change, but I will only see the change then when the tags are read again 50 minutes later?

I guess that’s where the “read after write” setting comes in

4

Yes.

5

Thanks! That makes a big difference. Somehow thought reads and writes would happen at each poll