Tag group properties questions

Ignition
1

Hi,

In the tag group editor there is a "read after write" option in the "OPC Setting".

Is the "read after write" supporsed to mean read the OPC PLC value after the Ignition tag has been written to? Or has it something to do with the OPC-UA module writing to the tags in the PLC and then read after that? What triggers the read after write action?

How does "read after write" behave when the tag group is set on a memory tag with no OPC comminucation?

If I have a bunch of memory tags and OPC tags and I just want them to "update" their values when they are wirtten to from an Ignition designer or client, because I know that is the only time they change, how would I best set up the tag group for them?

Thanks

2

"Read after write" only has any effect with OPC tags, and it causes an explicit read to be issued to the underlying OPC server after the write has finished.

It's something of a legacy option and should not be used with OPC UA tags unless the Tag Group is in Polled/Read mode.

This is already how tags behave, you don't need to set anything special.

3

This is already how tags behave, you don't need to set anything special.

But on both OPC tags and memory tags you have to set a tag group.

And the tag group has a rate at which it updates all the tags in the group.

What I'm looking for is basically a tag group setting that Ignores the rate - at least thats what I think I'm looking for :slight_smile:

4

Memory tags update immediately after writing.

OPC tags will not update until the change is received from the server. You could use Read After Write with Polled mode, or use "Optimistic Writes" with Subscribed mode.

5

Memory tags update immediately after writing.

Does this mean that whichever tag group is chosen for a memory tag makes no difference to its behaviour?

6

Yes, doesn't really matter.