Transaction Group with Scan Class in Read Mode

General Discussion
1

Hi,
I have an application where I am reading product data from a UDT into a Transaction Group that loads a stored procedure. The group runs periodically at 500ms and looks for a Boolean trigger inside the UDT to be =1.
I have a Direct Scan Class with a slow rate of 350ms, that I have setup in Read mode.
I run into issues when I add other similar transaction groups that pull similar data from other PLC’s, if that PLC happens to be offline. Whether I use the same scan class, or create a unique scan class for the other machine, if either machine happens to be offline, then the tags within the scan class for the online machine will not update for 20-30 seconds.
I had hoped that having separate scan classes would protect against this issue. If I set the two scan classes at different, non-overlapping rates, then the problem disappears. This is not ideal, because I may have up to 10 production lines running on this same application.
Is there some way to avoid having to use prime numbers in my scan class rates to keep from running into this issue?
If I switch to Subscribed mode in the scan class, then having PLC’s offline has no effect on the other systems. I am concerned about not having current data in the transaction group using this method though.

Just looking for tips and/or advice.

Thank you.

2

Have you considered using OPC Items, rather than tag references?
(docs.inductiveautomation.com/di … +OPC+Items)

3

Someone can correct me if I’m wrong, but I believe that the way OPC tags get into Ignition is something like this:

PLC
OPC Server
OPC Client (Ignition Tag Provider)
Ignition Client

I believe Subscribed vs Read affects the connection from OPC Client to the OPC Server not the connection from OPC Server to the PLCs.
I believe in Subscribed mode the OPC Server uses the rate of the scan class to check tag values on the PLC and then only passes values to the OPC Client if they have changed.

So if you’re using the Ignition OPC Server, then Subscription will not affect the accuracy of the tag values.