Is it one OPC subscription per OPC Connector?

I have a situation with a Rx3i PLC dropping OPC connections and I suspect that it is because the connections to the PLC have exceeded the maximum allowed connections of that PLC (which is 5)

This system normally runs with a CIMPLIICTY HMI connected via OPC. When a new, Ignition based system is also connected to the same PLC and we restart the CIMPLICITY system, we are losing the OPC link to the CIMPLICITY system.

I believe that the CIMPLICITY system is limited to 1 OPC subscription. The question I have is for a single OPC connector, does Ignition limit itself to a single subscription? Or will it make use of multiple subscriptions?

I have looked in the OPC Connector Advanced Configuration of another project and nothing obvious jumps out at me (but I could be missing the obvious)

Note that I am running under limited information at the moment:

  1. There are no diagnostics on the PLC end that indicate how many subscriptions are currently in use. :man_facepalming:
  2. I didn’t build this Ignition system, and I have yet to see inside it. So I have no idea what surprises might lie within it. :thinking:

Each tag group with an OPC tag pointing at that connection will get a subscription (or 2 if it’s a leased mode tag group).

OK …I have a follow on question now. I have a GW backup in my hands right now. I restored it to a test system and I am looking at the project. And I’m a little confused by what I see.

There are no tags listed in the Tag browser for the project. But there are 10 transaction groups, each with a different amount of tags in them (smallest is 5 largest is 100). These tags pointed to the single OPC connector for the GW, and each have their OPC path. The transaction groups are pretty well all timer based (and typically on a 1 second rate)

My question is then, because the tags do not seem to be being logged directly by the GW, does this mean that each transaction group could be generating a new subscription?

Yes, each group will have its own subscription, unless all of its OPC items are configured to be polled.

You should be able to see the subscriptions for each OPC UA connection in the gateway under Status > Connections > OPC Connections > Client:

OK … I don’t have access to the live system right now.

Am I correct in saying that if the points were passing explicitly through the GW and the transaction groups referred to those tags, then there would only be one subscription?

I’m not sure what you mean by that. Transaction groups can have their own OPC items that exist independently of any tags in Ignition.

You could create OPC tags in the gateway for every tag that every group needs and then modify all the groups to reference those OPC tags rather than having their own separate OPC items (I think…) but I’m not sure if that’s what you’re asking about. I also don’t think that’s generally recommended…

You could also just switch all the groups to be polled if this server can’t handle subscriptions.

That is what I asking about. I have a hard limit of 5 total subscriptions to the OPC server in my PLC. I’m already using one of them, so I need to get the subscription count from this system down to at least 4.

Aside from the question at hand, I would encourage you to be careful with “tag” terminology:

Ignition has what it calls tags, of various types. One type, an OPC tag, points at an OPC item in an OPC server via one of Ignition’s OPC client connections. An OPC item commonly points at a PLC tag, but not necessarily, and not necessarily using the “tag” terminology. (Modbus generally calls them registers, due to the addressing scheme. Some platforms call them “points”, etc.)

Transaction groups contribute to the confusion because they can monitor/record/use both Ignition tags (whether OPC or not) and OPC items, in any mix desired. I recommend not using the term “tag” to refer to OPC items.

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Also I’m missing where this switch could occur. I see in the TG options I have a choice of subscribe or read. Is that what you mean?

Yeah I think that’s it. Going by memory here and I was never that familiar with transaction groups in the first place.

In a TG’s OPC read mode, any trigger is still subscribed. You’ll probably want to point that at an Ignition tag instead of an OPC item.

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9 of the TGs are timer based, the other one is schedule based. So no OPC item triggers are involved.