Subscribe and polling

I am new to Ignition and I hope someone can explain the pollings and rates for me.

I have a Siemens PLC (S7-1500) with including OPC-server. Ignition (8.0.0) is used as an OPC-client.

Have I got this correct?:

  • OPC-server is polling the PLC for new values.
  • When Tag Group is set to Subscribe, OPC-server will send the changed value(s) to Ignition Gateway.
  • Ignition Client polls the Ignition Gateway and get the changed values.

In the Tag group settings there are

  • Rate
  • Leased/Driven rate
    These rates - what are these used for? Is it instructing how often the OPC-server (in PLC) should poll?
    Or is it instructing the Gateway to poll the OPC-server in spite that Data mode in Tag group is set to Subscribe?

In Tag group settings are also

  • Publishing Interval (Default: -1.0)
    In the PLC’s OPC-server there is a minimum publishing interval setting as well.
    When/How is the Publishing interval used in the Tag group? Will it override the setting in the PLC’s OPC-server?
    Or is it only used when Ignition is used as an OPC-server?

I am confused and would be very happy if someone can clarify this for me. :slight_smile:

Unless you set the Tag Group to “OPC Read Mode” then a subscription is made on the OPC UA server, it’s not polling.

The group’s Rate setting is used as the OPC UA Sampling Interval for each item. The Publishing Interval of the subscription is automatically calculated unless you set it to something other than -1.0.

The Sampling Interval for each item is supposed to represent how often the server polls its underlying datasource, if applicable. The Publishing Interval is how often accumulated changes for the subscription are published to the client. By default you get them essentially right away, but it’s possible to allow changes to accumulate and only be published periodically by increasing the Queue Size for each Monitored Item and setting a longer Publishing Interval.

When you use Leased Mode then the item gets moved back and forth between subscriptions with and the Sampling Interval gets switched between the slow and fast rates depending on whether some part of Ignition is using that tag.

Thanks for your answer. You write that the Tag Group must not be set to “OPC Read Mode” if I want to achieve “Subscription”. I can’t find that property. Do you mean that “Data Mode” must not be set to “Polled”?

So, if I use Subsription,

  • the Groups Rate is for how often the OPC Server(Siemens) is polling the PLC?
  • the Publishing Interval is for how often new values are sent from OPC-Server(Siemens) to the OPC-Client (Ignition Gateway)?

Sorry for maybe basic questions, but I think I really need to understand this to make good solutions.