Rate effects on Subscribed OPC tags

Ignition
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1

Hi all,

Looking for a bit of clarity on the underlying logic behind the rate on a subscribed OPC tag group.

  1. If a tag group has an OPC setting of 'Subscribed', is the 'rate' parameter redundant? I.e. Does the tag only update when it recieves a subscription update from the PLC / Device? (Lets ignore the case that the subscription fails as it wouldnt then 'fall over' to polling)
  2. Same question as above, but for a leased tag thats set to 'Subscribed'. Does the leased rate actually have an effect?
  3. If a tag is dervived from an source OPC tag, and the derived tag has an OPC setting of subscribed with rate 2000, does the derived tag poll the OPC tag with a rate of 2000? Or does the subscribed pass-through so it waits for subscriptions?

Trying to understand how it works so we can make sure the right tags are prioritised!

Thanks!
Alex

2
  1. The rate parameter is never redundant. It is handed to the OPC server to tell it how often the underlying item is to be sampled/polled. For native Ignition drivers, the rate is passed through to the driver, which typically will poll at that rate.

(Let's not ignore the subscription failure case: there is NO fail-over to polling. If you have a broken server--there are many--you have to switch the tag group to polling yourself, or you get no data.)

  1. Yes. But note that a non-leased rate of zero means to drop the item from the subscription entirely when not leased.

  2. Derived tags and Reference tags simply pass through whatever arrives at their source, IIRC, ignoring any rate. Expression tags in event driven mode do the same, but re-execute for any change in any of their references.

In short, always use subscribed mode with OPC servers that support it properly.