Recommended Update Rates for PI Connection to 800xA with Advant Connect
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Standard answer ....
DO NOT USE OPC DATA SUBSCRIPTIONS FROM ADVANT TO PI. USE OPC HISTORY INSTEAD
I strongly suggest you change the OPC-DA interface out for OPC-HDA. Your comments about subscription rates are completely correct. If you cannot do this, then yes - use 3 or 9 seconds. 1 second can kill the subscription tasks in the controller.
The same is true for Basic History logging. If you're not using TTD Logs, stick to 3 and 9 seconds.
This can be called by calling the Advant Master Adapter Statistics operation and study the amount of "Execution Timers" with PI connected and when not.
Check the attached SHC guide, chapter 12.6 (the whole of chapter 12 is advisable to execute during a SHC).
OPC data subscriptions from a third party client, i.e. OSI PI via the OPC surrogate process in 800xA to AC450 works great if you stick to the 1, 3 or 9 seconds subscription rates.
One connectivity server pair can easily subscribe on 6000 tags from a number of controllers with a mix of 1, 3 and 9 seconds subscription rates.
For instance 4000 tags on 9 sec 1500 on 3 sec and 500 on 1 sec divided on 6 or more controllers.
Out from today's experience OPC basic history is also preferred before TTD as it's more easy to re configure and one subscription will be shared to clients, basic history, and external OPC.
meaning that if you anyhow subscribe on i.e 5000 basic history tags on 3 seconds, you will not increase the load at all in the controller as the same subscription will be used to
provide the PI and the graphics with same data.
OPC subscriptions are also almost a must (vs TTD) if you have 1 sec history requirements for a few tags , as TTD rapidly gives you a very high TTD load when using 1 sec and it's common that you
get "holes" in the trend curves
2 sec is the fastest recommendation for TTD, but also here the TTD load is significant.
In the end you must off cause tune the number of subscriptions and the rate depending on the number of controllers, the controller load in each controller and the RTA board load.
OPC HDA may be more efficient but it's in the "gray zone" as it's not officially described and at least in the past, not officially supported from external systems.
OPC DA is described and supported in our documentation and product guide.