Effect of affinity setting on Load on AC400 controller
Could anybody please clarify on the understanding of the affinity setting and its relation with the controller loading as briefed below:
Here is the scenario of different affinity settings for Operator stations/800xa History Data collector nodes(DCNs).
Affinity setting in such a way that If two DCNs/Operator stations will point the single connectivity server (CSA)first then it will reduce the load on the controller, bacause it is only single subscription on the controller by one connectivity server. but it will load the connectivity server as it has to server both DCs (Is this understanding correct? ).
As a standard affinity setting, If different DCNs are pointing to different Connectivity servers, then it will share the the load between two connectivity servers. But it will load the controller, because number of subscriptions doubled on the controller, as both connectivity servers will be subscribing from the controller (Is this understanding is correct?).
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You are correct.
AFAIK, only MOD300 OPC DA server is unable of #1 (multiplexing multiple identical subscriptions into a single). 800xA for Advant Master OPC server do multiplex. However, some historians make use of Parallel Redundancy to reduce bad quality points in logs. With PR enabled, each historian subscribe over both OPC servers in parallel.
In most cases #2 is preferred to avoid a single point of failure. Also, it is beneficial to have more than one path between HMI and controller open in case of disturbance; with two separate feeds operators have "two active legs" to monitor and control the process with.
I promote #2 and making sure no adjacent workplaces share same data path (affinity).
Advant/Master controllers are more sensitive for bad clients. Eg subscriptions using unfavorable cyclic rates or unfavorable properties. Write operations should be kept at a minimum, and to MB300 DAT objects only.
I strongly recommend reading the "800xA for Advant Master Performance Guideline" document available on ABB Library and the (less detailed) performance chapter of the 800xA Configuration User's Guide.