Minimum Time for Trend Log (OPC Direct Log Type) Configuration
Is there any way to configure the system 800xA to register log data less than 1 sec minimum storage time for Trend Logs (Direct OPC).
I have some critical signals that I wish to acquire at least at the controller task time rate but the log does not stores data constantly at the configured rate scan time in the logs.
The Controller Task runs at 350 ms, controller is a redundant PM864A and system has a redundant Aspect/Connectivity Server. System 800xA is version 6.0
I have configured a Log Template with a single Direct OPC Trend Log:
Min Time: 175 ms by 3 day storage.
Max Time: 350 ms.
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The default MMS poll cycle is 500ms.
Basic History can record at "any" interval (called Min Time) eventually getting into trouble with disk IO queuing if too many logs need simultaneous writing/reading to/from disk. Memory can be a limiting factor as default blocking rate is 20 minutes between flushing unwritten data to disk.
To record faster than 500ms, the "Requested Rate" setting for the fastest MMS group need to be lowered in the AC 800M OPC server.
This will come with a pricetag of increased MMS transactions per second. Also, the AC 800M task tuning must reserve space (=time) between task execution to permit MMS transactions to take place. This is made with Task Offset.
With no or too little task offset, no or too few transactions will fit and the OPC server will have to wait to get served with data, =old data if time is of essence. The Actual Rate shown in the Update Rate window of the OPC server panel will then depart from Requested Rate and oscillate. This is bad and indicate too many transactions, too fast with not enough task offset or free CPU time available in the controller.
175 ms is more than 5x faster than default rate of 1000ms and 3x faster than you can read data out of an AC 800M with default settings.
You should be able to read at 500ms, a bit faster if you tweak rate the AC 800M OPC server settings, but I can not guarantee that you will reach your desired frequency without overloading the controller.
Consider moving also the second fastest MMS group to enable eg process graphics to read at 500 ms, or else all subscriptions will end up in the fastest group and make big contribution to the total number of transactions per second. The "Variable Communication Statistics" function of the AC 800M OPC server will break it down for you so that you can see where transactions come from and why. In Control Builder M -> Remote System -> Show MMS Communication you can see the total MMS figure per controller. Use these tools to monitor your controller!
I would also scale the official performance figures given in the System Guide. With 5x faster logging I would consider configuring 5x less logs and 5x less dense trends in the HMI.
ABB have dedicated ultra fast loggers running outside the realms of MMS and OPC.
Logging at 175ms is not what System 800xA and AC 800M was designed for.
You might need blessing from some ABB authority to run this fast with maintained product warranty. Do not experiment if you do not know what you are doing.
An AC 800M controller will prioritize running your application code, but too high CPU load is not wise. Keep total CPU load below 90%.