Opc server for ac 800M, Installation and Configuration


Setting Cache Update Rate



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3BSE035983-510 - en AC 800M 5.1 OPC Server

 
Setting Cache Update Rate
The cache update rate controls how often the OPC Data Access Server updates its 
internal cache with data from a certain controller. The cache update rate limits the 
update rate an OPC client can have on an OPC group. To ensure that subscribing 
OPC clients can access changed values at their requested update rate, the time 
(value) for cache update rate should be set shorter than the fastest OPC group update 
rate. To set the update rate, open the Update Rate dialog box by selecting Settings > 
Update Rate... from the OPC Server Configuration Panel.
Figure 6. Update Rate Dialog
The OPC Server should always have its update rate set twice as fast as the OPC 
client(s) update rate.
Settings > Update Rate is only available from the OPC Data Access Server, i.e. 
when the Data Access tab is selected.


Section 2 Start-up and Configuration
Setting Cache Update Rate
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27
 
To distinguish between fast and slow update rates there are five categories for 
simple values (integerrealbool) and a separate category for strings. The values in 
the Requested column determines the update rate of the internal cache in the OPC 
server, i.e. how often items in each category are read from the controller. When 
OPC Server for AC 800M is started, the values are set to default values.
The values in the Actual column shows the current update rate. This value will vary 
around the Requested value depending on the load. A zero value in this column 
means that there is no MMS communication for that category at the moment.
The requested update rates must be given in ascending order, starting at the High 
category. The lowest permitted update rate value for any category is 50 ms.
Example
An OPC client creates an OPC group with an update rate of 1000 ms. This means 
that items added to this OPC group will be reported to the OPC client once every 
second. In the OPC server, the MMS communication for this group is put in the 
High category, which means that items in this category will be read from the 
controller every 500 ms.
If any groups has been added, the update rate cannot be changed. The update rate 
configuration should therefore be made at start-up of the OPC Server for 
AC 800M, before any groups been added.


Clock Synchronization
Section 2 Start-up and Configuration
28
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