Rockwell Automation 1756-HIST2G Historian ME 2.1 - Manual (Page 224)

F ACTORY T ALK H ISTORIAN ME U SER ’ S G UIDE
214
specifications to determine which of the values that it has received get saved in
the Archive subsystem. By filtering out data that you do not need, you get more
efficient Archive storage. This process is also called compression testing. The
name of the compression testing algorithm used for this process is called the
swinging door compression algorithm.

To set compression specification attributes, you must set the Compressing
Flag to ON (1). Turn the Compression Flag to ON (1) for all real-time points
in the system.

Set compressing to OFF (0) for laboratory and manually entered points so all
values are archived. Setting compressing OFF means all points will be
archived.
Compression affects points of type Digital, since a new value is recorded only
when the current value changes. Points of type String have a similar behavior;
new points pass compression only when the value changes. Exception Deviation.
The FactoryTalk Historian ME uses exception specifications so that the interface
sends the data you are interested in to the server, rather than sending a lot of data
that is not meaningful. The process of using exception specification to control the
flow of data from the interface to the server is called Exception Reporting. It takes
place on the interface before the data is sent to the server.

Exception Deviation (ExcDev attribute)
Specifies in engineering units how much a point's value must change before
the interface considers it as a significant value, and sends it to the server. As a
general rule, you should set the exception slightly smaller than the precision of
the instrument system.
You can also specify Exception Deviation as a percentage of the Span
attribute. A typical value is 1% of Span. Exception Deviation should be less
than Compression Deviation by at least a factor of 2. When Exception
Deviation is expressed in percentage, it is known as ExDevPercent attribute.

Exception Minimum (ExcMin attribute)
Specifies a limit on how frequently the interface can report values to the
server. For example, if you want the interface to wait a full ten minutes before
reporting a new value to the server, then you would set the ExcMin attribute to
ten minutes. ExcMin is typically set to zero.