Rockwell Automation 1756-HIST2G Historian ME 2.1 - Manual (Page 115)

6 • C OLLECTING D ATA
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E XCEPTION M INIMUM
The Exception Minimum (ExcMin) attribute is a deadband after the previous value.
This is used to suppress noise. It is specified in seconds. A new data value that is
received before the end of the ExcMin interval will be discarded.
E XCEPTION M AXIMUM
The Exception Maximum (ExcMax) attribute puts a limit on the length of time that
values can be discarded due to exception testing. For example, it is possible for the
incoming data to be a single value for many days. If ExcMax is set to 28800 seconds
(8 hours) then a value will not be discarded due to exception if the previous event
timestamp was more than 28800 seconds before that. Note that the Data Collection
service does not manufacture data. If there are no incoming values within 28800
seconds, then nothing will be passed to the FactoryTalk Historian ME server.
Compression Filtering
The Snapshot subsystem uses compression filtering to determine which events it
should pass to the archive for storage. The point of compression filtering is to store
just enough data to accurately reproduce the original signal.
For example, in the following illustration, all the events fall on the same straight line.
In a simple case like this, you do not actually need to store all the points on the line. If
you store just two points, you can exactly recreate the point value for any other time.
This line can be reconstructed from any two of these events, so the most efficient
storage would be to store only the first and last events (A and B) rather than storing
all the events. Furthermore, no accuracy is sacrificed. If a user wants to retrieve the
value at any point along the line, it can be interpolated from the values that have been
stored.
TIME
ENG Units