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Appendix A
HMI Function Reference
© National Instruments Corporation
A-31
Read Historical Trend
Use the Read Historical Trend VI to read the historical data for a given tag from user specified
start and stop dates and times, up to the maximum number of points specified. If start
timestamp and stop timestamp are not wired, all historical data for the tag is returned, up to
the maximum points per trend specified. If max points per trend is left unwired, all points
between the start timestamp and stop timestamp are returned.
start and stop dates and times, up to the maximum number of points specified. If start
timestamp and stop timestamp are not wired, all historical data for the tag is returned, up to
the maximum points per trend specified. If max points per trend is left unwired, all points
between the start timestamp and stop timestamp are returned.
You can use this VI to read history information for analog, discrete or bit array tags. All
values are returned as floating point values.
values are returned as floating point values.
max points in trend is the maximum number of points to read. If the value
is less than zero, all points available between start timestamp and stop
timestamp are returned. Otherwise, the number of points in the trend is the
minimum of the actual number of data points between start timestamp,
stop timestamp, and max points in trend.
is less than zero, all points available between start timestamp and stop
timestamp are returned. Otherwise, the number of points in the trend is the
minimum of the actual number of data points between start timestamp,
stop timestamp, and max points in trend.
Citadel path in is the path to the directory containing the Citadel historical
database. If this path is empty, the VI attempts to use the historical data
directory configured in the active
database. If this path is empty, the VI attempts to use the historical data
directory configured in the active
.scf
file. If this has not been configured,
the VI prompts you to select a data directory.
tag name is the tag for which you want to read historical data. If the tag is
not logged in the historical database, you will get an empty trend.
not logged in the historical database, you will get an empty trend.
start timestamp is the date and time associated with the first data point to
be retrieved from the historical database. If this input is unwired, the data
is extracted starting at the first point available for the tag.
be retrieved from the historical database. If this input is unwired, the data
is extracted starting at the first point available for the tag.
error in (no error) is a cluster that describes the error status before this VI
executes. For more information about this control, see the section
executes. For more information about this control, see the section
in this appendix.
stop timestamp (now) is the date and time associated with the last data
point to be retrieved from the historical database. If this input is unwired,
the data is extracted up to the last point available for the tag.
point to be retrieved from the historical database. If this input is unwired,
the data is extracted up to the last point available for the tag.
Citadel path out is the path to directory containing historical data files.