Appendix A
HMI Function Reference
©
National Instruments Corporation
A-33
Read Historical Trends
Use the Read Historical Trends VI to read the historical data for a given set of tags from a
user specified start and stop date and time, up to max points per trend. If the inputs for start
timestamp and stop timestamp are not wired, all historical data for the tags is returned, up
to the max points per trend specified. If max points per trend is left unwired, all points
between 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.
max points per 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.
Citadel path in is the path to directory containing the Citadel historical
database. If this path is empty, the VI prompts the user for the citadel folder
path.
tag names is the list of tags for which you want to read historical data. If
one or more of the tags is not logged in the historical database, you will get
an empty trend for that tag.
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.
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
Not Reported by the BridgeVIEW Engine
in this appendix.
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.
Citadel path out is the path to directory containing the historical database.
