Programming instructions

Chapter 8 Controlling Your Acquisition with Triggers
LabVIEW Data Acquisition Basics Manual 8-8
© National Instruments Corporation
trigger channel
where the analog triggering signal will be connected
as well as the
trigger level
on the triggering signal needed to begin
acquisition. In other words, once you specify the channel of the triggering
signal, LabVIEW will wait until the slope and trigger level conditions are
met before starting a buffered acquisition. If you use channel names
configured in the DAQ Channel Wizard,
trigger level
is treated as being
relative to the physical units specified for the channel in the DAQ Channel
Wizard. Otherwise,
trigger level
is treated as volts.
The Acquire N Scans Analog Hardware Trig VI example, located in
labview\examples\daq\anlogin\anlogin.llb
, holds the data in a
memory buffer until the device completes data acquisition. The number
of data points you want to acquire must be small enough to fit in memory.
This VI only views and processes the information after the acquisition.
If you need to view and process information during the acquisition, use the
Acquire & Proc N Scans-Trig VI, located in
labview\examples\daq\
anlogin\anlogin.llb
. If you expect multiple analog trigger signals that
will start multiple acquisitions, use the example Acquire N-Multi-Analog
Hardware Trig VI, located in
labview\examples\daq\anlogin\
anlogin.llb
.
Software Triggering
With software triggering, you can simulate an analog trigger using
software. This form of triggering is often used in situations where hardware
triggers are not available. Another name for software triggering signals,
specifically analog signals, is
conditional retrieval
. With conditional
retrieval, you set up your DAQ device to collect data, but the device does
not return any data to LabVIEW unless the data meets your retrieval
conditions. LabVIEW scans the input data and performs a comparison with
the conditions, but does not store the data until it meets your specifications.
Figure 8-7 shows a timeline of events that typically occur when you
perform conditional retrieval.
The read/search position pointer traverses the buffer until it finds the scan
location where the data has met the retrieval conditions. Offset indicates
the scan location from which the VI begins reading data relative to the
read/search position. A negative offset indicates that you need pretrigger
data (data prior to the retrieval conditions). If offset is greater than 0, you
need posttrigger data (data after retrieval conditions).