Programming instructions

Chapter 8 Controlling Your Acquisition with Triggers
LabVIEW Data Acquisition Basics Manual 8-10
© National Instruments Corporation
The
conditional retrieval
cluster of the AI Read VI specifies the analog
signal conditions of retrieval, as shown in Figure 8-8.
Figure 8-8.
The AI Read VI Conditional Retrieval Cluster
Note
Remember that the actual data acquisition is started by running your VI and that
the conditional retrieval just controls the returning of data already being acquired.
When acquiring data with conditional retrieval, you typically store the data
in a memory buffer, similar to hardware triggering applications. After you
start running the VI, the data is placed in the buffer. Once the retrieval
conditions have been met, the AI Read VI searches the buffer for the
desired information. As with hardware analog triggering, you specify the
analog channel of the triggering signal by specifying its
channel index,
an
index number corresponding to the relative order of a single channel in a
channel list. You also specify the
slope
(rising or falling) and the
level
of
the trigger signal.
Note
The channel index might not be equal to the channel value. The Channel to
Index.vi can be used to get the channel index for a channel. You can find this VI
in Data Acquisition»Calibration and Configuration.
The AI Read VI begins searching for the retrieval conditions in the buffer
at the read/search position, another input of the AI Read VI. The
offset
, a
value of the
conditional retrieval
input cluster, is where you specify the
scan locations from which the VI begins reading data relative to the
read/search position. A negative
offset
indicates data prior to the retrieval
condition pretrigger data, and a positive
offset
indicates data after the
retrieval condition posttrigger data. The
skip count
input is where you
specify the number of times the trigger conditions are met. The
hysteresis
input is where you specify the range you will use to meet retrieval
conditions. Once the
slope
and
level
conditions on
channel index
have
been found, the read/search position indicates the location where the
retrieval conditions were met.