© National Instruments
|
3-7
The timing engine ignores the AI Start Trigger signal while the clock generation is in progress.
After the clock generation is finished, the counter waits for another Start Trigger to begin another
clock generation. Figure 3-4 shows a retriggerable analog input with four samples per trigger.
Figure 3-4.
Retriggera
b
le Analog Input
Note
Waveform information from LabVIEW will not reflect the delay between
triggers. They will be treated as a continuous acquisition with constant t0 and dt
information.
Reference triggers are retriggerable.
AI Reference Trigger Signal
A reference trigger can be used to acquire samples before and after a trigger event. This type of
acquisition is sometimes referred to as a pretriggered acquisition. An acquisition with pretrigger
data allows you to view data that is acquired before the trigger of interest, in addition to data
acquired after the trigger. The trigger of interest that transitions the device from taking pretrigger
data to taking posttrigger data is the AI Reference Trigger (ai/ReferenceTrigger) signal.
Figure 3-5 shows a typical pretriggered DAQ sequence. In the sequence, when the AI Start
Trigger pulse occurs, the sample counter is loaded with the number of pretrigger samples, which
is four in this example. The value decrements with each pulse on AI Sample Clock, until the
value reaches zero. The sample counter is then immediately loaded with the number of
posttrigger samples, which is three in this example. The sample counter for posttrigger samples
does not start decrementing until the AI Reference Trigger pulse has occurred.
Figure 3-5.
Typical Pretriggered DAQ Sequence
AI
S
t
a
rt Trigger
AI
Sa
mple Clock
1
1
3
4
2
0
3
3
2
AI Start Trigger
AI Reference Trigger
AI Sample Clock
Sample Counter
Don't Care
