Figure 4-9. posttriggered data acquisition example, Figure 4-10. pretriggered data acquisition example – National Instruments DAQ M Series User Manual
Page 38
Chapter 4
Analog Input
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Figure 4-9. Posttriggered Data Acquisition Example
Pretriggered data acquisition allows you to view data that is acquired before
the trigger of interest, in addition to data acquired after the trigger.
Figure 4-10 shows a typical pretriggered DAQ sequence. ai/StartTrigger
can be either a hardware or software signal. If ai/StartTrigger is set up to be
a software start trigger, an output pulse appears on the ai/StartTrigger line
when the acquisition begins. When the ai/StartTrigger pulse occurs, the
sample counter is loaded with the number of pretriggered samples, in this
example, four. The value decrements with each pulse on ai/SampleClock,
until the value reaches zero. The sample counter is then loaded with the
number of posttriggered samples, in this example, three.
Figure 4-10. Pretriggered Data Acquisition Example
If an ai/ReferenceTrigger pulse occurs before the specified number of
pretrigger samples are acquired, the trigger pulse is ignored. Otherwise,
when the ai/ReferenceTrigger pulse occurs, the sample counter value
decrements until the specified number of posttrigger samples have been
acquired.
1
3
0
4
2
ai/StartTrigger
ai/SampleClock
ai/ConvertClock
Sample Counter
n/a
0
1
2
3
1
0
2
2
2
ai/StartTrigger
ai/ReferenceTrigger
ai/SampleClock
ai/ConvertClock
Scan Counter