Chapter 4
Analog Input
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National Instruments Corporation
4-13
Analog Input Timing Signals
An acquisition with posttrigger data allows you to view data that is
acquired after a trigger event is received. A typical posttrigger DAQ
sequence is shown in Figure 4-7. The sample counter is loaded with the
specified number of posttrigger samples, in this example, five. The value
decrements with each pulse on AI Sample Clock (ai/SampleClock), until
the value reaches zero and all desired samples have been acquired.
Figure 4-7.
Typical Posttriggered DAQ Sequence
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.
Figure 4-8 shows a typical pretrigger DAQ sequence. The AI Start Trigger
signal (ai/StartTrigger) can be either a hardware or software signal. If
AI Start Trigger is set up to be a software start trigger, an output pulse
appears on the AI START TRIG line when the acquisition begins. When the
AI Start Trigger pulse occurs, the sample counter is loaded with the number
of pretrigger samples, in this example, four. The value decrements with
each pulse on AI Sample Clock, until the value reaches zero. The sample
counter is then loaded with the number of posttrigger samples, in this
example, three.
Figure 4-8.
Typical Pretriggered DAQ Sequence
1
3
0
4
2
AI Start Trigger
AI Sample Clock
Sample Counter
Don't Care
0
1
2
3
1
0
2
2
2
AI Start Trigger
AI Reference Trigger
AI Sample Clock
Sample Counter
