National Instruments AT-MIO-16X - Manual (Page 77)

Chapter 3 Theory of Operation
© National Instruments Corporation 3-11 AT-MIO-16X User Manual
In this sequence, the sample-interval counter, Counter 3, is programmed
to generate conversion signals only under a certain gating signal, such
as the DAQPROG signal. In addition, the sample counter, Counter 4, is
programmed to count the number of conversions generated. In this case,
the sample counter is programmed to count 10 samples, then stop the
acquisition sequence. A signal is generated at the end of the sequence
to indicate its completion. An interrupt request can be generated from
this signal if desired. Because the sample counter begins counting
immediately after the application of the trigger, this is a posttrigger
sequence. If samples are necessary before and after the trigger, then
a pretrigger sequence is needed. This sequence is described in the
following paragraphs.
Figure 3-6 depicts a pretrigger data acquisition sequence. It is called
a pretrigger sequence because the first trigger initiates the
sample-interval timer without enabling the sample counter.
Conversions occur after this initial trigger and are stored in the ADC
FIFO for later retrieval in the same way they are for a posttrigger
sequence. After a second trigger is received, the sample counter begins
counting conversions. In this example, there are three pretrigger
samples, and seven posttrigger samples. Only the number of posttrigger
samples is programmable.
Figure 3-6. Single-Channel Pretrigger Data Acquisition Timing
The pretrigger sequence is programmed in much the same way as a
posttrigger sequence. The sample-interval timer is programmed to
generate conversion pulses under a gate signal, and the sample counter
is programmed to count the number of conversions. The only difference
between pretrigger and posttrigger sequences for all data acquisition
modes is that the sample counter waits for a gating signal in the
pretrigger mode before beginning the count. For posttrigger sequences,
Trigger*
DAQPROG
DAQCMPLT
DAQCLEAR*
Interrupt
CONVERT
6 Sample CTR 5 4 3 2 1 7 6
Samp CTR Gate