National Instruments NI 447 Series, Руководство пользователя

Страница: 47 / 80

Chapter 3 Device Overview and Theory of Operation
© National Instruments Corporation 3-17 NI 447X User Manual
Figure 3-14. NI 447 X Modules with Shared Oversample Clock Using the SYNC Pulse
The NI 4472 cannot acquire data while it is in the reset state that occurs
immediately after it receives the SYNC pulse. During this period, the ADC
returns only zeros. The length of the reset period t
reset
(in samples) is a
function of sample rate. Table 3-2 shows the possible lengths of the reset
period.
Note The SYNC pulse is automatically handled by NI-DAQ through the RTSI 5/TRIG 5
line. Therefore, you do not need to make an explicit software call to generate or route the
SYNC pulse.
The Acquisition Start Trigger
After sharing the oversampling clock and issuing the SYNC pulse, the
ADCs on every NI 447
X in the system run in lock-step. At this point, the
only remaining task is to synchronize the beginning of the data acquisition
on each NI 447
X . You can choose RTSI <0..4>/TRIG <0..4> for the start
trigger.
Table 3-2. Reset Period as a Function of Sample Rate
Sample Rate Reset Period (t
reset
)
f
s
≤
51.2 kS/s t
reset
= 8,960 samples
f
s
> 51.2 kS/s t
reset
= 17,920 samples
NI 4472 #1
NI 4472 #2
Data Returned
Data Returned