Programming instructions

Chapter 9 Letting an Outside Source Control Your Acquisition Rate
LabVIEW Data Acquisition Basics Manual 9-2
© National Instruments Corporation
Some DAQ devices do not have scan clocks, but rather use
round-robin
scanning
. Figure 9-2 shows an example of round-robin scanning.
Figure 9-2.
Round-Robin Scanning Using the Channel Clock
The devices that always perform round-robin scanning include, but are not
limited to, the following:
•NB-MIO-16
•PC-LPM-16
• PC-LPM-16PnP
• PC-516
•DAQCard-500
•DAQCard-516
•DAQCard-700
• Lab-NB, Lab-SE
•Lab-LC
With no scan clock, the channel clock is used to switch between each
channel at an equal interval. The same delay exists between all channel
samples, as well as between the last channel of a scan and the first channel
in the next scan. (For boards with scan and channel clocks, round-robin
scanning occurs when you disable the scan clock by setting the scan rate to
zero and using the
interchannel delay
of the AI Config VI to control your
acquisition rate.)
Finally, remember that LabVIEW is
scan-clock oriented
. In other words,
when you select a scan rate, LabVIEW automatically selects the channel
clock rate for you. LabVIEW selects the fastest channel clock rate that
allows adequate settling time for the Analog-to-Digital Converter (ADC).
LabVIEW adds an extra 10-
µ
s to the interchannel delay to compensate for
any unaccounted factors. However, LabVIEW does not consider this
additional delay for purposes of warnings. If you have specified a scan rate
that is adequate for acquisition but too fast for LabVIEW to apply the
10-
µ
s delay, it configures the acquisition but does not return a warning.
channel interval
012301230123