Programming instructions

© National Instruments Corporation 9-1 LabVIEW Data Acquisition Basics Manual
9
Letting an Outside Source
Control Your Acquisition Rate
Typically, a data acquisition (DAQ) device uses internal counters to
determine the rate to acquire data, but sometimes you might need to capture
your data at the rate of particular signals in your system. For example,
you can also read temperature channels every time a pulse occurs which
represents pressure rising above a certain level. In this case, internal
counters are inefficient for your needs. You must control your acquisition
rate by some other, external source.
You can compare a scan of your channels to taking a snapshot of the
voltages on your analog input channels. If you set your scan rate to 10 scans
per second, you are taking 10 snapshots each second of all the channels in
your channel list. In this case, an internal clock within your device (the scan
clock) sets the scan rate, which controls the time interval between scans.
Also, remember that most DAQ devices (those that do not sample
simultaneously) proceed from one channel to the next depending on the
channel clock rate. Therefore, the channel clock is the clock controlling
the time interval between individual channel samples within a scan, which
means the channel clock proceeds at a faster rate than the scan clock.
The faster the channel clock rate, the more closely in time your system
samples the channels within each scan, as shown in Figure 9-1.
Note
For devices with both a scan and channel clock, lowering the scan rate does not
change the channel clock rate.
Figure 9-1.
Channel and Scan Intervals Using the Channel Clock
channel interval
0123 0123 0123
scan interval