Chapter 4
Analog Input
X Series User Manual
4-10
ni.com
Software-Timed Acquisitions
With a software-timed acq
u
isition, software controls the rate of the
acq
u
isition. Software sends a separate command to the hardware to initiate
each ADC conversion. In NI-DAQmx, software-timed acq
u
isitions are
referred to as having on-demand timing. Software-timed acq
u
isitions are
also referred to as immediate or static acq
u
isitions and are typically
u
sed
for reading a single sample of data.
Hardware-Timed Acquisitions
With hardware-timed acq
u
isitions, a digital hardware signal (AI Sample
Clock) controls the rate of the acq
u
isition. This signal can be generated
internally on yo
u
r device or provided externally.
Hardware-timed acq
u
isitions have several advantages over software-timed
acq
u
isitions:
•
The time between samples can be m
u
ch shorter.
•
The timing between samples is deterministic.
•
Hardware-timed acq
u
isitions can
u
se hardware triggering.
Hardware-timed operations can be b
u
ffered or hardware-timed single point
(HWTSP). A b
u
ffer is a temporary storage in comp
u
ter memory for
to-be-transferred samples.
•
Buffered
—In a b
u
ffered acq
u
isition, data is moved from the DAQ
device’s onboard FIFO memory to a PC b
u
ffer
u
sing DMA before it is
transferred to application memory. B
u
ffered acq
u
isitions typically
allow for m
u
ch faster transfer rates than HWTSP acq
u
isitions beca
u
se
data is moved in large blocks, rather than one point at a time.
One property of b
u
ffered I/O operations is the sample mode. The
sample mode can be either finite or contin
u
o
u
s:
–
Finite sample mode acq
u
isition refers to the acq
u
isition of a
specific, predetermined n
u
mber of data samples. Once the
specified n
u
mber of samples has been read in, the acq
u
isition
stops. If yo
u
u
se a reference trigger, yo
u
m
u
st
u
se finite sample
mode.
–
Contin
u
o
u
s acq
u
isition refers to the acq
u
isition of an
u
nspecified
n
u
mber of samples. Instead of acq
u
iring a set n
u
mber of data
samples and stopping, a contin
u
o
u
s acq
u
isition contin
u
es
u
ntil
yo
u
stop the operation. Contin
u
o
u
s acq
u
isition is also referred to
as do
u
ble-b
u
ffered or circ
u
lar-b
u
ffered acq
u
isition.
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