Chapter 4
Analog Input
X Series User Manual
4-44
ni.com
Analog Input Data Acquisition Methods
When performing analog inp
u
t meas
u
rements, yo
u
either can perform
software-timed or hardware-timed acq
u
isitions.
•
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.
–
B
u
ffered—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.
Note
(NI USB-6356/6366 Devices)
Some X Series devices internally transfer data in sample
pairs, as opposed to single samples. This implementation allows for greater data
thro
u
ghp
u
t. However, if an acq
u
isition on these devices acq
u
ires an odd n
u
mber of total
samples, the last sample acq
u
ired cannot be transferred.
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