Chapter 4
Analog Input
©
National Instruments
4-45
X Series User Manual
To ens
u
re this condition never occ
u
rs, NI-DAQmx adds a backgro
u
nd channel for finite
acq
u
isitions that have both an odd n
u
mber of channels and an odd n
u
mber of
samples-per-channel. The backgro
u
nd channel is also added when performing any
reference-triggered finite acq
u
isition. Data from the backgro
u
nd channel is only visible
when reading in RAW mode.
For maxim
u
m efficiency in b
u
s bandwidth and onboard FIFO
u
se,
u
se an even n
u
mber of
samples-per-channel or an even n
u
mber of channels for finite acq
u
isitions, so the
backgro
u
nd channel is not added.
•
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.
If data cannot be transferred across the b
u
s fast eno
u
gh, the
FIFO becomes f
u
ll. New acq
u
isitions overwrite data in the
FIFO before it can be transferred to host memory. The device
generates an error in this case. With contin
u
o
u
s operations, if
the
u
ser program does not read data o
u
t of the PC b
u
ffer fast
eno
u
gh to keep
u
p with the data transfer, the b
u
ffer co
u
ld
reach an overflow condition, ca
u
sing an error to be generated.
–
Hardware-timed single point (HWTSP)—Typically, HWTSP
operations are
u
sed to read single samples at known time intervals.
While b
u
ffered operations are optimized for high thro
u
ghp
u
t,
HWTSP operations are optimized for low latency and low jitter. In
addition, HWTSP can notify software if it falls behind hardware.
These feat
u
res make HWTSP ideal for real time control
applications. HWTSP operations, in conj
u
nction with the wait for
next sample clock f
u
nction, provide tight synchronization
between the software layer and the hardware layer. Refer to the
NI Developer Zone doc
u
ment,
NI-DAQmx Hardware-Timed
Single Point Lateness Checking
, for more information. To access
this doc
u
ment, go to
ni.com/info
and enter the Info Code
daqhwt
s
p
.
Note
(NI USB-634
x
/635
x
/636
x
Devices)
X Series USB devices do
not
s
u
pport
hardware-timed single point (HWTSP) operations.
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