Chapter 5
Analog Output
X Series User Manual
5-4
ni.com
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.
•
Buffered
—In a b
u
ffered generation, data is moved from a PC b
u
ffer to
the DAQ device’s onboard FIFO
u
sing DMA before it is written to the
DACs one sample at a time. B
u
ffered generation typically allow for
m
u
ch faster transfer rates than non-b
u
ffered 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 generation refers to the generation of a
specific, predetermined n
u
mber of data samples. Once the
specified n
u
mber of samples has been written o
u
t, the generation
stops.
–
Contin
u
o
u
s generation refers to the generation of an
u
nspecified
n
u
mber of samples. Instead of generating a set n
u
mber of data
samples and stopping, a contin
u
o
u
s generation contin
u
es
u
ntil yo
u
stop the operation. There are several different methods of
contin
u
o
u
s generation that control what data is written. These
methods are regeneration, FIFO regeneration and
non-regeneration modes:
•
Regeneration is the repetition of the data that is already in the
b
u
ffer. Standard regeneration is when data from the PC b
u
ffer
is contin
u
ally downloaded to the FIFO to be written o
u
t. New
data can be written to the PC b
u
ffer at any time witho
u
t
disr
u
pting the o
u
tp
u
t. Use the NI-DAQmx write property
RegenMode to allow (or not allow) regeneration. The
NI-DAQmx defa
u
lt is to allow regeneration.
•
With FIFO regeneration, the entire b
u
ffer is downloaded to
the FIFO and regenerated from there. Once the data is
downloaded, new data cannot be written to the FIFO. To
u
se
FIFO regeneration, the entire b
u
ffer m
u
st fit within the FIFO
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