Chapter 6
Digital I/O
X Series User Manual
6-14
ni.com
Digital Output Data Generation Methods
When performing a digital waveform operation, yo
u
either can perform
software-timed or hardware-timed generations.
Software-Timed Generations
With a software-timed generation, software controls the rate at which data
is generated. Software sends a separate command to the hardware to initiate
each
u
pdate. In NI-DAQmx, software-timed generations are referred to as
on-demand timing. Software-timed generations are also referred to as
immediate or static operations. They are typically
u
sed for writing a single
val
u
e o
u
t, s
u
ch as a constant digital val
u
e.
Hardware-Timed Generations
With a hardware-timed generation, a digital hardware signal controls the
rate of the generation. This signal can be generated internally on yo
u
r
device or provided externally.
Hardware-timed generations have several advantages over software-timed
generations:
•
The time between samples can be m
u
ch shorter.
•
The timing between samples can be 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.
•
Hardware-timed single point (HWTSP)
—Typically, HWTSP
operations are
u
sed to write 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
.
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