Chapter 6 Digital I/O
X Series User Manual 6-14 ni.com
Digital Output Data Generation Methods
When performing a digital waveform operation, you 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 update. 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 used for writing a single
value out, such as a constant digital value.
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 your
device or provided externally.
Hardware-timed generations have several advantages over software-timed
generations:
• The time between samples can be much shorter.
• The timing between samples can be deterministic.
• Hardware-timed acquisitions can use hardware triggering.
Hardware-timed operations can be buffered or hardware-timed single point
(HWTSP). A buffer is a temporary storage in computer memory for
to-be-transferred samples.
• Hardware-timed single point (HWTSP)—Typically, HWTSP
operations are used to write single samples at known time intervals.
While buffered operations are optimized for high throughput, HWTSP
operations are optimized for low latency and low jitter. In addition,
HWTSP can notify software if it falls behind hardware. These features
make HWTSP ideal for real time control applications. HWTSP
operations, in conjunction with the wait for next sample clock
function, provide tight synchronization between the software layer and
the hardware layer. Refer to the NI Developer Zone document,
NI-DAQmx Hardware-Timed Single Point Lateness Checking, for
more information. To access this document, go to
ni.com/info and
enter the Info Code
daqhwtsp.
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