Software-Timed Acquisitions; Hardware-Timed Acquisitions - National Instruments DAQ X Series User Manual

Chapter 4 Analog Input

Software-Timed Acquisitions

With a software-timed acquisition, software controls the rate of the
acquisition. Software sends a separate command to the hardware to initiate
each ADC conversion. In NI-DAQmx, software-timed acquisitions are
referred to as having on-demand timing. Software-timed acquisitions are
also referred to as immediate or static acquisitions and are typically used
for reading a single sample of data.

Hardware-Timed Acquisitions

With hardware-timed acquisitions, a digital hardware signal (AI Sample
Clock) controls the rate of the acquisition. This signal can be generated
internally on your device or provided externally.
Hardware-timed acquisitions have several advantages over software-timed
acquisitions:
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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.
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The time between samples can be much shorter.
The timing between samples is deterministic.
Hardware-timed acquisitions can use hardware triggering.
Buffered—In a buffered acquisition, data is moved from the DAQ
device's onboard FIFO memory to a PC buffer using DMA before it is
transferred to application memory. Buffered acquisitions typically
allow for much faster transfer rates than HWTSP acquisitions because
data is moved in large blocks, rather than one point at a time.
One property of buffered I/O operations is the sample mode. The
sample mode can be either finite or continuous:
– Finite sample mode acquisition refers to the acquisition of a
specific, predetermined number of data samples. Once the
specified number of samples has been read in, the acquisition
stops. If you use a reference trigger, you must use finite sample
mode.
– Continuous acquisition refers to the acquisition of an unspecified
number of samples. Instead of acquiring a set number of data
samples and stopping, a continuous acquisition continues until
you stop the operation. Continuous acquisition is also referred to
as double-buffered or circular-buffered acquisition.
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