Chapter 4 Analog Input
© National Instruments Corporation 4-11 M Series User Manual
switches between channels much less often and is affected much less by
settling time.
Analog Input Data Acquisition Methods
When performing analog input measurements, you either can perform
software-timed or hardware-timed acquisitions. Hardware-timed
acquisitions can be buffered or non-buffered.
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/SampleClock) 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.
• The time between samples can be much shorter.
• The timing between samples is deterministic.
• Hardware-timed acquisitions can use hardware triggering.
Hardware-timed operations can be buffered or non-buffered. A buffer is a
temporary storage in computer memory for to-be-generated samples.
Buffered
In a buffered acquisition, data is moved from the DAQ device’s onboard
FIFO memory to a PC buffer using DMA or interrupts before it is
transferred to application memory. Buffered acquisitions typically allow
for much faster transfer rates than non-buffered 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.
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