Analog Input Data Acquisition Methods - National Instruments DAQ X Series User Manual

Chapter 4 Analog Input

Analog Input Data Acquisition Methods

When performing analog input measurements, you either can perform
software-timed or hardware-timed acquisitions.
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Note (NI USB-6356/6366 Devices)
pairs, as opposed to single samples. This implementation allows for greater data
throughput. However, if an acquisition on these devices acquires an odd number of total
samples, the last sample acquired cannot be transferred.
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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:
– 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 hardware-timed single
point (HWTSP). A buffer is a temporary storage in computer memory
for to-be-transferred samples.
– 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.
Some X Series devices internally transfer data in sample
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