National Instruments NI 6232 User Manual download pdf (Page 37)

Chapter 4 Analog Input

NI 6232/6233 User Manual 4-10 ni.com

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 onboard FIFO memory of

a DAQ device 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.

Finite sample mode acquisition refers to the acquisition of a specific,

predetermined number of data samples. When the specified number of

samples has been written out, the generation 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.

If data cannot be transferred across the bus fast enough, the FIFO will

become full. New acquisitions will overwrite data in the FIFO before it can

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National Instruments NI 6232 User Manual Page 37