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Analog Input Data Acquisition Methods
When performing analog input measurements, you either can perform software-timed or
hardware-timed acquisitions.
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.
–
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 becomes full. New
acquisitions overwrite data in the FIFO before it can be transferred to host memory,
which causes the device to generate an error. With continuous operations, if the user
program does not read data out of the PC buffer fast enough to keep up with the data
transfer, the buffer could reach an overflow condition, causing an error to be
generated.
X Series User Manual
© National Instruments | 4-9
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