National Instruments Network Device DAQ S - Manual (Page 32)

Chapter 4 Analog Input
© National Instruments Corporation 4-5 NI 6124/6154 User Manual
Hardware-timed acquisitions have several advantages over
software-timed acquisitions:
– The time between samples can be much shorter.
– The timing between samples can be deterministic.
– Hardware-timed acquisitions can use hardware triggering. For
more information, refer to Chapter 11,
Triggering .
Hardware-timed operations can be buffered or non-buffered. A buffer
is a temporary storage in the computer memory where acquired
samples are stored.
– 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 ADE 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. For more information about DMA
and interrupts, refer to the
Data Transfer Methods section of
Chapter 10, Bus Interface .
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 acquisitions of a
specific, predetermined number of data samples. After the
specified number of samples has been collected into the buffer, the
acquisition stops. If you use a reference trigger, you must use
finite sample mode. Refer to the
AI Reference Trigger Signal
section for more information.
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. A continuous acquisition is also referred
to as double-buffered or circular-buffered acquisition.
If data cannot be transferred across the bus fast enough, the data
in the FIFO will be overwritten and an error will be generated.
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.
– Non-Buffered—In non-buffered acquisitions, data is read directly
from the FIFO on the device. Typically, hardware-timed
non-buffered operations are used to read single samples with
known time increments between them and small latency.