National Instruments PXI-6070E, Руководство пользователя

Страница: 48 / 265

Chapter 2 Analog Input
E Series User Manual 2-14 ni.com
Buffered
In a buffered acquisition, data is moved from the DAQ device 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, refer to the
Data Transfer Methods section of Chapter 9, 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. Once 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.
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 good latency and known time increments between
them.
Analog Input Triggering
Analog input supports three different triggering actions: start, reference,
and pause. An analog
or digital hardware trigger can initiate these actions.
All E Series devices support digital triggering, and some also support
analog triggering. Refer to Chapter 10,
Triggering , for more information
on analog and digital triggering. Refer to Appendix A, Device-Specific
Information , to find your device triggering options.