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Chapter 5 Analog Output
© National Instruments Corporation 5-3 NI 6238/6239 User Manual
Non-Buffered
In non-buffered acquisitions, data is written directly to the DACs on the
device. Typically, hardware-timed, non-buffered operations are used to
write single samples with good latency and known time increments
between them.
Buffered
In a buffered acquisition, data is moved from a PC buffer to the DAQ
device’s onboard FIFO using DMA or interrupts before it is written to the
DACs one sample at a time. 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 generation refers to the generation of a specific,
predetermined number of data samples. After the specified number of
samples has been written out, the generation stops.
Continuous generation refers to the generation of an unspecified number of
samples. Instead of generating a set number of data samples and stopping,
a continuous generation continues until you stop the operation. There are
several different methods of continuous generation that control what data is
written. These methods are regeneration, FIFO regeneration, and
non-regeneration modes.
Regeneration is the repetition of the data that is already in the buffer.
Standard regeneration is when data from the PC buffer is continually
downloaded to the FIFO to be written out. New data can be written to the
PC buffer at any time without disrupting the output.
With FIFO regeneration, the entire buffer is downloaded to the FIFO and
regenerated from there. After the data is downloaded, new data cannot be
written to the FIFO. To use FIFO regeneration, the entire buffer must fit
within the FIFO size. The advantage of using FIFO regeneration is that it
does not require communication with the main host memory after the
operation is started, thereby preventing any problems that may occur due to
excessive bus traffic.
With non-regeneration, old data will not be repeated. New data must be
continually written to the buffer. If the program does not write new data to