National Instruments Data Acquisition Device NI USB-621x - Manual (Page 74)

Chapter 5 Analog Output
© National Instruments Corporation 5-3 NI USB-621x User Manual
– Hardware-timed acquisitions can use hardware triggering.
Hardware-timed operations are buffered. During hardware-timed AO
generation, data is moved from a PC buffer to the onboard FIFO on
the USB-621x device using USB Signal Streams before it is written to
the DACs one sample at a time. Buffered acquisitions allow for fast
transfer rates 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. Once 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. Once 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 once the operation is started, thereby preventing any
problems that may occur due to excessive bus traffic.
With non-regeneration, old data is not repeated. New data must be
continually written to the buffer. If the program does not write new
data to the buffer at a fast enough rate to keep up with the generation,
the buffer underflows and causes an error.
Analog Output Digital Triggering
Analog output supports two different triggering actions:
• Start trigger
• Pause trigger