Software-Timed Generations; Hardware-Timed Generations - National Instruments NI 6711 User Manual

Chapter 3 Analog Output
Analog Output Series User Manual

Software-Timed Generations

With a software-timed generation, software controls the rate at which data
is generated. Software sends a separate command to the hardware to initiate
each DAC conversion. In NI-DAQmx, software-timed generations are
referred to as On Demand timing. Software-timed generations are also
referred to as immediate or static operations. They are typically used for
writing a single value out, such as a constant DC voltage.

Hardware-Timed Generations

With a hardware-timed generation, a digital hardware signal controls the
rate of the generation. This signal can be generated internally on your
device or provided externally.
Hardware-timed generations have several advantages over software-timed
generations:
• The time between samples can be much shorter.
• The timing between samples can be deterministic.
• Hardware-timed generations can use hardware triggering. For more
information, refer to Chapter 10, Triggering.
Hardware-timed operations can be buffered or non-buffered. A buffer is a
temporary storage in computer memory for acquired or to-be-generated
samples.
Buffered
In a buffered generation, 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 generations typically allow for much
faster transfer rates than non-buffered generations because data is moved in
large blocks, rather than one point at a time. For more information on DMA
and interrupt requests, refer to the
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 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.
Data Transfer Methods
3-4
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