National Instruments Data Acquisition Device E Series - Manual (Page 84)

Chapter 3 Analog Output
E Series User Manual 3-4 ni.com
AO Data Generation Methods
When performing an analog output operation, there are several different
data generation methods available. You can either perform software-timed
or hardware-timed generations. Hardware-timed generations can be
non-buffered or buffered.
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.
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
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. Refer to Chapter 9,
Bus Interface ,
for more information on data transfer methods.
One property of buffered I/O operations is the sample mode. The sample
mode can be either finite or continuous.