Chapter 5
Analog Output
©
National Instruments Corporation
5-3
–
Hardware-timed acq
u
isitions can
u
se hardware triggering.
Hardware-timed operations are b
u
ffered. D
u
ring hardware-timed AO
generation, data is moved from a PC b
u
ffer to the onboard FIFO on
the USB-621
x
device
u
sing USB Signal Streams before it is written to
the DACs one sample at a time. B
u
ffered acq
u
isitions allow for fast
transfer rates beca
u
se data is moved in large blocks rather than one
point at a time.
One property of b
u
ffered I/O operations is the sample mode. The
sample mode can be either finite or contin
u
o
u
s.
Finite sample mode generation refers to the generation of a specific,
predetermined n
u
mber of data samples. Once the specified n
u
mber of
samples has been written o
u
t, the generation stops.
Contin
u
o
u
s generation refers to the generation of an
u
nspecified
n
u
mber of samples. Instead of generating a set n
u
mber of data samples
and stopping, a contin
u
o
u
s generation contin
u
es
u
ntil yo
u
stop the
operation. There are several different methods of contin
u
o
u
s
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 b
u
ffer.
Standard regeneration is when data from the PC b
u
ffer is contin
u
ally
downloaded to the FIFO to be written o
u
t. New data can be written to
the PC b
u
ffer at any time witho
u
t disr
u
pting the o
u
tp
u
t.
With FIFO regeneration, the entire b
u
ffer is downloaded to the FIFO
and regenerated from there. Once the data is downloaded, new data
cannot be written to the FIFO. To
u
se FIFO regeneration, the entire
b
u
ffer m
u
st fit within the FIFO size. The advantage of
u
sing FIFO
regeneration is that it does not req
u
ire comm
u
nication with the main
host memory once the operation is started, thereby preventing any
problems that may occ
u
r d
u
e to excessive b
u
s traffic.
With non-regeneration, old data is not repeated. New data m
u
st be
contin
u
ally written to the b
u
ffer. If the program does not write new
data to the b
u
ffer at a fast eno
u
gh rate to keep
u
p with the generation,
the b
u
ffer
u
nderflows and ca
u
ses an error.
Analog Output Digital Triggering
Analog o
u
tp
u
t s
u
pports two different triggering actions:
•
Start trigger
•
Pa
u
se trigger
