Chapter 6
Digital I/O
©
National Instruments
6-3
X Series User Manual
Each of the X Series DIO lines can be
u
sed as a static DI or DO line. Yo
u
can
u
se static DIO lines to monitor or control digital signals. Each DIO can
be individ
u
ally config
u
red as a digital inp
u
t (DI) or digital o
u
tp
u
t (DO).
All samples of static DI lines and
u
pdates of static DO lines are
software-timed.
Hardware-Timed Acquisitions
With hardware-timed acq
u
isitions, a digital hardware signal
(di/SampleClock) controls the rate of the acq
u
isition. This signal can be
generated internally on yo
u
r device or provided externally.
Hardware-timed acq
u
isitions have several advantages over software-timed
acq
u
isitions.
•
The time between samples can be m
u
ch shorter.
•
The timing between samples is deterministic.
•
Hardware-timed acq
u
isitions can
u
se hardware triggering.
Hardware-timed operations can be b
u
ffered or hardware-timed single
point. A b
u
ffer is a temporary storage in comp
u
ter memory for
to-be-transferred samples.
•
Buffered
—Data is moved from the DAQ device’s onboard FIFO
memory to a PC b
u
ffer
u
sing DMA before it is transferred to
application memory. B
u
ffered acq
u
isitions typically allow for m
u
ch
faster transfer rates than non-b
u
ffered acq
u
isitions 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 acq
u
isition refers to the acq
u
isition of a
specific, predetermined n
u
mber of data samples. Once the
specified n
u
mber of samples has been read in, the acq
u
isition
stops. If yo
u
u
se a reference trigger, yo
u
m
u
st
u
se finite sample
mode.
–
Contin
u
o
u
s acq
u
isition refers to the acq
u
isition of an
u
nspecified
n
u
mber of samples. Instead of acq
u
iring a set n
u
mber of data
samples and stopping, a contin
u
o
u
s acq
u
isition contin
u
es
u
ntil
yo
u
stop the operation. Contin
u
o
u
s acq
u
isition is also referred to
as do
u
ble-b
u
ffered or circ
u
lar-b
u
ffered acq
u
isition.
If data cannot be transferred across the b
u
s fast eno
u
gh, the FIFO
becomes f
u
ll. New acq
u
isitions will overwrite data in the FIFO
before it can be transferred to host memory. The device generates
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