Programming instructions

Chapter 3 Basic LabVIEW Data Acquisition Concepts
© National Instruments Corporation 3-7 LabVIEW Data Acquisition Basics Manual
Default and Current Value Conventions
To use the DAQ VIs, you should know the difference between a default
input, a default setting, and a current setting. A
default input
is the default
value of a front panel control. When you do not wire an input to a terminal
of a VI, the default input for the control associated with that terminal passes
to the driver. In the
Help
window, default inputs appear in parentheses to
the right of the parameter names. A
default setting
is a default parameter
value recorded in the driver. The
current setting
is the value of a control at
any given moment. The default setting of a control becomes the current
setting and remains so until you change the value of the control.
In many cases, a control input defaults to a certain value (most often 0),
which means you can use the current setting. For example, the default input
for a parameter may be
do not change the current setting
, and
the current setting may be
no AMUX-64T boards
. If you change the value
of such a parameter, the new value becomes the current setting.
Common DAQ VI Parameters
The
device
input on analog I/O, digital I/O, and counter VIs specifies the
number assigned to your DAQ device in the DAQ configuration software.
Your software assigns a unique number to each DAQ device. The
device
parameter usually appears as an input to the configuration VIs. Another
common configuration VI output,
task ID
, assigns your specific
I/Ooperation and device a unique number that identifies it throughout
yourprogram flow. The
task ID
can also contain group information about
the channels and gain used in your operation.
Some DAQ VIs perform either the device configuration or the
I/Ooperation, while other DAQ VIs perform both configuration and the
operation. The VIs that handle both functions have an
iteration
input.
When your VI has the
iteration
set to
0
, LabVIEW configures the
DAQdevice and then performs the specific I/O operation. For iteration
values greater than 0, LabVIEW uses the existing configuration to perform
the I/Ooperation. You can improve the performance of your application
bynot configuring the DAQ device every time an I/O operation occurs.