Main loop, Main loop -16 – National Instruments NI-DNET User Manual
Page 47
Chapter 4
NI-DNET Programming Techniques
4-16
ni.com
Main Loop
If your application essentially accesses all DeviceNet input/output data as
a single image, you would normally wait for read data to become available
on one of the input connections (such as a strobed I/O connection), read all
input data, execute your application code, then write all output data. The
wait is important because it helps to synchronize your application with the
overall DeviceNet network traffic.
In single-loop applications such as this, you normally set the
PollMode
parameter of
ncOpenDnetIntf
to
Automatic
or
Scanned
so that all poll
command messages are sent out in quick succession.
Within a single-loop application, error handling is often done for the entire
application as a whole. In the C programming language, this means that
when an error is detected with any NI-DNET object, you display the error
and exit the application. In LabVIEW, this means that you wire all error
clusters of NI-DNET VIs together.
If your application uses different control code for different DeviceNet
devices, you might want to split your application into multiple tasks. You
can easily write a multitasking application by creating a notification for the
NI-DNET
Read Avail
state. This notification occurs when either input
data is available (to synchronize your code with each device’s
I/O messages), or an error occurs. In the C programming language, you
create this notification callback using the
ncCreateNotification
function. In LabVIEW, you create this notification callback using the
ncCreateOccurrence
function.
In multiple-loop applications such as this, you normally set the
PollMode
parameter of
ncOpenDnetIntf
to
Individual
so that each poll
command message can be sent out at its own individual rate.
Within a multiple-loop application, error handling is done separately for
each task. In the C programming language, this means that when an error
is detected, you handle it for the appropriate task, but you do not exit the
application. In LabVIEW, this means that you only wire the error clusters
of NI-DNET VIs that apply to each task, and thus you write different
sub-diagrams that are not wired together in any way.