How to implement start & stop ai triggers when # scans unknown

Question

I can't seem to find an implementation example for this application.
Some are close but seem to have a big gap on close examination.
The application is to log an unknown number of samples for a
single rotation of a variable speed drum. The drum is connected
to a tachometer which generates 1 pulse per revolution and is the
trigger source.
The user will initiate the data collection and only a single
rotation's
worth of data need be collected and displayed.
The board is a PCI 6111E sampling a single input at ~2 Megahz.
All the examples I've found using a start and stop trigger depend
on knowing how many scans to acquire.
Is there any way to get around that? It seems so simple to
have a
start and stop pulse, a scan rate constant, and just get
the data out of a buffer. But it does not seem to be that simple
or am I missing something here?
There is also an issue with the identical tach signal being used
for both the start and stop trigger. I think that can be gotten
around by using a divide by n counter and feeding that to
trigger config vi but if anyone has any suggestions for that,
they would be greatly appreciated.
Gene Couch

Answer 1

I'm afraid I can't help you beyond that. We use NI boards and NI-DAQ library
of functions in custom control systems with software written in C++. I have
not had opportunity to use LabView. Sorry.
gcouch wrote:
>On 5 Jun 2000 09:38:46 -0500, "J. Wolters" >wrote>Thank
you for the response but I'm not sure what the DAQ_Rate function>is. With
LabVIEW 5.1 it does not appear in the DAQ pallette and I>cannot find it in
the example vi's. Do you know where it can be>found?>>>>>>Have you looked
that DAQ_Rate function? You should be able to use this function>>to obtain
your # of samples.>>>>>>gcouch wrote>>I can't seem
to find an implementation example for this application.>Some>>a
re close but
seem to have a big gap on close examination.>>The application>>is to log
an unknown number of samples for a >single rotation of a variable>>speed
drum. The drum is connected>to a tachometer which generates 1 pulse>>per
revolution and is the>trigger source.>>The user will initiate the data>>collection
and only a single>rotation's>worth of data need be collected and>>displayed.>>The
board is a PCI 6111E sampling a single input at ~2 Megahz.>> >>All the examples
I've found using a start and stop trigger depend>on knowing>>how many scans
to acquire.>>Is there any way to get around that? It seems>>so simple to>have
a start and stop pulse, a scan rate constant, and just>>get>the data out
of a buffer. But it does not seem to be that simple>or>>am I missing something
here?>>There is also an issue with the identical tach>>signal being used>for
both the start and stop trigger. I think that can>>be gotten>around by using
a divide by n counter and feeding that to>trigger>>config v
i but if anyone
has any suggestions for that,>they would be greatly>>appreciated.>>Gene Couch>>>