Experimental Physics and Industrial Control System
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At 10:26 AM 3/8/2004, Brian Tieman wrote:
Ned,
I've been using the Active-X implementation for a month or two now. It
feels more like the Portable Server in that it's easy to create a PV and
have it served without the complications of supporting a full
record--which is a good thing! Since most of what I use LabView for is
quick and dirty, I would be happy with this type of interface as opposed
to full record support.
The biggest problem I have with this Active-X interface and LabView is
that CA calls queue up. There's no support for anything like threads or
even events. The created PV must be in a loop and checked periodically to
see if a client requests a change. Also, once a change has been detected
and the LabView process starts doing its thing, any subsequent client
requests are ignored until the first request is completed. The client
requests seem to be queued in some manner--which can be a real problem.
I haven't used the ActiveX interface to epics, but there are events, which
are called occurences, and threads available in LabVIEW itself. It's pretty
straightforward to set up a parallel structure in a vi that sits and waits
for events from other areas of the diagram, or to launch a vi that sits and
waits for events from other vis. There are also semaphores, queues, and
some other types of synchronization. There is also support for threads, but
not very flexible. There are about 5 threads to which vis can be assigned
ahead of time. Within each thread LabVIEW does cooperative multitasking, to
which there are limitations (for example I think any called dll takes over
its thread until it's finished).
I've used the LabVIEW "datasocket" server, which I think is based on
activeX. Basically I have a small server application that launches a few
subvis executing in separate threads and waiting for new values in some
datasocket items, which are passed off to queues in other subvis that
handle the input. This is a pretty small scale server.
dale brewe
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dale L. Brewe PNC-CAT/U. of Washington
ph: 630 252 0582 fax: 630 252 0580
Mail:
Argonne National Laboratory
Bldg 435E sector 20
9700 S. Cass Ave.
Argonne, IL 60439
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- References:
- [Fwd: LabView/EPICS] Ned Arnold
- Re: [Fwd: LabView/EPICS] Brian Tieman
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ANJ, 10 Aug 2010 |
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