Experimental Physics and
Industrial Control System

Brian Tieman <[email protected]> · Fri, 16 Feb 2001 15:54:42 -0600

Hi all,

I've been using the portable channel access server to provide EPICS
server level support for various CCD acquisition systems for a couple of
years now.  Now I've hit a problem I don't know how to solve.

The way our acquisition works, performing a put_callback on a "trigger"
pv causes our camera to start acquisition.  The acquisition program (the
part which uses the PCAS) starts acquisition, but does not return from
the write operation until the data has exposure has completed.  This
forces the caller to wait until acquisition completes as the
put_callback doesn't finish until acquisition does.

This works very well for the scan record in general use at the APS.  The
scan triggers the camera, but does not go on to the next point until the
put_callback was successful--which isn't until after the exposure is
complete.

The problem is that this works very well as long as the callback
completes in less than 30 seconds.  If the callback does not complete,
the connection appears to close and the callback gets satisfied by
default.  It looks like there is an inherant 30 second timeout
somewhere, but I'm not sure who is responsible for it.

To try and get around this problem I've tried making the acquire PV an
asynchronous pv.  This solves the connection problem--the channel never
disconnects.  However, it never satisfies a callback request either.  I
can see the PV value change at the end of the exposure time so I believe
the asynchronous PV is created and works fine--it just doesn't seem to
satisfy a callback request properly.

I don't see any discussion in the documentation about how callbacks are
handled.  Does anybody have any ideas?  Is it even possible for an
asynchronous PV--from PCAS or elsewhere--to satisfy callback requests?

Brian Tieman
Advanced Photon Source

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System