Experimental Physics and
Industrial Control System

[email protected] (Tim Mooney) · Tue, 21 Oct 1997 18:35:22 -0500

John P. Hammonds writes:

> I am looking at using EPICS for a data acquisition system for an array
> of detectors.  We are thinking of using subArray records to hold the
> data but need some flexibility that does not seem to be worked in.  
> 
> First of all we will routinely rebin the array from one run to the next
> so that it would be nice to resize the array on the fly.
> We often combine data on the fly to conserve memory.  Again, we change
> how this is done on the fly.
> 
> To get around the resizing problem I was thinking about defining one
> large subArray record that would predefine the entire data space for the
> crate.  I would then use a subArray record that points into the larger
> dataspace to allow access data for each detector.  For now I can limit
> myself to the 16KB limit for CA transfers within this limit.  This also
> allows data to be binned together on the fly since two of the smaller
> subArrays can point to the same space.  
> 
> One problem that I have with this is that the smaller subarrays seem to
> copy the data from the larger subArray to the smaller one.  This seems
> to be a waste of both time (for copy) and memory for this purpose.  What
> I was thinking of doing was making the BPTR of the smaller array point
> directly to the BPTR of the larger array (+the INDX offset).
> 
> Has anyone done this or does anyone see a problem/improvement with this
> approach?

I think you need a custom record and custom device support.  Device
support could allocate the data array and incoming-data buffers, spawn
and send messages to a binning task, point the task at incoming data,
and give record support pointers to the binned-data array.  Record
support could tell device support how to bin data, and would not
allocate any memory.  It's cvt_dbaddr() routine would just return a
pointer into the data array owned by device support.

Tim Mooney

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System