Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Wed, 28 Mar 2012 09:30:25 -0600

Hi Peter,

On 2012-03-28 Peter Milne wrote:
> Maybe it's quite easy to do this outside the IOC (eg with Python Channel
> access), but ideally it would happen inside an IOC so that it's
> completely transparent to a client. The Subarray record is an obvious
> choice, but it appears to take a contiguous block of data, there doesn't
> seem to be a concept of a stride value.
> 
> eg assuming the Waveform Record refers to data
> [samples][channels] with dimension (typical) [1024][96], the extract
> routine needs to run through the data set with a stride of 96.
> 
> It looks like subarray could be used to extract [sample-N][:] but not
> the required [:][channel].

A possibly simpler alternative to using Mark Rivers' areaDetector module might 
be to use an aSub record and write your own C code that it runs to extract the 
samples for each channel. A single aSub record has 21 output value fields 
(which can be configured as arrays of any of the basic types) so it's easy to 
see how to extract up to 21 channels from the input waveform. However it is 
also possible to use some of the input fields as outputs, so it should be 
feasible to have one aSub record handle a nice round 32 channels and still 
leave enough input fields for some configuration values.

I'm not trying to steer you away from areaDetector which is great for doing 
complicated real-time processing on camera images and other 2-D data, but if 
you already have an EPICS driver that populates the waveform record, just 
writing an aSub subroutine would probably involve less software development 
effort. If you decide to go this route you would need to call db_post_events() 
on the input channels you set to trigger Channel Access monitors on those 
fields.

HTH,

- Andrew
-- 
Never interrupt your enemy when he is making a mistake.
-- Napoleon Bonaparte

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System