Re: labCA and EPICS_CA_MAX_ARRAY_BYTES

[Till Straumann <strauman@slac.stanford.edu>]

On 05/25/2010 05:31 PM, Mark Rivers wrote:
> No, there is no limit, just set EPICS_CA_MAX_ARRRAY_BYTES.  We are sending 16MB images over CA with no problem.
>
> Amd 16K of doubles is not 1K values, it is 2K.  A double takes 8 bytes on every architecture I know of.
>    

IIRC it can be a little less because the EPICS_CA_MAX_ARRAY_BYTES include
the extra info (status, timestamp etc) -- details depend on the DBR type.

In labCA PVs are normally retrieved as DBR_TIME_xxx.

A limitation of the underlying ezca library is that when you fetch
units or limits (stuff that isn't provided with DBR_TIME_xxx) then
ezca uses a DBR_CTRL_xxx with the native element count of the PV
even though the array elements themselves are discarded.
This is unfortunate when you have a slow connection.

E.g., if you call

lcaGetUnits(pv)

and your PV has a large native element count then the entire
array is transferred and discarded (and the call will fail if
your EPICS_CA_MAX_ARRAY_BYTES is too small).

Luckily, if you ever run into that problem you can obviously
work around in most cases by reading the '.EGU' field

lcaGet(pv + '.EGU'); // this is scilab notation

HTH
T.
> Mark
>
>
> ________________________________
>
> From: tech-talk-bounces@aps.anl.gov on behalf of emmanuel_mayssat@lynceantech.com
> Sent: Tue 5/25/2010 4:15 PM
> To: Till Straumann
> Cc: EPICS Tech-Talk
> Subject: Re: labCA and EPICS_CA_MAX_ARRAY_BYTES
>
>
>
> Is there still a limit on the length of arrays?
> I understand that the client and ioc needs to be configured with
> EPICS_CA_MAX_ARRAY_BYTES
>
> Note that the number of bytes is not the numbers of values.
> 16K of double variables is only 1K values....
>
> is this still a limitation in 3.14.11?
> --
> E
>
> On 10:27 Tue 25 May     , Till Straumann wrote:
>    
> > On 05/25/2010 09:55 AM, John Dobbins wrote:
> >      
> > > Is it possible to use labCA to retrieve arrays larger than 16K?
> > >
> > >        
> > Yes - I routinely use that.
> > You have to make sure EPICS_CA_MAX_ARRAY_BYTES is set
> > in the environment *before* you start matlab or scilab (RTM).
> >
> > On the IOC, too, you need EPICS_CA_MAX_ARRAY_BYTES but
> > since you tested with caget that's obviously set up correctly.
> >      
>
>