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<== Date ==> <== Thread ==>

Subject: Re: Waveform Record Device Support
From: Dirk Zimoch <dirk.zimoch@psi.ch>
To: graham.cox@stfc.ac.uk
Cc: tech-talk@aps.anl.gov
Date: Mon, 07 Jun 2010 11:30:20 +0200
Hi Graham,

The waveform record allocates memory for the waveform at boot time, according to NELM. All links (including local db links on the same IOC) store the address of the memory at the time they connect. Thus the memory must not be re-allocated after booting. Channel access clients receive the size information when they connect. There is no support in Channel Access for dynamic array sizes.

As you can see, no part of EPICS is prepared to deal with changing number of elements. You have to set NELM to the maximum possible number of elements. The device support may change NORD to indicate the actual number of valid elements. But a client has to connect to an array with NELM elements. It has to monitor NORD in addition to VAL to find out the real array size. The array is filled up with 0 if NORD < NELM.

Best regards,
Dirk

graham.cox@stfc.ac.uk wrote:
Hi All,

I am integrating a turn-by-turn BPM system into EPICS and would like to
store the raw BPM pickup data in a waveform record.  Due to the nature
of the system, each run can result in a different number of samples for
each BPM pickup, so the array size is unknown in advance and could
potentially be different every run.

Is it possible to resize the number of waveform record elements in my
device support, or do I have to size the waveform record in advance for
the maximum number of samples that could be expected.  Most of the
examples of waveform device support I see read the nelm field and return
this fixed amount of data via the records bptr.

Thanks,

Graham


Replies:
RE: Waveform Record Device Support Krempasky Juraj
References:
Waveform Record Device Support graham.cox

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