Re: analog output record - constant DOL

[Rolf Keitel <rolf@triumf.ca>]

Hi Andrew,

I haven't given other links any thought.
For DOL on output records, however, one could store the constant link value, which is evaluated on record init in a record private structure and pick it up from there when processing.
Ideally this private value should be updated, if the value of the constant link is changed. Mhhh?

- rolf -

On 6/14/2013 8:30 AM, Andrew Johnson wrote:

Hi Rolf,

On 2013-06-14 Rolf Keitel wrote:

I am setting  DOL to the desired constant value and OMSL=closed_loop and
everything seems to work as desired.
When I do a caput to one of these ao records, however, the record
outputs from this moment the value received from the caput and ignores
the DOL value.

This seems counter-intuitive to me. I had expected that in closed_loop
mode, the VAL field would be *always* obtained from the DOL link.
Does anyone remember what the rationale is for treating a constant link
differently from a db or ca link?

Constant links are only ever read once, at record initialization time, and
this is true for all link fields of all record types.  I assume this was done
to avoid having to call strtod() or its equivalent every time the IOC reads
the link, which could be a significant drag on performance for something like
a CALC or SEQ record which have many links that are often constants.  A link
field that has not been set is currently a constant link too.

There is an opportunity to reconsider this design in some work I'm doing for
3.15/3.16 and v4 (link support), but I would still want to avoid converting
the value from its string every time the link gets read.  The ability to
specify string and array constants also comes to mind as desirable, but don't
hold your breath waiting.

- Andrew

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