Re: analog output record - constant DOL

[Andrew Johnson <anj@aps.anl.gov>]

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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depends upon his not understanding it. -- Upton Sinclair