Experimental Physics and
Industrial Control System

Ben Franksen via Tech-talk <tech-talk at aps.anl.gov> · Thu, 26 Mar 2020 09:44:17 +0100

Am 26.03.20 um 09:22 schrieb Ben Franksen via Tech-talk:
> Am 26.03.20 um 07:09 schrieb Ralph Lange via Tech-talk:
>>> # not working: error --> dbJLinkInit: Link type 'db' not found
>>>
>>> record(ai, "${SYS}:dbInput") {
>>>
>>>   field(INP, {db:"${SYS}:constFloatPi"})
>>>
>>> }
>> The key is the second sentence in the documentation you refer to:
>> The IOC continues to support the older link types that do not use JSON to
>> specify their link addresses.
>
> Hi Ralph,
>
> I think your answer, while not exactly wrong, is a bit cryptic, so let
> me explain.
>
> The docs list the available link JSON types. There is no "db" link type
> listed, which explains the error you get. What Ralph alluded to is the
> reason /why/ there is no JSON "db" link type. This is because, strange
> as it may sound, '{db:"${SYS}:constFloatPi"}' is actually a valid name
> for a record and thus for a PV. A long, long time ago it was decided
> that a PV name may contain any byte that is not reserved for a special
> purpose by EPICS base (such as '.' and a few others), instead of a
> priori limiting this set to something reasonable.

Hmmm. What I wrote here doesn't sound very convincing after all.

If {db:"${SYS}:constFloatPi"} is indeed a valid PV name, and I just
verified that in base-3.14 this is indeed the case, and we want to
continue to support such PV names, then EPICS should accept it as such
and not try to interpret it as a JSON link. But if it was decided that
in fact this is no longer a valid PV name in base-3.15 and later, then
there is no reason not to allow to specify a PV link with JSON notation.

Cheers
Ben

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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System