Re: PV Access

["Kivel Niko \(PSI\) via Tech-talk" <tech-talk at aps.anl.gov>]

Hi Ben

thanks for the more detailed explanation.

In the doc, this example is given:

{calc: {expr:"A*B", args:[{db:"record.VAL"}, 1.5], prec:3}}

which uses the link type 'calc', which must be a valid link type according to the documentation. However, I receive the same error. Whenever I use anything author than 'pva'.

Also, if (and it doesn't sound that strange) "{db:"${SYS}:constFloatPi"}" is as valid PV, so is "{pva:"${SYS}:constFloatPi"}", which works perfectly fine.

Maybe it has change when this feature was introduced?

best

Niko

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Dr. Niko Kivel

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email: niko.kivel at psi.ch

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From: Tech-talk <tech-talk-bounces at aps.anl.gov> on behalf of Ben Franksen via Tech-talk <tech-talk at aps.anl.gov>
Sent: Thursday, March 26, 2020 09:22
To: EPICS Tech Talk
Subject: Re: PV Access

 

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.

Cheers
Ben

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