Experimental Physics and
Industrial Control System

Michael Davidsaver via Tech-talk <tech-talk at aps.anl.gov> · Fri, 18 Dec 2020 10:53:22 -0800

On 12/17/20 9:15 PM, Paduan Donadio, Marcio via Tech-talk wrote:
> Hi, Michael.
> 
>  
> 
> Thank you for the tip about using PVA link support. I’ve created a group with some PVs and the result looks like what I was trying to achieve with NTMultiChannel.
> 
>  
> 
> Two questions:
> 
> 1 – Is there a way to call a function every time a group processes? I’d like to create a device-support-like thing to read the fields and values of each channel and calculate things.

At present, no.  The "group" doesn't really exist as a concept in the database.
I have some pending work which would provide a way to do this by adding the
ability to have complex types as record fields (record in the dbCommon sense).

I think this work is to the point where a motivated developer could try it out.
Though at this stage familiarity with record support code is a must.

eg. A record type where VAL is an NTTable structure.

https://github.com/mdavidsaver/pva2pva/blob/vfield/pdbApp/columnarinRecord.cpp

There are prerequisite changes needed in Base.

https://github.com/mdavidsaver/epics-base/pull/1

> 2 – Is there a way to pvget individual fields of a group? Something like pvget grp:name.X.

No.  When accessing through the group name, clients will always see the group structure.

> Thank you,
> 
>  
> 
> Márcio Paduan Donadio
> 
> Control Systems Engineer - SLAC
> 
>  
> 
> *From: *Michael Davidsaver <mdavidsaver at gmail.com>
> *Date: *Thursday, December 17, 2020 at 7:28 AM
> *To: *Paduan Donadio, Marcio <marcio at slac.stanford.edu>, tech-talk at aps.anl.gov <tech-talk at aps.anl.gov>
> *Subject: *Re: Intended use of NTMultiChannel
> 
> On 12/16/20 9:11 PM, Paduan Donadio, Marcio via Tech-talk wrote:
>> I have a NTMultiChannel created using the C++ API.
> 
> Which c++ API specifically?
> 
>> I pvput to channelName passing a valid PV name. I placed a printf inside the process() method just to check if the record is processing and I can confirm it does.
>>
>>  
>>
>> When I pvget I expected to see the other fields of the record populated with values from the PV in channelName. Is it intended that inside process() I write the commands to bring the data for each channel and write to the fields of the NTMultiChannel? Is there a default “record support” that has it already implemented and I can use?
> 
> If you are referring to pvDatabaseCPP, then probably not.  It is more analogous to PCAS.
> 
> You might look at the PVA link support as a way of having an IOC act as a PVA client.
> 
> https://epics-base.github.io/pva2pva/qsrv_page.html#qsrv_link <https://epics-base.github.io/pva2pva/qsrv_page.html#qsrv_link>
> 
> 
>> Some data:
>>
>>  
>>
>> Epics base 7.0.3
>>
>>  
>>
>> $ pvget GMDTest:NTScalar
>>
>> GMDTest:NTScalar 0
>>
>>  
>>
>> $ pvput GMDTest:NTMultiChannel 'channelName=["GMDTest:NTScalar"]'
>>
>> Old : epics:nt/NTMultiChannel:1.0
>>
>>     any[] value
>>
>>     string[] channelName []
>>
>>     int[] severity []
>>
>>     int[] status []
>>
>>     string[] message []
>>
>>     boolean[] isConnected []
>>
>> New : epics:nt/NTMultiChannel:1.0
>>
>>     any[] value
>>
>>     string[] channelName ["GMDTest:NTScalar"]
>>
>>     int[] severity []
>>
>>     int[] status []
>>
>>     string[] message []
>>
>>     boolean[] isConnected []
>>
>>  
>>
>> $ pvget GMDTest:NTMultiChannel
>>
>> GMDTest:NTMultiChannel epics:nt/NTMultiChannel:1.0
>>
>>     any[] value
>>
>>     string[] channelName ["GMDTest:NTScalar"]
>>
>>     int[] severity []
>>
>>     int[] status []
>>
>>     string[] message []
>>
>>     boolean[] isConnected []
>>
> 

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System