Experimental Physics and
Industrial Control System

Michael Davidsaver via Tech-talk <tech-talk at aps.anl.gov> · Wed, 2 Nov 2022 09:48:13 -0700

On 11/2/22 07:04, Joao Paulo Martins via Tech-talk wrote:

Hello,

I’m trying to instantiate a NTNDArray with some NTAttributes in my IOC using QSRV group PV definitions, as the example in pva2pva (https://github.com/epics-base/pva2pva/blob/master/iocBoot/iocimagedemo/image.db <https://github.com/epics-base/pva2pva/blob/master/iocBoot/iocimagedemo/image.db> ).

There is a definite question of "can" vs. "should" with this example.

I include image.db to both demonstrate and test what is possible in
a pathological case.  (NTNDArray has every kind of PVA weirdness)

...

record(mbbi, "$(P)FOO") {
     info(Q:group, {
         "$(N):Array":{
             "attribute[0].value":{+type:"any", +channel:"VAL"},
             "attribute[0].timeStamp":{+type:"meta", +channel:"TIME"}

The "meta" mapping attaches a sub-structure at the referenced position.
So as specified, this creates "attribute[0].timeStamp.timeStamp" and friends.

What you intend should be possible with:

             "attribute[0]":{+type:"meta", +channel:"TIME"}

However, at the moment this results in an error:

TST:image1:Array: Error Group not created: leaf field may not have sub-script : attribute[0]

I see this as a bug in QSRV.

https://github.com/epics-base/pva2pva/issues/51

That said.  I tend to see "attribute[]" as a bit of a hack to work
around CA limitations.  With PVA it is straightforward to place
additional PVs at the top of a QSRV group, alongside the NDNTArray.

eg.

record(waveform, "$(N):ArrayData") {
    ...
    info(Q:group, {
        "$(N):Array":{
            +id:"epics:nt/NTNDArray:1.0",
            "mycamera.value":{+type:"any",
                     +channel:"VAL",
                     +trigger:"*"},
...
record(mbbi, "$(N)FOO") {
    info(Q:group, {
        "$(N):Array":{
             "othersignal":{+channel:"VAL",
...

This better takes advantage of PVA protocol features to only send
fields marked as changed.  With "attributes[]" the entire array must
be sent every time any field of any element changes.

Also, this avoids limitations implicit in the definition of NTAttribute.
eg. how to represent an enumeration like your "mbbi".  (where is the
"choices" list?)

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System