Experimental Physics and
Industrial Control System

Ben Franksen via Tech-talk <tech-talk at aps.anl.gov> · Tue, 6 Oct 2020 11:24:49 +0200

Am 06.10.20 um 10:25 schrieb Ralph Lange via Tech-talk:
> Since we started developing a number of "pure" pvAccess servers,
> users/developers have been asking how to do state machines that connect to
> them.
> 
> Is there any experience or plan to add support for pvAccess to the
> SNL/Sequencer?

I have no concrete plans. There are basically two reasons:

(1) I can't see how to reconcile the SNL data model with PVA. In CA you
have the basic primitive types which are mapped to the basic SNL types;
or arrays of those which are similarly mapped to SNL arrays; and then a
fixed set of meta data on top of that. PVA provides a richer set of data
types that does not map easily to a language that is, give or take, a
variant of C. Also, meta data is not as uniform in PVA as in CA. Even if
support is restricted to Normative Types, I am not sure we can map all
of them to SNL types in a reasonably useful way.

(2) The PVA API is C++ and (IIRC) quite heavily uses C++ features. It
may be possible to provide a wrapper that translates those parts of the
API we'd need for the sequencer into C. However, I wonder if this is
really a good idea. It may be more sensible to design something new that
is written entirely in C++ and provides the features we want from SNL as
an embedded language, that is, as a library, rather than a separate
programming language.

Besides, you may have noticed that development of the sequencer has
stalled in the last few years. This is mostly because the direction in
which I wanted to go with SNL (for version 2.3) made it very hard to
maintain compatibility. Also I got stuck on design issues I never
managed to find a satisfying solution for.

That said, if someone manages to hack PVA support into the sequencer,
however limited in scope, I am willing to review/accept contributions.
Any such efforts should probably be based on the 2.2 branch as I am not
sure 2.3 will be continued.

Cheers
Ben

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System