Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Fri, 26 Feb 2010 11:04:50 +0100

On Friday 26 February 2010, Andrew Johnson wrote:
> On Thursday 25 February 2010 17:20:28 Benjamin Franksen wrote:
> > I absolutely agree. It's just that in this case the language forces me
> > to use shared variables instead of message passing *even though the
> > underlying mechanism of synchronization is basically message oriented*.
> 
> Could you not have used an event flag or two to communicate start/done
>  events between your state sets?  If you want to publish whether the
>  slave is busy or not as a PV it can maintain that itself, but that
>  shouldn't be the mechanism used to control it.  If the slave can accept
>  multiple commands it could wait on more than one flag, or read a shared
>  command variable after the command event flag goes off.

Right. This is what I do now.

> > My particular problem was directly caused by the (forced) separation
> > (via global shared variables) between the synchonization (monitor
> > event, pvPut) and access to the value transported. This is something
> > that has bugged me ever since I learned about SNL. And it could be
> > fixed by making a number of language-level changes (the underlying
> > implementation of the sequencer and the generated C-code could remain
> > almost the same):
> >
> >  o Allow variables local to a state or a state-set. Forbid or at least
> >    discourage global variables. (Global constants are of course fine
> > and declaring them should be more directly supported.)
> >
> >  o All communication between state-sets should be via PVs or event
> > flags (what was the reason, again, not to call them semaphores?) which
> > both may be declared at the top-level. One would need some sort of pv
> > or channel data type that gets bound to a PV (at the top-level or local
> > to a state set) in a way similar to the current 'assign..to' clause.
> >
> >  o Allow access (write or read) to a PV object only with a procedure
> > call that gets resp. returns a value; events from monitored PVs store
> > their payload into a local variable that must be explicitly bound in
> > the state declaration.
> 
> The state notation compiler and sequencer is no longer a bundled part of
>  Base, so it could (and probably should) be gradually replaced by
>  something better (preferably with a compiler/translator that doesn't use
>  a lex/yacc parser written in C).  I would definitely encourage that kind
>  of development...

Cool. I'll ask my employer if they would fund this work (I don't think the 
chances are great though).

Cheers
Ben
-- 
"...And you know what incurable diseases do: they invite the quacks and 
charlatans in, who in this case take the form of Software Engineering 
gurus." (Dijkstra, EWD1305)

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System