Experimental Physics and
Industrial Control System

[email protected] (William Lupton) · Fri, 3 May 96 08:28:51 HST

Dear all,

  Thanks to Steven, Jeff and Andrew for their replies. I guess I _knew_
that if I mentioned the reason for the question then I would get
comments on the proposed application too! Thanks for those, and yes, of
course one would have to be very careful at the switchover point.

Andrew said:
> If you know that the Unix CA server will always be running, you could 
> always have the Unix server reflect PVs from the IOC into a different 
> namespace, and have all your clients attach to the Unix names and never to 
> the original IOC names.  This way the Unix server knows exactly when it 
> needs to be a simulator (because it gets connect and disconnect messages 
> from the IOC), and the clients couldn't care less if an IOC gets turned 
> off because they're never talking directly to it. There will be a 
> performance hit here of course, but for slow records it shouldn't matter.

  This is close to the existing non-EPICS system that we are trying to
emulate.  Things are a lot easier if all control and status flows
through a single point in both simulate and non-simulate modes because
the simulate mode can be a single global flag at that single point. I
was wondering whether there was a solution which still allowed clients
to connect directly to the online system.

  In conclusion, Jeff points out that to support this sort of thing in
CA would probably require a protocol change (perhaps not a major one?)
and this is unlikely to happen. I think that we are likely, in
_decreasing_ order of probability, to:

1. change the requirements (e.g. handle simulation mode on a per-client
   basis and not support automatic rollover)

2. go for a hybrid system (e.g. use a Unix server at remote sites and
   use existing IOC-based simulation facilities at the telescope)

3. go for a scheme like the one that Andrew suggests

  William

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System