Re: Processor Synchronization

[Marin David Condic <condicma@bogon.pwfl.com>]

Brian Orpin wrote:
> 
> OK maybe I am being thick here but isn't the discrete a single point of
> failure?  If that goes which processor takes over and how does it know it
> should and that it isn't just the link that has gone down?  If both
> processors check for instability and take over if it is detected do they
> need to be closely synchronised?

Of course it is impossible to describe the entirety of our architecture
in a newsgroup post, so expect some inaccuracy. Also, I am describing an
"in general" architecture that is used by our controls. The control for
the F119 engine (Advanced Tactical Fighter) actually has three
processors per channel and two channels. (much more complex sync
problem!)  The Joint Strike Fighter is yet again even more complex. So
it is hard to give you a schematic here in a post which can't be picked
apart.

Yes, the discrete between both channels would be a single point failure,
but not one that necessarily causes the control to fail
catastrophically. If the discrete goes down (assuming you have no other
means of communicating between the two channels, which we do - a
manchester data link.) you can at least reason about what to do next: I
know I'm alive. The other guy may be dead or alive. If he's dead and I
take control, that's cool. If he's alive and I take control, is that
bad? Are the actuators dual active? Are there any ways of determining if
the other side is still controlling the actuators?, etc... In any event,
you're not necessarily putting a pilot in the drink or blowing up a
billion dollar space payload because a wire broke loose between the two
channels.

I hope that sheds a little light on it. It really becomes a fascinating
problem when you get into the FDA aspects of the control and the system
it runs!

MDC
-- 
Marin David Condic
Real Time & Embedded Systems, Propulsion Systems Analysis
United Technologies, Pratt & Whitney, Large Military Engines
M/S 731-95, P.O.B. 109600, West Palm Beach, FL, 33410-9600
Ph: 561.796.8997         Fx: 561.796.4669
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