Re: embedded aviation computers.

[Bob_Deep <bobd@cssgroup.com>]

And I thought the requirements of my software coustomers was bad... 
Shesh!

I think I'd be very happy with linux in risky enviroments on good solid
hardware.  It may be big, but it is *very* stable.  (I've never seen
linux crash when the hardware is good, and I've watched it run for
months on end under heavy use without any problems.)

Of course I'd test the living daylights out of what ever I fielded, and
the testing would be proportional to the ammount of risk involved should
the application fail.

Good hardware and good testing are extremly expensive things, so if you
can scale down the hardware to a minimum and do away with the largeness
of linux by writing your own custom application, that may be cheaper for
smaller applications by making the hardware cheaper and the testing
easier because the application is smaller.

I seriously doubt that a complex system, such as a moving map display or
flight director is going to be easily hosted on custom hardware without
using some off the shelf OS for embeded systems.  

Jerome Kaidor wrote:
> 
> > First off, the group looks dead, what's up, or not up as the case may be?
> >
> >
> > In the never-ending quest....
> >
> > All these projects related to custom hardware etc are very interesting,
> > but I really wonder if we'd not be a lot better off to grab semi-standard
> > eqpt and get moving.
> >
> > Decent linux code sould be portable to any reasonable platform as it
> > becomes reasonable in cost, reliability and support.
> >
> *** My only problem with all of this is reliability.  Whups, just a second,
> my PC just died....<reset>.   While Linux is surely more robust than Windows
> of any flavor, I would still not trust it to run my flight instruments!
> It's just too big.
> 
>    Imagine, you have a standard PC, running a large OS, such as Linux or
> Windows.  How can you convince yourself that the software is correct?
> There's so much of it!  And PC hardware is not necessarily designed
> to high standards of timing and temperature range, not to mention vibration
> resistance.
> 
>    My first job in programming was for a piece of railroad equipment.  The
> safety of life and property was involved, so the hardware design was
> extremely robust.  Clock rates were kept low, and ample margins used for
> all timings.  The backplane had traces on one side, a solid ground on
> the other, and guard traces ( all connected to ground ) between the signal
> traces.  The clock pulses on this thing looked like out of a textbook!
> The engineer tested it with a 5W CB radio with a rubber duckie antenna -
> he stuck the antenna inside the chassis and scoped the clock lines.  If
> he saw RF, he went back to the drawing board.
> 
>    The software was written to the same high standard.  Our CPU spent
> something like 30% of its time doing integrity tests.  Things like the
> memory and I/O were constantly being incrementally tested.  Critical
> software items like the stack were constantly checked for reasonableness.
> For example, the main loop checked the stack pointer each time around - it
> needed to always be the same number.  The stack was "guarded" with a known
> value somewhat below the top - if it was about to blow, that known value
> would change, and our program could take action.  A hardware watchdog timer
> on a short 200-ms fuse constantly watched the system.
> 
>    There was no OS, and we laboriously verified every section of the program
> for correctness.  It got so good that we had trouble fixing bugs, because
> the system would fix itself so fast!  If something really bad happened, the
> system would reset, and we had it to where it could perform a full reset and
> be back in normal operation in about a half a second.   But we gradually
> weeded out even all the causes of these failures.
> 
>    And THAT's the caliber of hardware and software I would want running my
> flight instruments.  I don't think it's attainable with a "big" OS, even
> Linux.  Maybe with something like VRTX or Nucleus OS, although I even have
> my doubts about those.   At least with Linux, you get the source.
> 
>                       - Jerry Kaidor ( jerry@tr2.com )
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-- 
      -=  Bob =-
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