From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID
	autolearn=no autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,a1eff3a9508d6cba
X-Google-Attributes: gid103376,public
From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: Space Station S/W in Ada -- No Tasking?
Date: 1998/05/06
Message-ID: <EACHUS.98May6171227@spectre.mitre.org>#1/1
X-Deja-AN: 350972724
References: <354dadfd.2883074@news.mindspring.com>
Organization: The Mitre Corp., Bedford, MA.
Newsgroups: comp.lang.ada
Date: 1998-05-06T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <354dadfd.2883074@news.mindspring.com> munck@Mill-Creek-Systems.com (Robert Munck) writes:

  >  "To make troubleshooting easier, the software that runs
  >  the trio of computer networks aboard the space station is
  >  written to operate in synchronous, or serial, fashion 
  >  rather than the faster but more complex asynchronous."

    While the rest of the discussion on this sounds correct, I think
that what was being implicitly rejected here is the way that the Space
Shuttle computers do voting.  In the Space Shuttle, voting is based on
whether three different computer systems come up with about the same
answer at about the same time.  If no two agree, the results of a
fourth are arbitrarily accepted.  (Is that both right and concise?)
Since the computers do not get their data synchronously, the actual
data values, and the control inputs computed from them, will be
slightly different.

    In the ISS, where voting is required, two out of three computers
will have to agree, but based on identical data, and bit for bit
compares.  The Space Shuttle approach does provide more reliability
where the algorithms are not known to be stable, but is a maintenance
nightmare.  (All computers getting the same overflow is no help, and
the SS flight guidance software does go through about 20 different
flight regimes during landing.  At the boundary between some of those
modes, the flight control algorithms are known to be unstable.  So
that approach is not only appropriate to the shuttle, it seems to be
necessary.)
--

					Robert I. Eachus

with Standard_Disclaimer;
use  Standard_Disclaimer;
function Message (Text: in Clever_Ideas) return Better_Ideas is...