Originally Posted by
Blythy
As an example, on the Space shuttle, there were four identical computers which voted against each other in the case of discrepancy. However, there was a 5th computer (limited to ascent and reentry only) which was different hardware and different software in the event of something which had the same root cause in the software / hardware.
Not entirely true. All 5 computers are AP-101. It had a different subset of functions for ascent and descent, written by Rockwell (IBM was the main contractor for the hardware and flight software). It wasn't a complete rewrite of the flight software. The reason given for not having different hardware was that is would have
cost too much. The software itself was an OS written in assembly, and the main code written in HAL/S, possibly on different versions of compiler.
Has there ever really been an aircraft with 2 completely separate hardware and software teams?
Info was gotten from
Computers in Spaceflight: The NASA Experience Chapter 4-3
and The Space Shuttle Primary Computer System, Communications of the ACM September 1984 Volume 27 Issue 9