Conservative FBW Architecture: If it ain't broke, don't fix it
Quote from ve3id [Feb24/03:30]:
Chris Scott said:
"PS: Isn't it remarkable that, 20 years on, the SECs may still be employing an Intel 80186 chip, now ancient history in the home-PC world? We were all buying PCs with 80286/80386 chips, even as the A320s were first going into service."
I couldn't let this go! NO, It is not remarkable, it is just good engineering. If a chip performs the designed task within spec it should stay in the design forever. The engineering world is not motivated by specsmanship and marketing like the consumer electronics world is. If you put a more modern chip in there, what benefit is it going to give you? Absolutely none, but what about the risk of mask errors introducing bugs that the original programmers did not test for because the new chip has circuits that were not even known back then? Very probable.
If you re-design with a new chip, you have to re-test all system components, and that costs a lot of money.
I'm just an ex-driver, so thank you for confirming my sentiment from an engineering perspective. "Remarkable" was not written in the pejorative sense of the word; more in admiration.
In the A320, the mother of civil digital FBW, the designers took a deliberate decision to go for well-proven COMMERCIAL chips, which were already being used in large numbers in a wide range of applications. For the SECs (spoiler-elevator computers), they chose the 80186. This, I am now told, was a close derivative of the 8086, which had brought PCs to offices and homes across the world, and whose bugs were well understood. Airbus and Sextant Avionique had the option of developing what they called "mathematically correct" micro-processors. Without wishing to sound complacent, their decision not to do so has so far stood the test of 20 years' service and (presumably) well over a million flight cycles.
If the missions of the SECs and ELACs (elevator-aileron computers) are not much more demanding in 2008 than in 1988, and replacement chips are still available, why indeed go for a more complex chip? As you put it, ...... It's not a matter of keeping up with the Joneses!
Despite all their differences, I expect the "Joneses" themselves would be in broad agreement.