Build your own Apollo Guidance Computer
wdonzelli at gmail.com
Wed Dec 20 18:35:23 CST 2006
> I think a bigger factor here was the consistentcy of switching time. All of
> Seymour's design are incredibly tight on switching time. He liked to line
> up signals so they would reach the next gate at the same time without
> requiring a clock. Having only one gate switch time would ease design. In
> all of his designs that I have worked on, he made flip flops and adders out
> of individual gates. Again to have exact control of the switching time.
This makes no sense. In every ECL family I have seen, when gates are
assembled onto one die to make a logic function, the sum is always
faster (often by a good amount) than if the same design was done using
individual gates. Even if the individual gates are hand picked for
speed, the complete logic function will be faster. And there is
nothing to keep anyone from hand picking the complex logic functions
for speed, as well.
There is also the speed gain of having more computing logic per board.
For example in an individual gate design, the extra 2 inches of
microstrip traces on the board, plus all the extras involved with
getting the signal on and off each chip package, can add up to a
significant part of a propagation delay of a gate. And, with more
complex boards, the machine could get smaller, with a speed increase
gained there as well, as the backplane (backnest?) would get smaller.
> And to eliminate any un-necessary logic. There are no unused gates in any
> of his computers.
This is certainly a valid reason, and for some of his later designs I
can see where an ECL ALU (100181, for example) may have too many extra
things. Ok, perhaps a bad example, as I do not think Cray designs use
ALUs, or 100K, but it is what springs to mind. But, with something
like an and-or-invert gate, or even a multiplexer or demultiplexor,
the off the shelf designs are basically minimized.
I know the Cray-1s ECL was something like MECL III or 100K, and
certainly whoever the chip maker was could have supplied some basic
logic functions beyond a gate or two. I think the extra effort to use
them would have been minimal, with a great payoff.
> Miserable servicing? As someone who spent literally years tuning wires in
> Seymour's designs, I have to agree. His machines were very demanding to
> impossible to maintain. Just before it died, I spent a few hours on the
> 8600. None of us on that machine believed it could be maintained! And the
> math models all gave the MTBF as a negative number!
I once heard that one of the big boxes in a computer room that hosted
a Cray would have a fancy tag like "Disk Control" or something, but
that was actually where the field engineer lived.
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