🚀 go-pugleaf

In article <26481@nigel.udel.EDU> berryh@udel.edu (John Berryhill) writes:

| Define "moderate."  I assume from the title that we're talking about
| distances less than a few meters.  If you can detect significant
| phase dispersion for signals of a couple of hundred megabits per second
| over that distance, I tip my hat to you.  It's true that LED's aren't
| going to run at several gigabits, but neither are the chips that you
| want to get the data into and out of.  It is excruciatingly difficult
| to built a computer that uses internal data rates approaching 200 MBPS.

  Sounds right to me. I suspect that a little led will run just fine at
100 mbit, and probably at 200 as well. I saw a prototype device sitting
in a PC backplane which connected the two outer boards with an led and
sensor, using a hole cut through the middle board. This bypassed the PC
bus (which was only used for power and control anyway). I believe this
hack was running at 50 mbit, although I certainly didn't measure it. It
was carrying three million 16 bit A/D values per sec, so that sounds
right.

| The fastest commercial LED's that I've seen have risetimes of about
| 2 ns.  I've made surface emitters with a rise time of 560 ps and
| Wolfgang Harth has made ones that are < 300 ps.

  This sounds like confirmation of what I said. We can step up a good
bit from current speeds.

| The reflexive answer than lasers are "right" and LEDs are "wrong"
| is reminiscent of gallium arsenide having been the "material of the
| future" for the last thirty years.  As long as improved performance
| can be squeezed out of silicon, or lately out of Si-Ge alloy BJTs,
| then it's going to be cheaper and easier to deal with than GaAs.  

  This is somewhat like magnetic media, in that something better has
been just a few years away for several decades now. Magnetic bubbles and
CCDs have their place, but not as replacements for mass storage.

  And there's always a new technology coming. I think Indium Phosphite
is the one  now? I don't expect to see it in my next computer, or even
in my next Cray (although Cray might have GaS for some critical parts).
-- 
bill davidsen	(davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen)
            "Stupidity, like virtue, is its own reward" -me