Path: utzoo!attcan!uunet!samsung!zaphod.mps.ohio-state.edu!rpi!crdgw1!crdos1!davidsen From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr) Newsgroups: sci.electronics Subject: Re: Electro-optic bus Message-ID: <2387@crdos1.crd.ge.COM> Date: 3 Aug 90 16:19:07 GMT References: <1965@trlluna.trl.oz> <7914@tekgvs.LABS.TEK.COM> <26481@nigel.udel.EDU> Reply-To: davidsen@crdos1.crd.ge.com (bill davidsen) Organization: GE Corp R&D Center, Schenectady NY Lines: 42 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