Xref: utzoo comp.arch:17309 sci.electronics:13039 sci.physics:13712 comp.lsi:1093 Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!zaphod.mps.ohio-state.edu!brutus.cs.uiuc.edu!ux1.cso.uiuc.edu!ux1.cso.uiuc.edu!aglew From: aglew@oberon.crhc.uiuc.edu (Andy Glew) Newsgroups: comp.arch,sci.electronics,sci.physics,comp.lsi Subject: Re: Electro-optic bus Message-ID: Date: 24 Jul 90 01:39:37 GMT References: Sender: usenet@ux1.cso.uiuc.edu (News) Distribution: comp Organization: University of Illinois, Computer Systems Group Lines: 105 In-Reply-To: aglew@oberon.crhc.uiuc.edu's message of 16 Jul 90 21:59:01 A while back I posted about an electro-optic bus to comp.arch. It generated a flurry of responses, but nobody provided the real data that one correspondent rightly suggested would be critical to an electro-optic bus. I am hoping that sci.electronics or comp.lsi or sci.physics readers may be able to fill in the gaps. (For these newsgroups the original post is included at the end of this post). We know that optics can achive high bandwidth, ie. throughput. But what is the latency of an electro-optic system? Eg. what would be the time to covert from a - GaAs electronic (voltage) signal, - to optics (typically via a laser diode, hopefully on the GaAs chip) and then to receive - native Si optical receiver. - amplify the signal to CMOS levels. For good measure add the time to drive a wire between the Si and GaAs chip. What is the ballpark for this electrical->optic->electrical conversion? Tens of nanoseconds? Sub-nanosecond? Any experience people have in timing discrete electro-optical systems would be helpful, providing at least an upper bound. The original post: >Newsgroups: comp.arch >From: aglew@oberon.crhc.uiuc.edu (Andy Glew) >Subject: Electro-optic bus >Organization: University of Illinois, Computer Systems Group >Distribution: comp >Date: 16 Jul 90 21:59:01 > >Here's a throwaway - the electro-optic star bus: > > We all know that electrical interconnects such as busses have >major performance problems - loading, skews, etc. [*]. >The longer and the more things you talk to, the slower. > > We all know that optics has much more bandwidth than electrical >busses - right? (Maybe I should put a smiley here?) > > We all know that the biggest problem with optics is that Si >doesn't interface to it well. GaAs does, but GaAs doesn't achieve >Si's levels of integration [**]. > > What is, perhaps, less well known is that Si can build fairly good >*receivers* for optics; Si just cannot build good transmitters. Even >MOSIS's CMOS processes can build fairly good receivers. > > > Throw-away idea: take, say, 4 Si microprocessors. Give them each a >point-to-point (easier to make fast) electrical interconnect (wires), >from the Si chips, to a GaAs chip. Let the GaAs chip take these 4 >sets of electrical signals, and compress them all onto a faster >optical bus that is sent back to the 4 microprocessors. > Ie. use the GaAs chip as the hub of a star, with incoming signals >in electronics, and outgoing signals in optics. > The Si chips all receive the optical signals with native Si >receivers. > Since all of the signals are broadcast on the optics, we >effectively have a bus. If the Si receivers and logic can be made fast >enough, then we could snoop and do all those sorts things that are >nice to do on busses. Ie. bus bandwidth is no longer the constraint - >the constraint is how fast the Si receivers and/or the Si cache >snooping logic can be made. > > > > >Waiting anxiously to be shot down... > > >[*] this is not to say that there aren't people who believe that > existing bus technology cannot be milked. Such as making existing > 25 MB/s busses run at more than 4 MB/s under real usage patterns. > But that's another story. > >[**] I am, of course, neglecting the problem of coupling optical fibers > (if they were fibers) to the Si chips. >-- >Andy Glew, aglew@uiuc.edu -- Andy Glew, andy-glew@uiuc.edu Propaganda: UIUC runs the "ph" nameserver in conjunction with email. You can reach me at many reasonable combinations of my name and nicknames, including: andrew-forsyth-glew@uiuc.edu andy-glew@uiuc.edu sticky-glue@uiuc.edu and a few others. "ph" is a very nice thing which more USEnet sites should use. There is an info-ph mailing list, or, contact Steve-Dorner@uiuc.edu.