Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!swrinde!ucsd!ogicse!intelhf!ichips!omews54!colwell From: colwell@omews54.intel.com (Robert Colwell) Newsgroups: comp.arch Subject: Re: The Future of Buses (and Futurebus) and the Cray YMP Message-ID: <1990Dec13.194434.13692@omews63.intel.com> Date: 13 Dec 90 19:44:34 GMT References: <36734@cup.portal.com> <1178@shakti.ncst.ernet.in> <1990Dec13.153204.3294@bellcore.bellcore.com> Sender: news@omews63.intel.com (News Account) Reply-To: colwell@omews54.UUCP (Robert Colwell) Organization: Intel Corp., Hillsboro, Oregon Lines: 31 In article <1990Dec13.153204.3294@bellcore.bellcore.com> mo@messy.UUCP (Michael O'Dell) writes: >Very fast computers don't have busses. They do everything the >hard way: very fast point-to-point interconnects. > >Yes, the multiport memory controller on such machine is *quite* complex. Mike is quite right. The last machine we designed at Multiflow had a 15 nS cycle time, and it was no picnic even to get off one gate array, traverse a few inches of PC board, and make setup on the input flop to another. Getting across a backplane was much harder. To get enough connectivity we used high density connectors, but then crosstalk (with these very fast edges) can become a problem. And the connectors exhibit considerable impedance changes vis a vis the etch on the board or the backplane, not to mention their capacitance. The overall effect was that the gate array pins themselves were 3 unit loads, the backplane connectors were 3 loads each (and each signal saw at least two of them) and the destination array exhibited 3 loads. There are better impedance-controlled connectors coming out now using flexstrips and the like, but their reliability and longevity has yet to be proven, they're quite expensive, and they're largely single-sourced. Essentially, all important backplane buses had to be single-source, single-dest, driven by 50 ohm drivers, properly terminated at the destination. So is this any longer a bus, or just a collection of wires that share a common name? Bob Colwell mipon2!colwell@intel.com 503-696-4550 Intel Corp. JF1-19 5200 NE Elam Young Parkway Hillsboro, Oregon 97124