Date: Fri, 14 Aug 1998 15:28:07 -0700 (PDT) From: Matthew Jacob <mjacob@feral.com> To: "Justin T. Gibbs" <gibbs@plutotech.com> Cc: Ross Harvey <ross@teraflop.com>, matti.aarnio@sonera.fi, aic7xxx@FreeBSD.ORG Subject: Re: AHA2790UW has speed-limit problems ? Message-ID: <Pine.LNX.4.02.9808141527290.26869-100000@feral-gw> In-Reply-To: <199808142158.PAA03463@pluto.plutotech.com>
next in thread | previous in thread | raw e-mail | index | archive | help
It's exactly this kind of discussion that has encouraged me to think that Fibre Channel is a better technology when you're getting to these speeds. The cost differential between this and parallel SCSI is getting to be pretty small. On Fri, 14 Aug 1998, Justin T. Gibbs wrote: > >> Shorten the cables. Ensure that the distance between all connectors on > >> the cable are equal. Use a forced perfect terminator. > > > >Shortening the cables might help, but what is the principle behind equalizing > >the inter-tap distance? > > Assuming that the drives on the chain offer similar capacitive loads, you > want the cable distance to be equal between ports to maintain a constant > impedance throughout the length of the bus. > > >(2) segments less than 0.3m (stub clustering lumps loads > > and aggregates the impedance mismatch); > > ... > > >I suspect that people think equalizing segment lengths helps because it forces > >them to use the recommended minimum stub (load) spacing. But what makes this > >win is the elimination of stub clusters, not the equalization of segment runs. > > This is a common misconception. The requirement, although dictated as an > explicit number by the SCSI spec, is really trying to say, the ratio of > stub length to stub spacing should be at least 1:3. I have a fairly > in-depth testing report provided by Adaptec to OEMs using their parts on > MBs, where they show with signal analysis that keeping the ratio is what > matters. Considering that the stub length of most drives is ~1" of PCB > trace, the inter-stub spacing can be as low as ~.1m. > > > --Ross Harvey > > -- > Justin > > > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe aic7xxx" in the body of the message > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe aic7xxx" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?Pine.LNX.4.02.9808141527290.26869-100000>