Date: Fri, 14 Aug 1998 15:53:14 -0600 From: "Justin T. Gibbs" <gibbs@plutotech.com> To: Ross Harvey <ross@teraflop.com> Cc: gibbs@plutotech.com, matti.aarnio@sonera.fi, aic7xxx@FreeBSD.ORG Subject: Re: AHA2790UW has speed-limit problems ? Message-ID: <199808142158.PAA03463@pluto.plutotech.com> In-Reply-To: Your message of "Fri, 14 Aug 1998 14:31:17 PDT." <199808142131.OAA20416@random.teraflop.com>
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>> 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
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