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Newsgroups: comp.os.386bsd.bugs Path: sserve!newshost.anu.edu.au!harbinger.cc.monash.edu.au!bunyip.cc.uq.oz.au!munnari.oz.au!spool.mu.edu!agate!howland.reston.ans.net!gatech!news-feed-1.peachnet.edu!paperboy.wellfleet.com!noc.near.net!monk.proteon.com!jfw From: jfw@proteon.com (John Woods) Subject: Re: FreeBSD 2.0 does not detect first SCSI disk Message-ID: <D1HAHq.Dy5@proteon.com> Sender: news@proteon.com Nntp-Posting-Host: kerplop.proteon.com Organization: Proteon, Inc., Westborough, Ma. References: <3dc2pd$82f@hdxu03.telecom.ptt.nl> <D1EMH4.r7K@ns1.nodak.edu> Date: Tue, 27 Dec 1994 16:44:13 GMT Lines: 23 ortmann@plains.NoDak.edu (Daniel Ortmann) writes: >This sounds like the classical problem of leaving the terminating >resistors on one of your devices. The SCSI cable is a transmission >line, and the terminating resistors will absorb the signal, keeping >it from reaching the further devices. Right problem, wrong description. Terminating resistors in the middle of a SCSI chain do not "absorb the signal" (though they do absorb some of it). What they DO do is to cause reflections: the transmission line leading up to the mistermination point sees a mismatch (because you have the terminators in parallel with the impedance presented by the remainder of the cable), so you get some reflections back from that point. Some of the signal is dissipated in the resistors. Some of the signal DOES propagate down the remainder of the cable (where it reflects off the mismatch at the open end of the cable (if it is unterminated), and bounces back to the mistermination point, again dividing between reflecting, dissipating, and continuing on!). The exact behavior is a fairly simple exercise in RF transmission line theory, but the simple answer in the digital world is: If it hurts when you do that, DON'T DO THAT! (That said, I've successfully operated woefully screwed up SCSI chains, and had other SCSI chains crap out entirely with even the slightest problem. But I've never had trouble when everything was done right. :-)