Date: Fri, 19 Sep 1997 11:26:11 -0700 (PDT) From: "Rodney W. Grimes" <rgrimes@GndRsh.aac.dev.com> To: dan@math.berkeley.edu (Dan Strick) Cc: FreeBSD-hardware@FreeBSD.ORG, dan@math.berkeley.edu Subject: Re: SCSI3 cables Message-ID: <199709191826.LAA11167@GndRsh.aac.dev.com> In-Reply-To: <199709190107.SAA26720@math.berkeley.edu> from Dan Strick at "Sep 18, 97 06:07:37 pm"
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I agree with 98% of your message, just one small point I want
to make.
...
> However, using gold plated connector shells with embedded LEDs
> does not make your electrons move any faster. Gold plated
> contacts and teflon insulation can make the physical cable
^^^^^^ ^^^^^^^^^^^^^^
> and the connections a little more robust, but they aren't
> worth the extra cost in most applications. You don't need
> silver wire. The manufacturer should have used an adequate
> wire size (28 awg of better) and the cable should have a
> "characteristic impedance" similar to the termination
^^^^^^^^^^^^^^^^^^^^^^^^^^
> resistance (usually about 115 ohms these days; was about
> 132 ohms before "active" terminators).
Use of Teflon(TM) is not just to increase physical strength,
infact that is a secondary reason. The number 1 reason is
that to make a ribbon cable that has a continuous characteristic
impendance you must control wire spacing very carefully at all
times. PVC just can't do this, when you bend PVC cable the
wire spacing changes all over the place, with Teflon which is
more rigid this does not happen. _Always_ use Teflon when
doing 20 Mhz SCSI busses is the rule here at AAI.
--
Rod Grimes rgrimes@gndrsh.aac.dev.com
Accurate Automation, Inc. Reliable computers for FreeBSD
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