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To: Wilko Bulte <wilko@yedi.iaf.nl>
cc: gibbs@plutotech.com (Justin T. Gibbs), asami@cs.berkeley.edu,
        scsi@FreeBSD.ORG
Subject: Re: 3940uw with long cables 
In-reply-to: Your message of "Sat, 07 Jun 1997 20:31:18 +0200."
             <199706071831.UAA00806@yedi.iaf.nl> 
Date: Sun, 08 Jun 1997 16:33:46 -0600
From: "Justin T. Gibbs" <gibbs@plutotech.com>
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>As Justin T. Gibbs wrote...
>
>> Perhaps.  There are a few important things to keep in mind when
>> tuning the bus length.
>> 
>> 1) Having equal distances between all devices is sometimes more
>>    important than total bus length since the varying RC constants
>>    can cause strange signal skew conditions if the lengths aren't
>>    matched.
>
>In addition to this: don't use both the internal and the external
>connector of a SCSI adapter (assuming the internal & external 
>connector are on the same bus). The differences in external versus
>internal (flat)cable are also no-good.

If you get quality external cables, along with high quatlity
connectors, this need not be an issue.  When dealing with cabling,
it is the impedance of the cable and its length that are important,
not its "external" or "internal" nature.  I would only recommend
a single external cable though, for example to an external enclosure
that strings multiple devices together via ribbon cable, since it
is almost impossible to meet requirement 1 (spacing) using external
cables with mixed internal/external devices.  Having two "clusters"
of devices like this is how Adaptec recomends filling a fast20-wide
bus since most external drive enclosures still don't seat 15 devices.

>> 6) Honor the 6m and 3m bus length limits.  6m is the longest for 10MHz
>>    and 3m is the longest for 20MHz single ended SCSI.  This assumes
>>    proper stub/gap matching, so real life buses may need to be shorter.
>
>Hm. The company I work for (==DEC) uses the rule that a single ended
>F10 (10MHz) bus can be a max of 3 meters.

Yes, my mistake.  The actual numbers are:

6m   - 5MHz sync
3m   - 10MHz sync (Fast10)
1.5m - 20MHz sync (Fast20)

>Wilko
>_     ____________________________________________________________________
> |   / o / /  _  Bulte  email: wilko@yedi.iaf.nl - Arnhem, The Netherlands
> |/|/ / / /( (_) 	Do, or do not. There is no 'try' - Yoda
>--------------------------------------------------------------------------
>

--
Justin T. Gibbs
===========================================
  FreeBSD: Turning PCs into workstations
===========================================