Date: Sun, 3 Nov 1996 23:54:47 -0800 (PST) From: "Rodney W. Grimes" <rgrimes@GndRsh.aac.dev.com> To: gibbs@freefall.freebsd.org (Justin T. Gibbs) Cc: jkh@time.cdrom.com, hardware@FreeBSD.org Subject: Re: OK, what's the deal with 2940W controllers and internal connectors? Message-ID: <199611040754.XAA03238@GndRsh.aac.dev.com> In-Reply-To: <199611040242.SAA00127@freefall.freebsd.org> from "Justin T. Gibbs" at "Nov 3, 96 06:42:33 pm"
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> According to Adaptec, you are supposed to use only two or the three
you CAN use ANY two OF the three connectors...
> connectors on the card. I'm sure the EE members of the crowd can
> tell you why.
All three connectors on the card are electrically and MECHANICALLY
within the ``stub'' length allowed by the SCSI spec, thus you
don't have a Y (as some one else put it) as long as you only use
2 of the 3 connectors.
The SCSI spec allows a small unterminated stub to exists on the
SCSI bus. Your 2940UW looks something like this:
XW--IW--IN
XW is the eXternal Wide connector,
IW is the Internal Wide connector,
IN is the Internal Narrow connector, and
-- is a very short distance.
Now as you can see:
XW--IW--IN
| |
| ---------ND
WD
When we add drives using both internal connectors we have this
little stub bus out to the XW connector, no big deal, the SCSI
spec allows it. Note that the 2940UW must terminate ONLY the
high 8 bits of the wide bus in this configuration, and both WD
and ND must be terminated.
Other configurations can be found in the manuals, and as usual
termination configuration is left as an excercise to the user...
--
Rod Grimes rgrimes@gndrsh.aac.dev.com
Accurate Automation, Inc. Reliable computers for FreeBSD
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