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From: chugins@hpcupt1.HP.COM (Chris Hugins)
Newsgroups: comp.periphs.scsi
Subject: Re: SCSI A/B/C switcher
Message-ID: <100140002@hpcupt1.HP.COM>
Date: 4 Sep 90 16:55:57 GMT
References: <3908@ursa-major.SPDCC.COM>
Organization: Hewlett Packard, Cupertino
Lines: 21

I believe that SCSI will have severe problems with a switch-box.  There are
several reasons for this, not the least of which is the "glitching" which
would occur on the bus.  Many devices' transceivers may "glitch" the bus
at power-on and/or power-off.  No CRC is employed on data, therefore any
other device active on the bus may have the data being transfered be
corrupted in an undetectable manner.  Some manufacturers have recently
been using "glitchless" transceivers so to address this problem.

SCSI expects its bus to be terminated at each end.  "Breaking" a SCSI
bus with a switch-box may give interesting results.

You may dedicate a SCSI bus to your shared device.  Thus, you would have
no problems with other devices being active on the bus.  Also, you would
know that I/Os are quiescent to this device when you are messing with the
switch.  The processors that are not switched to the device would have
unterminated SCSI busses, but they wouldn't be used at that time.

That does waste some connectivity however.

Chris T. Hugins
chugins@hpisoa2.hp.com