LVD SCSI (start round 2)
Gene Milligan
Gene_Milligan at notes.seagate.com
Fri Oct 18 08:30:43 PDT 1996
* From the SCSI Reflector (scsi at symbios.com), posted by:
* Gene Milligan <Gene_Milligan at notes.seagate.com>
*
I would be the last to say (or near last) that the LVD related definitions have
been scrubbed enough to cover all the requirements for those designers that
have not actually been in the meetings. I would certainly not be the last to
say they should.
But let's keep in mind the scope of mixing devices. LVD is only used when all
devices are LVD capable. The multimode is a market entry and inventory benefit,
not a configuration handle per se. Dynamic switching is for hardware protection
not for dynamic reconfigurations per se. Not working until the wrong
configuration is an acceptable minimum capability providing parts are not
damaged.
Having a dedicated single ended drive is not the only way to bring down a
working Ultra 2 system. An inadvertent unplugging of the terminator will also
bring it down. Incidentally, I presume a person who does not have sufficient
information to know not to plug a dedicated single ended drive into a working
Ultra 2 system is just as likely to configure it in the first place with a
single ended terminator. I don't think we have evaluated the electrical
consequences of such a configuration error.
I postulate, that outside of the factory, the most frequent mixing of single
ended and LVD units will not be plugging a single ended unit into a LVD system,
the frequent occurrence will be end users adding an eighteen Gigabyte LVD drive
to their single ended system as an upgrade. With multimode drives that should
be just fine.
So the message is don't add any complexity to facilitate dynamic handling of
high performance methods of reconfiguration from wrong installations. Just make
it clear how to achieve no damage and the capability to recover in due course.
Errors may very well be an adequate indicator of the wrong configuration. As
far as I know that is the case now with the installed base. Hot plugging of
correct configurations, including with multimode, is fair game for transparent
performance.
Gene
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