LVD SCSI (start round 2)

Gene Milligan Gene_Milligan at
Fri Oct 18 08:30:43 PDT 1996

* From the SCSI Reflector (scsi at, posted by:
* Gene Milligan <Gene_Milligan at>
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 

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 

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