Design flaw in the SCSI spec?

Paul Koning paulkoning at
Wed Jan 8 12:06:40 CST 2020

> On Jan 8, 2020, at 12:54 PM, Liam Proven via cctalk <cctalk at> wrote:
> With his express permission, I'm forwarding a mail from a public list.
> I am interested in Gene's comments about the design of SCSI, but I
> don't know enough electronics to judge.
> I thought others here might.
> I have trimmed the mail a little to the relevant parts.
> -- 
> Liam Proven - Profile:
> ...
> And all that pickityness can be laid at the feet of a bean counter
> between the interface card designer, who specified a $2.00 schotkey
> diode for buss isolation, which had a maximum voltage drop across it of
> perhaps .1 volts, and changed to have an 8 cent Si diode with .666 volts
> drop across it, thereby lowering the logic one voltage by .45 volts.

Allowing accountants to do electrical engineering makes just as much sense as allowing sociologists to do brain surgery.  That's true whether you're building a disk interface or an airplane.

But why is such a corporate organizational screwup any reflection on the SCSI standard?  

I suppose it depends on the spec, and in particular the conformance requirements.  A properly constructed spec is written such that two conforming implementations will interoperate.  Ethernet and the DECnet specs are examples of this, and in fact that property was an explicitly stated requirement in the DECnet architecture team.

Unfortunately "conformance implies interoperability" is not nowadays all that common a standards property.  I've even found myself involved in a standard development effort where the standard document editor explicitly told me that "conformance implies interoperability" was NOT a goal for that standard.  I never quite understood why he thought that was reasonable, or why he was permitted to "hold the pen" on a major technical spec.


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