Design flaw in the SCSI spec?

Liam Proven lproven at
Wed Jan 8 11:54:58 CST 2020

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.

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---------- Forwarded message ---------
From: Gene Heskett <gheskett at>
Date: Wed, 8 Jan 2020 at 17:29
Subject: Re: External Disk Intrusion
To: <ubuntu-users at>

On Wednesday 08 January 2020 09:01:35 Liam Proven wrote:


> The thing is that actual SCSI cabling faults are very hard to
> diagnose. We used to have internet jokes about them and the need for
> sacrificial goats or chickens, but of particular colours for different
> cable types, pentagram drawn in blood, holy candles, etc.

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.

Since the logic one at the logic chips inputs had to be at lease 2.2
volts, and the nearest set of termination resistors gave 3.0 volts when
this buss isolation diode was replaced with a short, but with the cheap
Si diode in there gave a logic one voltage closer to 2.4 most of the
circuits noise margin was used up and considering that same bean counter
crossed out the 5% terminators in favor of 20% tolerance, the result was

The situation was much better when active terminations came into use, but
by that time the scsi buss's reputation was doomed.

But back in the day of the bottom 190 market tv stations were generating
their on air gfx needs with a bank of Amiga computers, every scsi card
that came in the door, if the engineer was worth his paycheck, that
diode was replaced the first time the Amiga it was in, crashed.  End of
problem except for one Trump Card, where in traceing that card, I found
the termpacks had been soldered in bass ackwards. It was easier to cut
and jumper the supplies traces than to unsolder the packs and turn them
around. So I did that in addition to swapping out the infamous diode.

We, in a middle 160's market, were the first to put our news on the air,
out as a webpage, which led to legal problems because CBS fussed about
their copyrights, so we had to filter any web content that came from
CBS, but that didn't last long after their bean counters discovered
there was money to be made.  That was all driven by ARexx and delivered
by dialup from that Amiga in those Jurrasic (in web time) days.

Who am I to criticise the makers of that stuff?  I have an 8th grade
diploma, but I am also a CET. Look that up if you care.

Cheers, Gene Heskett
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
If we desire respect for the law, we must first make the law respectable.
 - Louis D. Brandeis
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