Design flaw in the SCSI spec?

Eric Smith spacewar at
Wed Jan 8 12:49:21 CST 2020

On Wed, Jan 8, 2020 at 10:55 AM 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.

> 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.
The complaint is with the design of a specific host adapter, not of SCSI
itself, and specifically with the diode that host adapter uses to provide
termination power.

The complaint is only relevant to the use of passive termination, though
the specific host adapter might include passive termination on board. The
problem can be solved by removing the on-board passive termination and
using an active terminator.

A schottky diode will have a minimum drop around 0.3V (not 0.1V!), and
typical with an actual termination load the drop will be closer to 0.4V to
0.5V. Better than a normal silicon rectifier, but not hugely better.

Using real-world values of 5V termination power, 220/330 ohm passive
terminator, 0.45V for the schottky diode drop, and 0.7V for a silicon diode
drop, the resulting termination voltages are 2.73V with a schottky diode
((5.0 - 0.45) * 0.6) and 2.58V with a silicon diode ((5.0 - 0.7) * 0.6).
While 2.73V is clearly better than 2.58V, the difference isn't as huge as
Gene suggests.

The SCSI data lines are normally actively driven, so during data transfer
the termination voltage is not the only thing pulling the line up to a
logic high.

Even in the 1980s, a schottky diode cost under ten cents, not $2. It was
maybe a few pennies more than a silicon diode. It is highly unlikely that a
"bean counter" was involved in this decision at all. An engineer made the
decision, presumably believing that the slightly higher voltage drop of the
silicon diode didn't make a big difference.

Is a schottky diode better for powering a bus terminator? Certainly. Is it
a disaster to use a silicon diode in stead? In my opinion, no.

I suspect that the Amiga SCSI host adapter had other issues that are much
more significant with regard to system reliability.


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