Design flaw in the SCSI spec?

[Richard Pope]

Hello all,
     I used Amigas for several decades and I really liked SCSI. The 
problems with the Amiga and SCSI were related to firmware and not the 
hardware. It was important to use the latest SCSI Roms on the 2091 and 
the 590. The A3000 did have a problem with the hardware. If you upgraded 
the SCSI controller chip it fixed the A3000 problem. I still use SCSI 
with my windoze machine.
GOD Bless and Thanks,
rich!

On 1/8/2020 12:49 PM, Eric Smith via cctalk wrote:
> On Wed, Jan 8, 2020 at 10:55 AM Liam Proven via cctalk <
> cctalk at classiccmp.org> 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.
>
> Eric
>