8 Jan
2020
8 Jan
'20
9:54 a.m.
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: https://about.me/liamproven
Email: lproven at cix.co.uk - Google Mail/Hangouts/Plus: lproven at gmail.com
Twitter/Facebook/Flickr: lproven - Skype/LinkedIn: liamproven
UK: +44 7939-087884 - ?R (+ WhatsApp/Telegram/Signal): +420 702 829 053
---------- Forwarded message ---------
From: Gene Heskett <gheskett at shentel.net>
Date: Wed, 8 Jan 2020 at 17:29
Subject: Re: External Disk Intrusion
To: <ubuntu-users at lists.ubuntu.com>
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
predictable.
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
Genes Web page <http://geneslinuxbox.net:6309/gene>
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8 Jan
8 Jan
10:06 a.m.
On Jan 8, 2020, at 12:54 PM, 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.
I thought others here might.
I have trimmed the mail a little to the relevant parts.
--
Liam Proven - Profile: https://about.me/liamproven
...
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.
paul
10:56 a.m.
On 08/01/2020 18:06, Paul Koning via cctalk wrote:
Allowing accountants to do electrical engineering
makes just as much
sense as allowing sociologists to do brain surgery.
Off topic, but allowing accountants to do accountancy isn't always much
better.
I recall the Christmas when our accountant insisted on an inventory of
the electronics repair workshop. We kept our small components in a bank
of drawers some 1.5m (around 5 feet) wide by about 1m (3 feet or so)
high. Each drawer is just over 50mm (2") by 50mm. A drawer would hold
several tens of 1/10W resistors, or some loose transistors or
capacitors, or half a dozen ICs, or... And he wanted each one counted.
And a cost given for each single item. And insisted that estimates
were unacceptable for either count or cost.
A few years later, when the company was about to be wound up, a
colleague and I bought the bank of drawers and several other items for ?100.
--
Pete
Pete Turnbull
11:17 a.m.
On 08/01/2020 18:56, Pete Turnbull via cctalk wrote:
I recall the Christmas when our accountant insisted on an inventory of
the electronics repair workshop.? We kept our small components in a
bank of drawers some 1.5m (around 5 feet) wide by about 1m (3 feet or
so) high.? Each drawer is just over 50mm (2") by 50mm.? A drawer would
hold several tens of 1/10W resistors, or some loose transistors or
capacitors, or half a dozen ICs, or... And he wanted each one counted.
?And a cost given for each single item.? And insisted that estimates
were unacceptable for either count or cost.
A few years later, when the company was about to be wound up, a
colleague and I bought the bank of drawers and several other items for
?100.
I had a similar experience.
"We need you to list each item, with item. We'll check a sample later".
There were many hundreds of boards to list (as well as many systems,
where the requirement was more reasonable).
So I listed each board and when they came to check I picked (at random,
of course) a bag of M9047 cards.
"As you can see, they have an ID on the handle, but no serial number".
They checked, they agreed with my analysis and were perfectly satisfied.
Just simple people skills required :-)
Antonio
--
Antonio Carlini
antonio at acarlini.com
10:17 a.m.
Before I go delving into my pile of SCSI X3T10 documentation and
interface chip datasheets, exactly *which* flavor of SCSI are we talking
about here?
--Chuck
10:32 a.m.
On Wed, Jan 08, 2020 at 10:17:29AM -0800, Chuck Guzis via cctalk wrote:
Before I go delving into my pile of SCSI X3T10
documentation and interface
chip datasheets, exactly *which* flavor of SCSI are we talking about here?
Given the reference to the Amiga, almost certainly SCSI-1, i.e. 8 bit wide
single-ended HVD, clocked asynchronously at low single-digit MHz.
The A2091 (and thus presumably the A590) was a pretty hateful controller, but
the main sources of pain were its shoddy firmware and the limited Zorro-II bus
rather than the SCSI interface. Perhaps the third-party GVP controllers swapped
it around so the firmware was great but the SCSI side sucked.
10:45 a.m.
On 1/8/20 10:32 AM, Peter Corlett via cctalk wrote:
On Wed, Jan 08, 2020 at 10:17:29AM -0800, Chuck Guzis
via cctalk wrote:
SCSI-1 was pretty chancy to begin, but even SCSI-2 had a bit of voodoo
associated with it.
--Chuck
Before I go delving into my pile of SCSI X3T10
documentation and interface
chip datasheets, exactly *which* flavor of SCSI are we talking about here?
Given the reference to the Amiga, almost certainly SCSI-1, i.e. 8 bit wide
single-ended HVD, clocked asynchronously at low single-digit MHz.
The A2091 (and thus presumably the A590) was a pretty hateful controller, but
the main sources of pain were its shoddy firmware and the limited Zorro-II bus
rather than the SCSI interface. Perhaps the third-party GVP controllers swapped
it around so the firmware was great but the SCSI side sucked. 
10:49 a.m.
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
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
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.
10:58 a.m.
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
11:15 a.m.
On 1/8/20 10:58 AM, Richard Pope via cctalk wrote:
The A3000 did have
a problem with the hardware. If you upgraded the SCSI controller chip it fixed the A3000
problem.
I guessed it was a WD
http://eab.abime.net/showthread.php?t=73936
yup..
Amigas weren't the only systems that had problems with WD33C93s
11:27 a.m.
Al,
Yep!!
GOD Bless and Thanks,
rich!
On 1/8/2020 1:15 PM, Al Kossow via cctalk wrote:
On 1/8/20 10:58 AM, Richard Pope via cctalk wrote:
The A3000 did have
a problem with the hardware. If you upgraded the SCSI controller chip it fixed the A3000
problem.
I guessed it was a WD
http://eab.abime.net/showthread.php?t=73936
yup..
Amigas weren't the only systems that had problems with WD33C93s
3:44 p.m.
On 1/8/2020 9:54 AM, Liam Proven via cctalk 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.
The electronics of the SCSI interface were designed initially by the
precursor to Adaptec.? At the time that the SCSI committee was formed,
there was work by a number of companies.? Among them were HP, and the like.
I know the design for the 40mhz flavor was done by Emulex with Bill
Roberts, one of the founders involved in the design work at Emulex.
I'm not sure that bean counters factored into anything there, but
business did, as Emulex saw dominating the parallel bus market emerging
as SCSI as a critical goal for the corporation.
I went to an ANSI meeting with Bill Roberts when he presented work that
Emulex did that defined the 20mhz flavor.? Thru the process of the
definition of SCSI-I there was a continuous evolution of the performance
of the bus, and of the protocol to best exploit the additional performance.
The one thing that I do recall was that AMP would always be present with
a new and wonderful connector tweak or design if needed.? They got to
supply the bulk of the product as a result.
thanks
Jim
3:57 p.m.
On 1/8/20 6:44 PM, jim stephens via cctalk wrote:
On 1/8/2020 9:54 AM, Liam Proven via cctalk wrote:
And then you have DEC. I have often heard DSSI described as
SCSI done right.
bill
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.
The electronics of the SCSI interface were designed initially by the
precursor to Adaptec.? At the time that the SCSI committee was formed,
there was work by a number of companies.? Among them were HP, and the like.
4:04 p.m.
On Wed, Jan 8, 2020 at 4:57 PM Bill Gunshannon via cctalk <
cctalk at classiccmp.org> wrote:
And then you have DEC. I have often heard DSSI
described as
SCSI done right
That's reportedly what DEC thought of it, but in practice it was SCSI done
differently so as to be incompatible. The only technical "advantage" of
DSSI was that it better matched some of DEC's other proprietary storage
standards, thus enhancing vendor lock-in and profitability.
If you were another company trying to decide whether to use SCSI or DSSI in
your products, even if licensing and related cost weren't at issue, the
_only_ reason to use DSSI would be to interoperate with DEC.
I was a fan of DEC, but they did plenty of things for vendor lock-in rather
than technical reasons.
3:59 p.m.
On Wed, Jan 8, 2020 at 4:44 PM jim stephens via cctalk <
cctalk at classiccmp.org> wrote:
The electronics of the SCSI interface were designed
initially by the
precursor to Adaptec. At the time that the SCSI committee was formed,
there was work by a number of companies. Among them were HP, and the like.
There was no one common electronic design for SCSI. The electronic designs
were unique to many companies. The earliest implementations were
board-level interfaces consisting mostly of TTL. Early single-chip
implementations included those from NCR and Adaptec, though many others
came later.
While there were many companies involved in the standardization of SCSI,
the original SCSI 1 specification, ANSI X3.131-1986, was a relatively minor
elaboration on SASI, which had been developed exclusively by Shugart
Associates, hence the name SASI being an acronym of Shugart Associates
System Interface. Shugart Associates was an early maker of 8 inch floppy
drives and invented the 5 1/4 inch floppy.
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