External Disk Intrusion

[Gene Heskett]

On Wednesday 08 January 2020 11:41:46 Liam Proven wrote:

> On Wed, 8 Jan 2020 at 17:29, Gene Heskett <gheskett at shentel.net> wrote:
> > 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.
>
> TBH that level of component electronics is over my head. Would you
> mind if I quoted that on the ClassicCmp list and asked for comments?
> Anonymously or with credit, however you prefer.
>
However you prefer Liam, be my guest. You may get some conflicting 
arguments but 99% of them can be discounted by their being originated by 
folks who haven't a clue in the first place.  Ask them to prove their 
arguments with posted scope traces. I'd be glad to take a look. A scsi 
buss, properly setup, can be error free at full speed AND tens of meters 
long. It is after all a transmission line and all the math rules for 
those apply.  Violate those math rules, and you will have malfunctions.

Keep in mind this list like most is software related, but we're 
discussing (or cussing) hardware here, which is a different critter.  
Those with little or no knowledge of the physics involved, are out of 
their area of expertise.

Take care, and have a better 2020 Liam.

> --
> Liam Proven - Profile: https://about.me/liamproven
> Email: lproven at cix.co.uk - Google Mail/Hangouts/Plus:
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> 702 829 053


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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