[Coco] Coco to PC cable

Gene Heskett gene.heskett at verizon.net
Thu Mar 12 11:07:07 EDT 2009


On Thursday 12 March 2009, Steven Hirsch wrote:
>On Wed, 11 Mar 2009, Gene Heskett wrote:
>> But school is in session for a moment folks.  I get down and dirty here.
>>
>> The legendarily fickle scsi interface doesn't have to be, if so darned
>> many compromises weren't inherent in the average setup.
>> 1) SCSI is a wired OR interface, built on the TTL signal level specs, eg
>> anything below .60 volts is guaranteed to be a logic zero, and anything
>> above 2.4 volts is guaranteed to be a logic one.  In between, its a gray
>> area.
>
>Excellent analysis, Gene!  Thanks for taking the time to write that up.
>It's important to note that more modern LVD SCSI interfaces _always_ use
>active termination.  Between that and the additional rejection of
>common-mode noise, they tend to be hugely more reliable.
>
>All that said, I actually have had very problems with single-ended SCSI-1
>drives over the years.  In my entire collection of vintage machines (more
>than most folks would believe can fit in one ranch house), the biggest
>PITA was an SGI Iris Indigo.  Despite conventional wisdom about always
>terminating the physical ends of the bus, it simply refused to recognize
>an external drive unless I _removed_ the terminator resistor pack from
>that unit.  Go figure.

Maybe it was internal?  Or it was designed by the same folks that made the 
Amiga TrumpCard?

>A lot of my luck probably stems from a penchant for short cabling.

I had a heck of a time with only a 10" cable in an amiga once, TrumpCard 
interface.  When I got down and dirty troubleshooting that, I cut the traces 
to the termpacks and reversed the supply hook ups, the pcb was laid out wrong 
so the 220 ohmers were connected to ground.  That was easier than removing and 
reversing all the soldered in termpacks by far. TrumpCard people were first 
class jerks anyway, and I didn't miss an opportunity to call them up one last 
time and call them something.  It actually worked pretty well after that.

BTW, I am a C.E.T. since 1972. :)

>The
>only SCSI drives I own that run > 3' from the controller are an LVD
>Ultra-80 RAID enclosure hooked to an IBM ServeRAID controller.  And, for
>reasons mentioned above, this is dead reliable.

I probably have about 3 feet between Marks TC^3 controller, and that Seagate 
Hawk 1GB on mine.  The TC^3 I believe has active terms, and I know the Seagate 
is termed, but don't recall if its resistor packs or active, probably r-packs 
at it's age since it is almost old enough to buy a beer these days. :)

>Steve

Thanks Steve.

-- 
Cheers, Gene
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