[Coco] Coco to PC cable
snhirsch at gmail.com
Thu Mar 12 07:19:23 EDT 2009
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.
A lot of my luck probably stems from a penchant for short cabling. 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.
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