[Coco] Coco to PC cable

[Steven Hirsch]

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.

Steve


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