[filmscanners] RE: Which SCSI Card for SS4000

["Austin Franklin" <darkroom@ix.netcom.com>]

Henning,

> SCSI is often not logical when connections get a bit complicated, or
> even when they're simple. On my last computer I had 4 SCSI chains; 2
> simple ones allowing 7 devices and 2 of the 15 device kind. Some
> combinations needed termination in the middle,

There is never any reason to provide termination in the middle of a SCSI
chain.  If you do, then something is very wrong.  The typical problems
people have are device being terminated that shouldn't, bad termination, not
using ACTIVE terminators at both ends, cable problems...like using very long
cable to one device and short cables to others...  A host of many
problems...but NEVER should the bus be terminated in the middle.  It may
"work", or appear to "work" (meaning it'll work on odd phases of the moon or
something like that), but it isn't a technically sound solution.

> Just try different cables and
> termination combinations.

This is what I believe gets people in trouble.  SCSI termination rules are
really very simple.  Actively terminate the ends ONLY.  Most devices
themselves provide active termination when enabled.  Keep cable lengths
equal, and don't go over 18'.  Use decent cables, though I know it's tough
to know what a decent cable is...

I've dealt with many many many many SCSI problems, and the above summarizes
the problems I've found with 99.99999999999%.  Sometimes there is a device
problem, like the device violates the SCSI spec by having too much stub
length inside the box...or doesn't really terminate correctly...

> If a SCSI device can't be seen by the computer or doesn't respond
> consistently, it's usually a termination or cable issue.

That is very true.

One other issue is "TRMPWR", termination power.  This is a rather confusing
issue.  It is typically provided to the SCSI chain by some device...and it's
important when you are using external termination.  If you are terminating
the ends of the SCSI bus with devices, they should be active termination
internally, so TRMPWR is really not important in this case.

Quite a few times I found TRMPWR not set-up correctly, especially on disk
drives, as their description of how to set it up is typically confusing.
It's also fused, so if the fuse is blown, and you think you are providing
TRMPWR, you may not be.  If you are using external termination, get an
ACTIVE terminator with a power LED...this will show if you have TRMPWR on
the SCSI bus.

Austin

----------------------------------------------------------------------------------------
Unsubscribe by mail to listserver@halftone.co.uk, with 'unsubscribe 
filmscanners'
or 'unsubscribe filmscanners_digest' (as appropriate) in the message title or 
body