[filmscanners] RE: Which SCSI Card for SS4000

["Laurie Solomon" <laurie@advancenet.net>]

>> 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.

I probably shouldn't attemt to interpret Henning; but I will. :-)  I think
that what he is referring to is the fact that in most of the 15 device
adapter cards there is really in fact two independent strings of 7 devices
each; and each string needs to be terminated at each end which sometimes
appears to users as beign terminated in the middle since they see the two
strings as being one sting.  This appears also to be the case even in 7
device adapters where you can have both internal and external devices.
While the total number of devices is 7, these cards in effect use two
separate chains (an internal chain and an external chain) and both chains
need to be terminated at each end with the ends attached to the card being
terminated by termination supplied on the card.  To the user, this again
appears as if there is termination in the middle of the chain if they are
using both internal and external devices on the same card.

Typically, the user should not have to do anything special on many of
today's cards when it comes to termination on the end attached to the card
since the card usually does take care of this; but the directiopns often
talk about such terminiation and often in somewhat obtuse terms such that
the user may very well think that they need to do something about
termination on the card end of the cable (internal and/or external).  Some
of the older jumpered cards would reguire the user to change jumpers; but
this is not the case with the newer jumperless cards.

-----Original Message-----
From: filmscanners_owner@halftone.co.uk
[mailto:filmscanners_owner@halftone.co.uk]On Behalf Of Austin Franklin
Sent: Tuesday, September 10, 2002 7:22 AM
To: laurie@advancenet.net
Subject: [filmscanners] RE: Which SCSI Card for SS4000


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

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