Re: How many drives can a sun3 scsi bus really support?

James.

Not bizarre, unterminated.  As Robert noted SCSI is rf transmission line
phenomena.  On a Sun 3/60 and a Sun 3/160 I have on occasion had 6 drives,
in the old shoeboxes.  The drives were the embedded scsi, not the sun esdi
drives, so I only had two of the boxes.

The SCSI spec from the 3/50 era was 5 meters, approx 18 feet.  The DB-50
connector used at this time is not impedance controlled, so the number of
them is VERY important.  Using 3 cables (6 foot) exceeds the spec, so with 3
boxes the performance can be unpredictable.

Most of the problems I have encountered have come in two primary groups.
One, more than terminator on the chain.  This unpredictably and
substantially reduces the usable length of the bus,  Two, the DB-50
connector was used for many non-scsi applications, thus the cable was not
transmission line spec.  The scsi line drivers (hardware_ are quite robust,
so these cables would work well for one box, but when used later in the
chain, or when a good cable was used after in the chain, things would not
work.  Since the connector is not impedance controlled, each connector
increases the effective of the buss (shortened the physical usable length).
Remember that there is physical bus length inside the boxes, so 3 6 foot
cables is out of spec.

In practice, I found that carefully searching all terminators and removing
them or disabling them and using an active terminator at the end of the run
would cure most problems

The bus is fairly robust.  Up to 12 feet you can get with almost anything
reasonable.  At six feet you can almost use  coat hangers.  If everything is
as it should be and within spec, it is possible to make 21 feet.  For the
most part the number of devices is not important, but the number of boxes
and cable length is.

the newer sub-D (sun-4) cables and connectors are built to a higher spec and
will perform better.  The active terminators will outperform the passive
ones.

Good Luck
best
James E. Taylor
Note:  I am visually handicapped, so there be some context typos that I have
missed and of course the spell checker missed.  If this obscures any
information I apologize.
----- Original Message ----- 
From: "James Birdsall" <[email protected]>
To: <[email protected]>
Cc: <[email protected]>
Sent: Sunday, November 02, 2003 4:48 PM
Subject: Re: How many drives can a sun3 scsi bus really support?


> I once had four in three shoeboxes: three disks and a tape drive. It
> required a lot of shuffling to find an order and combination of cables
that
> would work. If I connected them with all short (1.5' cables), the last
disk
> in the chain wasn't visible. That last disk would only become visible if
the
> final cable in the chain was a six-foot cable. Bizarre.
>
> --James B.
>