Re: [aclug-L] SCSI revisited

[John Goerzen <jgoerzen@xxxxxxxxxxxx>]

OK, here's the basic story.  All of those are SCSI-2, so that part is
not really relevant.  The differences are in bus speed and bus width.
SCSI-2 is the protocol used.

Fast SCSI runs at 20 MB/s.  There are two possible connectors to use
with it (in external) or one ribbon cable that's used in internal
configurations.  In external, you can use a Centronics (incorrectly
aka "printer" cable) connector (these are huge) or the so-called
"mini-50" 50-pin connector (more common these days).  This is
sometimes called "narrow SCSI".

Ultra Wide SCSI has a bus speed of 20 MB/s but has twice the bus width 
of Fast SCSI (16 bits instead of 8), so it effectively performs at
40MB/s.  For both internal and external use, the connector is
trapezoidal in shape, although the internal connectors are slightly
different than the external ones.  The connectors are all 68-pin.  The 
reason I make the difference between bus width and signalling speed is 
this: it is possible to conect narrow devices onto a wide SCSI bus.
With the proper card and adapter, it is possible to have your wide
devices running at 40 MB/s and your narrow devices running at 20 MB/s
-- all on a single chain.  This is because the only difference between 
narrow and wide is the bus width.  The protocol and signalling speed
remain the same.

"Fast Wide SCSI-2" is somewhat of a misnomer.  Some vendors use it to
mean ultra wide SCSI.

There is a new thing called Ultra-2 Wide SCSI.  This runs the bus at
40 MB/s on a wide chain, for an effective speed of 80 MB/s.  It does
this by using so-called LVD (Low Voltage Differential) signalling,
which means that if you put any non-U2W devices on the chain (they
cannot comprehend this new signalling), your chain automatically will
cut its speed in half.

One thing to keep in mind is that only the very high-end SCSI drives
are approaching 20 MB/s, so if you only have one drive, Fast SCSI will 
be fine for you.  However, one of the great benefits of SCSI is that
you can hook 7 devices (or 15 for Wide SCSI) to a single SCSI card.
As you add drives, the higher SCSI bandwidth becomes more important,
since each drive will of course use up some of the bandwidth.

"spiff" <spiff@xxxxxxxxxxxxxx> writes:

> ok, perhaps now let's do a bit with things like
> Ultra Wide SCSI
> Fast SCSI-2
> Fast Wide SCSI-2
> and I am sure there are others that I don't know of...
> 
> 
> what are the differences/advantages of one over another?  sorry if this is
> remedial to some, but would much rather have a real definition that is
> understandable then go reading for weeks about this or that and still remain
> confused   :)
> 
> 
> 
> thanks again,
> Seth aka spiff
> 
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-- 
John Goerzen   Linux, Unix consulting & programming   jgoerzen@xxxxxxxxxxxx |
Developer, Debian GNU/Linux (Free powerful OS upgrade)       www.debian.org |
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