SCSI hardware question

[Ben Scott]

On 2/24/06, Dan Coutu <coutu at snowy-owl.com> wrote:
> I'm a bit fuzzy on the specifics of different types of SCSI ...

  Common problem.  I recommend http://www.scsifaq.org

> I believe that I'm seeing intermittent problems due to hardware being
> configured wrong.

  Also common with SCSI, alas.

> Using an Adaptec 29160 SCSI controller. This provides a variety of SCSI
> connection possibilities. The external connector is a 68 pin LVD. There
> are three internal connectors that provide the following: 68 pin LVD,
> 68 pin Ultra Wide, 50 pin Ultra SCSI.

  Do you know how many SCSI buses (channels) it provides?  Some
controllers provide multiple, completely separate buses, similar to
how many motherboards provide two completely separate IDE channels.

  Also, Adaptec (possibly others) has a technology that lets one have
multiple physical channels on the same logical bus.  This enables a
lower-cost, single-channel SCSI controller to host both LVD and SE
devices without dragging all the LVD devices down to SE mode.  One bus
is for LVD, one is for SE.  You only get one set of SCSI IDs for the
logical bus.  Sounds like that might be what you have.  The
termination requirements are made more complex by this.

  I suppose it's possible all four connectors are mapped to a single
logical bus.  In that case, most likely, the wide LVD bus passes
through the controller (and will need terminator packs at each end),
but the SE buses terminate at the controller, and thus only need a
terminator pack at the non-controller end.

  It's also possible there are four entirely separate buses, each with
their own set of IDs and termination requirements.

  Or something in between.

  You'll have to check the manual to find out which it is.

> Are these devices, the tape, REC, and controller, compatible with each
> other as detailed above?

  Pretty much all SCSI devices (except HVD (high-voltage
differential)) should be compatible with each other.  The SCSI spec is
very clear on that.  However, SCSI manufactures are notorious for
implemented the spec's incompletely or incorrectly, so even if
everything should work, it often doesn't.

> I'm not at all clear on the impact of the tape drive being single ended
> (SE), for example. The differences between Ultra2 and Ultra3 are also
> unclear.

  Wide = 16-bit, 68-pin or 80-pin
  Narrow = 8-bit, 50-pin

  Wide is negotiated.  You can put narrow devices on a wide bus, so
long as everything is terminated properly.

  SE = Single Ended
  LVD = Low Voltage Differential

  SE and LVD refer to signalling methods.  SE is a simple digital
signal, high or low on one wire.  LVD uses the same style of
differential signalling (comparing two complemented voltages) that
twisted-pair Ethernet uses.  This results in better signal integrity,
which enables faster/longer buses.  SE came first.  I believe LVD is
only available on wide buses.

    LVD is required to be negotiated by the spec.  All LVD devices are
thus also SE devices.  (The newest SCSI standards may have relaxed
this requirement for the fastest speeds; I'm not up-to-date.)  If all
devices on a physical/electrical bus support LVD, the bus runs in LVD
mode.  If even one SE device is present, the bus runs in SE mode.

  That being said, it isn't unheard of to find devices that have fatal
brain cramps if the SCSI bus negotiates at SE when the device was
designed around LVD speeds.

  All the "Fast" and "Ultra" designations just refer to the clock rate
the bus is running at.  These are also negotiated. Table here:

http://bscott192.home.comcast.net/scsi/SCSI_Parameters.html

  Finally (and I've said this before): IOMega sucks, so if you're sure
your buses are all okay, I'd start looking there.

-- Ben