[Gllug] More than 2 scsi tape drives

[Russell Howe]

On Wed, Jan 31, 2007 at 10:50:41PM +0000, t.clarke wrote:
> >From memory I believe some internal SCSI tape drives have an internal
> scsi 'end terminator'   (all scsi cables have to have a terminator at the
> end of the chain).   However, usually there is a switch or link on the tape
> drive to disable/enable the scsi terminator.    So, what you need is basically:
> 
> SCSI ADAPATER----------------DRIVE_1--------------------------DRIVE_2---|
> 
> where -| represents the scsi terminator.

Yup. Note that internal drives tend to have a single SCSI connector,
whereas external units tend to have two ("in" and "out", they're often
called, although I doubt there's much of a distinction between the two).

By far the most obvious way to do this is to turn termination off on all
your devices and install physical terminators at the ends of the bus.
Oh, and use an active terminator (these often have a light on them)
rather than a passive one.

Internal SCSI cables tend to have T-pieces along their length, allowing
the bus to continue past the devices all the way to what is often a
moulded-on terminator at the end of the cable. External cables tend to
just be point-to-point, and if you have an external device with a single
connector, you'll need a T-piece (sometimes called a Y-connector).

> If drive 1 is terminated when it shouldnt be drive2 wont see the signals I
> guess !!

If only it were that simple - a terminator doesn't stop the signal from
passing, and if a SCSI system is incorrectly cabled, with devices both
sides of a terminator, it is entirely possible that all devices will be
visible!

See http://www.scsifaq.org/ for a huge FAQ on all things SCSI (well,
most things - last I checked, it didn't really cover more recent
developments such as iSCSI, Fibrechannel, SAS, etc). Its coverage of
'traditional' SCSI was very thorough, though.

-- 
Russell Howe       | Why be just another cog in the machine,
rhowe at siksai.co.uk | when you can be the spanner in the works?
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