Re: SCSI

[David Wright <d.wright@open.ac.uk>]

Quoting Karl E. Jrgensen (karl@jorgensen.com):

> > If I can get to understand Scsi and find it a viable alternative to
> > IDE, it was my intention to attach a CD writer and some new 
> > harddrives externally (the board provides for this). Here again
> > I'm lost - some people I've spoken to insist only one device can 
> > be attached externally whilst the literature clearly says up to 7.
> SCSI takes at least 7. Later versions of SCSI (where the last address bit is used) can take up to 15.
> 
> > Furthermore, here in the UK no-one appears to stock any 'boxes'
> > or whatever for more than one device. A stock explanation seems
> Why should they? If they can sell one controller per device? :-) They probably would argue that it's "just overengineering to be safe" :-)
> On a serious note though, some cheaper cards do take quite a bit of CPU overhead, and I ended up with such a card when I bought by SCSI CD-Rewriter. Although it *can* take 7 devices (I now have a 18Gb disk, a CD-RW and a tape station running off it), it doesn't seem to have an impressive bandwith... Something to do with PCI-busmastering I've been told...
> 
> > to be "haven't seen any for years - there's no call for them" 
> > (One 'charming' individual said he knows nothing technical
> > about computer parts "I only sell them".

I'm not sure what John means by "boxes" for more than one device. A
SCSI card will typically have one external connector and possibly more
than one internal connector to cater for different connector-types.
The documentation will warn you not to use more than any two
connectors at the same time (else you have a star configuration,
not a chain).

To connect more than one scsi device to the external connector,
they have to be daisy-chained, and most devices will carry two
connectors to allow this.

The order of daisy-chaining can be important when you have lame
8-bit devices that don't carry through the wires for bits 9--16,
or which have tacky 25-way D connectors which are not real
SCSI connectors anyway.

Cheers,

-- 
Email:  d.wright@open.ac.uk   Tel: +44 1908 653 739  Fax: +44 1908 655 151
Snail:  David Wright, Earth Science Dept., Milton Keynes, England, MK7 6AA
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