On Sat, May 06, 2000 at 09:04:14PM +1000, DaZZa wrote:
> On Sat, 6 May 2000, Minh Van wrote:
> OK - your basic types of SCSI are pretty much like this {some of the
> fiures might be wrong - but you'll bet the idea}.
>
> SCSI - also known as SCSI1 - up to 7 drives, 50 pin connector, 5 Mb/s
> transfer speed
Virtually non-existant any more. 5MBytes/sec
>
> SCSI2 - 7 drives, 50 pin connector, 10 Mb/s transfer speed.
If people are refering to "SCSI" they generally mean SCSI-2. This is
sometimes refered to as Narrow SCSI too (to differentiate it from Wide).
Transfer speed is 10MBytes/sec
> Ultra SCSI - 7 drives, 50 or 68 pin connector, 20 Mb/s transfer speed
You mean Wide SCSI. And it's 20 MB/sec. Wide scsi actually works at the
same bus speed as Narrow, but is twice as wide (thus the name!)
> Ultra Wide SCSI - 14 drives, 68 pin connector, 40 Mb/s transfer speed
Although technically Ultra Wide, this is often just refered to as Ultra.
40MBytes/sec
> Ultra Wide LVD SCSI - 14 drives, 68 pin connector, 80 Mb/s transfer speed
LVD stands for low voltage differential (more on Diff later). 80MBytes/sec
> Most standards are backward compatible - except LVD, which will actually
> run on a normal Ultra SCSI card, just slower.
All devices (disks and controllers) are backwards compatible. This means
that you can connect an Ultra Wide device to a SCSI-2 card and it will
work - but it will only work at 10MB/s. You can also connect a SCSI-2
drive to an Ultra Wide controller, but then the entire channel (including
any Ultra devices on it) will only work at 10MB/sec or 20MB/sec.
If you're mixing Narrow (SCSI-2) and Wide (Wide/Ultra Wide/etc) devices on
the one chain then everything past the first Narrow device will only run
at 10MB/sec, whilst everything before it will run at 20. Thus you should
hook up all your wide devices first, then all of your narrow devices.
Most of the standard above are also available in "differential" modes
(eg, Differential Ultra Wide SCSI).The difference between diff and normal
scsi is the voltages used. SCSI signals are either 0 volts or +5 Volts
(I think it's 5 - i could be wrong). Differential is either -5 volts or
+5 volts. This allows it to be used over longer distances (We've got some
20 metre Diff Wide cables at work - i think the max for Diff Wide is about
28 metres?), but also makes it incompatible with normal scsi, so you can
not hook a differential drive onto a non-diff controller, and vica versa.
The exception to this is LVD. LVD (Low Voltage Differential) drives are
generally (I think always, but i'm not sure) auto-sensing between Diff and
non-diff, which means that if you hook an LVD drive onto a Diff bus it will
work as diff, and if you hook it onto a non-diff bus it will work as
non-diff. This means that you can basically plug them into anything and they
will work.
Just to confuse the matter even further you will occasionally find things
which can be switched between diff and non-diff. One good example is SCSI
Tape jukeboxes, where the robot itself (but not the drives) can often be
switched between differential and normal.
> So your Ultra SCSI card will run SCSI, SCSI-2 and Ultra SCSI drives - but
> _NOT_ Ultra Wide or LVD drives. Since you can;t buy Ultra SCSI drives for
> love nor money these days, your options are really second hand, or upgrade
> your SCSI controller.
Any SCSI card will run any drive (except in the diff/non-diff setup i
mentioned above). You may not get the performance you would with the right
card, but if you've only got one or two devices on the chain it's not going
to be too much of a problem.
Of course, then there's FC-AL (1062.5 Megabits/second, or just over 100
MBytes/sec, over Fibre), which is incompatible with the lot (and this
generally includes each vendors FCAL being incompatible with anothers).
Scott.
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