[Posted and mailed]
Quote:> OK, don't laugh at me for asking this, but I am new to SCSI. I know there
> are different versions of SCSI devices, such as Ultra, Fast-Wide, etc. Is
> there a good guide somewhere on the internet which describes the different
> versions? I'm getting lost.
I'm sure there are, but I don't have any URLs handy, except to suggest
relevent FAQs at ftp://rtfm.mit.edu, which organizes them by newsgroup.
A very brief rundown:
SCSI-1: The original. 5MB/s over 50-pin cables. 8 devices (one of which
is the host adapter itself).
SCSI-2: Essentially SCSI-1 with some added software commands to
explicitly support devices like CD-ROMs and tape drives.
Fast SCSI-2: SCSI-2 with speedups, to allow 10MB/s speeds over 50-pin
Fast/Wide SCSI-2: Fast SCSI-2 with a second auxiliary cable, permitting
20MB/s speeds and 16 devices (including the host adapter
itself). Not very popular in its "base" form, but
these devices usually borrow the 68-pin cable from
SCSI-3: Various additions to SCSI-2, including a 68-pin cable. The last
I heard, it had yet to be ratified, but its improvements have
"leaked out" in the various other standards.
UltraSCSI: Another speed increase allowing a speed doubling, this time to
20MB/s on a 50-pin cable.
UltraWide SCSI: UltraSCSI over a 68-pin cable, permitting 40MB/s.
There are other, even more recent and/or * combinations, but these
are the ones that would be of most interest to the average desktop user.
Some points to note:
- SCSI devices are interchangeable over this entire range, though mixing
Wide and Narrow devices requires special adapters. I'm also not sure if
a Wide device will work with a Narrow host adapter, though you can
certainly use Narrow devices with Wide adapters (and most Wide adapters
include a Narrow connector for just this purpose).
- When mixing SCSI devices, the slower speed prevails FOR THAT DEVICE. If
you put, say, a SCSI-1 tape drive on a bus that also includes an
UltraWide hard disk, the disk won't be reduced to 5MB/s transfers.
- The speeds stated aren't likely to be met by an individual device of
that type. Even today, it's pretty rare for an individual hard disk to
exceed 10MB/s in sustained real-world transfer, though some do. You'll
get the higher speeds only when combining transfers from multiple
devices, as in a server or RAID setup, or when doing heavy
Quote:> Also, if I have a SCSI-2 interface, can I plug in non SCSI-2 devices into
> it (such as Fast-Wide, SCSI-1, etc.)?
Yes, though as I mentioned above, I'm not sure if a Wide device will work
with a Narrow host adapter.
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