Very Computer

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.

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.)?

Thanks,

--Brian

You may want to check comp.periphs.scsi for the SCSI FAQ.

From a pratical point of view, there are 8 bit (standard) and 16 bit (wide)
SCSI busses, each of them supports different speeds, too.

You can hook up fast/ultra/standard/... devices together on the same bit bus
(up to a max of 8 (16  for wide) devices on the same bus).
The transfer speed will be negotiated individually for each device. So a
slow standard SCSI scanner will not affect the transfer rate to an
ultra-fast HD on the same bus.

There are adapters available to switch between 8 and 16 bit, but they are
rather expensive.

Also there are two more variants, which are a bit more *:
- differential SCSI: Mostly used in mainframes and work stations. Although
they use the same connectors, they are NOT compatible with anything else!!!
- serial SCSI: very new, very fast, very expensive, not readily available
yet to the PC market. This will be the break in the compatibility chain: you
can't hook up a serial SCSI device to a parallel bus.

jue
--
Jrgen Exner; microsoft.com, UID: jurgenex
Sorry for this anti-spam inconvenience

[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
              cables.
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.
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
  disk-intensive multitasking.

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.

--
Rod Smith

http://www.veryComputer.com/~rodsmith
NOTE: Remove the digit and following word from my address to mail me

Can I just point out the existance of Ultra2 SCSI.  This does seem to be
available now and apparently provides 80 Mb/s.  Prices for drives here in
the UK are between 5% and 20% more than for Ultra-Wide.  Controller cards
(the only one I have found prices for is an Adaptec) seem to cost about 50%
more than Ultra-Wide.

I get the impression that the interface is the same as for Ultra-Wide
(i.e. 68 pin) and you can connect 15 devices.  I also seem to remember
reading somewhere that you only get 80 Mb/s if all the devices on the bus
are Ultra2 compatible (can anyone confirm or deny this?)

--
----------------------------------------------------------------------------

Richard Simpson
Farnborough, Hants, Uk                 Fax: 01252 392976

First of all, no disk (ok, U2W solid state disks will...) will run at
80 MB/s :-)

The main difference between Ultra and Ultra2 is that the electrical specs
have changed, to allow higher transfer rates and longer cables. The downside
of this is that SCSI-2 and Ultra devices are not compatible with Ultra2
systems. Now, you can still connect those old devices to an Ultra2 channel;
in this case the channel will operate as a normal Ultra channel to ensure
compatability. So you are right --- in order for an Ultra2 channel to
operate as Ultra2, there must be no non-Ultra2 device connected to it.
In particular, this means that you no longer get the benefits of Ultra2
(long cables, more devices...) when you connect a single non-Ultra2
device.

Of course there is a way around this --- SCSI bridges can let you split
an Ultra2 channel into two segments, with Ultra2 on one segment and
Ultra on the other.

Christian

--
Christian Stieber        http://www.informatik.tu-muenchen.de/~stieber