At 10:16 23-07-98 -0300, Ubiratan C. Peleteiro wrote:
>What is the basic diferences between SCSI-1, SCSI-2 and SCSI-3?

SCSI1 - First (now very outdated) SCSI standard
        SCSI1 didn't include a lot of features like built in "parity"
checking,
        the "wide" bus, "fast" clock speeds (see below), ...
        Avoid if you can SCSI1 devices, or older *25 pin* devices.

SCSI2 - Revision of above. The present standard. Aging. Doesn't include
        many things that are standard with most presently manufactured
        SCSI devices. But this is still the only "official" standard.

SCSI3 - Proposed (not yet official) new standard which will contain
        all the features that we have come to associate with SCSI.
        SCSI3 is used irresponsibly by manufacturers and vendors to refer
        to various things like "ultra" and even "wide". Don't think that
        a SCSI3 device is necessarily better than a SCSI2 device. Look at
        the specifications of the devices. As a matter of fact, just ignore
        SCSI2 and SCSI3 and look at the specs.

Things are pretty much backwards compatible. A SCSI2 controller will
support SCSI2 and SCSI1 devices. A "SCSI3" controller will also support
SCSI2 and SCSI1 devices. Remember, SCSI3 could refer to pretty much any
feature but wouldn't guarantee *every* advanced feature.

More information:

Regular SCSI =  8 data lines in parallel, 50 wires (one byte per clock cycle)
Wide SCSI    = 16 data lines in parallel, 68 wires (two bytes per clock cycle)

Regular SCSI - the SCSI bus runs at  5 MHZ (Million cycles per second)
Fast SCSI    - the SCSI bus runs at 10 MHZ
Ultra SCSI   - the SCSI bus runs at 20 MHZ
Ultra2 SCSI  - the SCSI bus runs at 40 MHZ

From the above, multiply the MHZ by one or two bytes (for narrow or wide)
to get transfer rates:

Narrow         5 MB/sec  (Max cable total length of 20 feet.)  8 devices max
Wide          10 MB/sec  (Max cable total length of 20 feet.) 16 devices
Fast (Narrow) 10 MB/sec  (Max cable total length of 10 feet.)  8
Fast Wide     20 MB/sec  (Max cable total length of 10 feet.) 16
Ultra         20 MB/sec  (Max cable total length of 5 feet.)   8
Ultra Wide    40 MB/sec  (Max cable total length of 5 feet.)  16
Ultra2        80 MB/sec  (Max cable total length of 30 feet.) 16

You might be able to extend the maximum cable length slightly using
highest quality cables and connectors. Cable length often limits you
to less than the maximum number of usable devices on a SCSI bus since
each device should be a foot apart (along the cable) from adjacent devices.

Ultra2 allows longer cable lengths because it uses "lvd" (low voltage
differential) signals. (Lvd devices *may* work as regular "single ended"
devices, except they then become regular ultra instead of ultra2.) Lvd
is *not* the same as the older (and never compatible with single ended)
"differential" SCSI. Differential and lvd use two lines for each data
signal (one line positive and the other line negative), taking the
"difference" between them as the signal. Single ended (regular) SCSI uses
only one line for each signal and is more susceptable to noise.

"SCA" devices have 80 pin connectors that combine the 68 pins along with
power and ID selectors. You can use an adaptor to connect a SCA device to
a normal cable. SCA is to allow for hot swaping in servers.

Transfer rates refer to bandwidth for the whole bus. No hard drive now
in existance offers 40 MB/sec. You would need two top of the line ultra
wide drives (each capable of 20 MB/sec) transferring data *simultaneously*
to get 40 MB/sec total...that is, to "saturate" an ultra wide bus.
Ultra2 is becomming common. 80 MB/sec would let you run four top of the
line drives on one bus without going much over limit. Fibre channel is
also becomming popular. This has a bandwidth of 100 MB/sec or much,
allows for much longer cable lengths, and many times as many devices on
the bus.

I have not mentioned "termination" and SCSI "ID"s at all.

Regards,
Bill