SCSI, SCSI-2, Wide/Ultra, etc SCSI????

SCSI, SCSI-2, Wide/Ultra, etc SCSI????

Post by Christoph Traxle » Tue, 26 Nov 1996 04:00:00



Hello netfolks,

I know this may be a FAQ, but can somebody either answer this Q or point
me to the appropriate literature?

What is the difference between the various SCSI flavors? Can I, e.g.,
connect a Wide SCSI HD to an ordinary SCSI interface?

Thank you,

Chris


work: Inst. f. Theor. Physik  home: Ammerweg 3
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SCSI, SCSI-2, Wide/Ultra, etc SCSI????

Post by Paul Uiterlinde » Tue, 26 Nov 1996 04:00:00



> Hello netfolks,

> I know this may be a FAQ, but can somebody either answer this Q or point
> me to the appropriate literature?

> What is the difference between the various SCSI flavors? Can I, e.g.,
> connect a Wide SCSI HD to an ordinary SCSI interface?

> Thank you,

> Chris


> work: Inst. f. Theor. Physik  home: Ammerweg 3
>       Heinrich-Buff-Ring 16         D-35435 Wettenberg
>       D-35392 Giessen               +49 641 9805921
>       +49 641 702-2802

Chris,

You'll find most of your questions answered in the SCSI FAQ:
  http://www.cis.ohio-state.edu/hypertext/faq/usenet/scsi-faq/top.html
  http://www.paranoia.com/~filipg/HTML/LINK/F_SCSI.html
  http://www.ultranet.com/~gfield/scsi/scsi-faq.part1.txt

And yes: you can connect a wide SCSI HD to an ordinary SCSI interface.

Paul.

 
 
 

SCSI, SCSI-2, Wide/Ultra, etc SCSI????

Post by Roderick W. Smi » Tue, 26 Nov 1996 04:00:00




Quote:

> And yes: you can connect a wide SCSI HD to an ordinary SCSI interface.

Not if "ordinary SCSI" is defined as Fast SCSI-2 or SCSI-1; those are
8-bit busses, and Wide SCSI uses a 16-bit bus, so the connectors won't
physically connect.  Now, if you define Fast/Wide SCSI-2 (or various
protocols that build upon that) as "ordinary SCSI," it'll work, but
that's trivial.  Also, most Fast/Wide SCSI-2 host adapters have
connectors for Narrow (non-Wide) Fast SCSI-2 (and SCSI-1) devices, so
it *IS* usually possible to connect a Narrow device to a Wide host
adapter.

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SCSI, SCSI-2, Wide/Ultra, etc SCSI????

Post by Andrew E. Miles » Tue, 26 Nov 1996 04:00:00


: I know this may be a FAQ, but can somebody either answer this Q or point
: me to the appropriate literature?

Symbios (formerly NCR, and formerly owned by AT&T at one time),
is an influential party in the SCSI standards process. Their
ftp site is used by the standards committee.
ftp://ftp.symbios.com/

: What is the difference between the various SCSI flavors? Can I, e.g.,
: connect a Wide SCSI HD to an ordinary SCSI interface?

SCSI (Small computer system interface) is a descendant of the ancient
and now obsolete SASI interface introduced by Shugart Associates (defunct).

Basically, there is:
  Narrow SCSI =  8 bit bus (up to 8 narrow devices)
  Wide SCSI   = 16 bit bus (up to 16 wide devices, or 8 narrow and 8 wide)
If a controller supports wide, then it _may_ also support narrow.
Narrow controllers only work with narrow devices.

Historic Note: Wide SCSI used to use dual narrow SCSI cables.
Now, they all use a high-density 'D' connector (introduced in SCSI-3 spec)
on a single cable that is half as wide as a "narrow" cable.

Update: SCSI-3 isn't limited to wire cables - optical fiber is available.
Since fiber is serial in nature, Narrow and Wide are meaningless.

Then for speed there is:
  Plain SCSI =  5 MHz (synchronous)
  Fast SCSI  = 10 MHz (synchronous)
  Ultra SCSI = 20 Mhz (synchronous)
SCSI devices negotiate the best speed - a speedy device can operate
slower, but a slow device cannot speed up of course.

Speed Example: Ultra-Wide-SCSI = 16 bits * 20 MHz = 40 Mb/sec maximum.

Most disk drives cannot sustain more than 6Mb/sec read/writing to
the disk platter, but the _CAN_ burst in and out of the drive cache.

All devices made today conform to the SCSI-2 standard, or the SCSI-3
draft (I don't know if it has been finalized yet). SCSI-1 devices can
only be found at the odd surplus store.

SCSI-3 devices are compatible with SCSI-2 and SCSI-1 devices.
SCSI-2 devices are compatible with SCSI-1 devices.
SCSI-1 is the minimum functionality required by SCSI devices.

Note: a SCSI controller is actually a special kind of DEVICE!
(it can tell other devices what to do)

Finally, there is single-ended and differential bus SCSI.
99% of all devices are single-ended. Differential devices are not
common, but allow for longer cable lengths. Single-ended and
differential SCSI are _NOT_ compatible - you can cause damage
by mixing them up.

Example of a super-duper SCSI controller: Adaptec AHA-2940UW
   http://www.adaptec.com/
I own one, and it works great. I've also used several other Adaptec
controllers, and have never had a problem with any of them.

--

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SCSI, SCSI-2, Wide/Ultra, etc SCSI????

Post by J.C. Archambe » Wed, 27 Nov 1996 04:00:00


: I know this may be a FAQ, but can somebody either answer this Q or point
: me to the appropriate literature?

Your favorite search engine will be able to point you to the SCSI FAQ.

: What is the difference between the various SCSI flavors? Can I, e.g.,
: connect a Wide SCSI HD to an ordinary SCSI interface?

For the most part yes.  But one rule of thumb that I've found out the
hard way.  Your host adapter needs to be able to handle the transfer
rate of the fastest device on the bus.  Don't expect an old Adaptec
AHA-1542B to be able to handle a fast or ultra SCSI drive unless there
is a way to step down the drive to non-fast SCSI via a jumper or
intelligently negotiates the transfer speed to what the adapter can
handle.

Differential and non-differential SCSI is only interchangeable via a
mythical adapter that I don't know really exists or not.  Narrow and
wide can be interchanged all you want.  Most wide devices will work in
narrow mode.  Narrow adapters will handle narrow and wide devices.

 JCA
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