Re: The sad Truth about iMac:

[dempson@actrix.gen.nz (David Empson)]

coaxial <coaxial@escape.ca> wrote:

> In article <david.921023197@wraith>, david@uow.edu.au (David Wilson) wrote:
> > SCSI-3 has not been released so where did they get SCSI-5?
> 
> I don't know. I found it in my most recent version of "inmac" catalog,
> which is published by MicroWarehouse (the Canadian version). Despite the
> name of the catalog, it has absolutely nothing to do with Mac's, it's a
> catalog for PC hardware, software, networking & supplies.
> 
> Here's how they (MicroWarehouse) listed it:
> 
> SCSI I ...............up to 5Mbps
> SCSI II (wide)........up to 10Mbps
> SCSI III (fast).......up to 20Mbps
> SCSI V (ultra)........up to 40Mbps
> 
> Perhaps they screwed up?

Note that these speeds are megabytes per second (written MBps), not
megabits (written Mbps).

They've invented their own names for the higher speeds, and they have
also made at least two errors.  The names I'm more familiar with:

Standard SCSI      5 MB/s
Standard/Wide      10 MB/s
Fast SCSI          10 MB/s
Fast/Wide          20 MB/s
Ultra SCSI         20 MB/s
Ultra/Wide         40 MB/s
Ultra2 SCSI        40 MB/s
Ultra2/Wide        80 MB/s

All the narrow variants use a 50-pin cable and the centronics-style
connector.  The wide variants use a 68-pin cable and the corresponding
connector.  Wide does 16-bit transfers and can support 16 devices on the
bus, while narrow does 8-bit transfers and supports 8 devices.

The higher speeds impose stricter rules on the cables, presumably
affecting the length, electrical characteristics of the wire, internal
shielding requirements, etc.

There is also a third categorisation: "single-ended", "differential" and
"low voltage differential" (LVD).  I think this just affects the
interpretation of the signals on the cable: a signal and ground pair are
replaced with a signal+ and signal- pair, for example.

We use the single-ended standard narrow SCSI variety on the Apple II,
though the computer limits the transfer rate to a maximum of 1 MB/s.


The terms "SCSI-1" and "SCSI-2" refer to revisions of the standard,
which includes details like the command sets and device types.  SCSI-1
was rather limited (hard drives were one of the few device types that
was clearly documented).

SCSI-2 standardised a lot more devices including CD-ROM, and defined
some of the faster speeds.  There is a SCSI-3 standard, but I don't
think the final version has been released yet.

The "fast", "wide" and "differential" variants are all documented under
SCSI-2 (possibly even under SCSI-1).  I think "ultra", "ultra2" and the
LVD option are later refinements, but I haven't gone looking at my
(draft) SCSI-2 standard for a while, so I can't be sure of this.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand