[G4] SCSI ID question

[Simon Muir]

I realised this didn't go to the list earlier. SCSI is becoming less
relevant for desktop Macs these days but there still appears to be a lot
of duff gen about! There's one typo I spotted below: SCSI only comes in
8- and 16- bit versions. There's no 6-bit! For the propellor headed,
there was a 32-bit version defined in the SCSI-2 spec, but never
implemented, as it would have used two cables!

-----Original message-----
Subject:    [G4] SCSI ID question
To:         Ron Steinke <ronsteinke at mac.com>
From:       Simon Muir <simonm at muircom.demon.co.uk>
Date:       Tue, 2 Mar 2004 22:00:00 +0000
Message-ID: <9XNB4IPgPQRAFwUv at muircom.demon.co.uk>

On the subject of "[G4] SCSI ID question," Ron Steinke said:
 >The SCSI socket on the mother board gives you the ability to attach up
 >to eight SCSI devices to that circuit (SCSI IDs from "0" to "7" equals
 >8 devices).

No it doesn't, at least it's not quite that simple.

You can have SEVEN devices, plus the host bus adapter on a standard
narrow SCSI bus. You can have FIFTEEN on a wide SCSI bus (plus the HBA).
It's because the data lines represent SCSI IDs during the initial stages
of a SCSI transaction - 8-bit equals eight lines, and 16-bit equals 16.

That said, you never want to have that many, for any number of good
reasons, not least performance. As a rule of thumb, the faster the bus,
the shorter the cables need to be and the less devices you want on it.

The other key thing is bus priority. SCSI IDs actually matter, because
they determine which device wins if two devices want the bus at the same
time. In 6-bit (narrow, old-fashioned) SCSI, ID7 is highest priority,
and ID0 lowest. Encouraging people to put the bootable disk at ID0 (in
the IBM-PC world) was rather dumb!

In wide SCSI they had to keep backwards compatibility, so it's a little
more awkward:

HIGHEST   7 6 5 4 3 2 1 0 15 14 13 12 11 10 9 8  Lowest

That way, a narrow device using the bus can't be confused by SCSI ID's
it can't 'see' (15 thru 8). Both wide and narrow devices can be made to
work fine on buses of the opposite type, provided connectors and SCSI
IDs are chosen carefully.

 >The primary limiting factor here is the number of power
 >connections that are available and the power output of the power
 >supply.

This is true for internal devices, but irrelevant if external devices
are being used.

 >As long as you provide the SCSI devices with different identities (no
 >duplicate numbers), you should see each device as a separate icon on
 >your desktop when you boot-up.

It's a REALLY good idea to put your usual bootable disk at ID 6 (wide or
narrow, it doesn't matter), and any other bootable disks at 5, filling
downwards to zero. This is especialyl true if you plan to use OS10,
which is heavily based on BSD UNIX underneath the hood.

 >You can then treat each device as an
 >individual site and choose to store your data in whichever location you
 >want. If you want, you can install separate operating systems on each
 >drive and choose to start up from different drives as the need arises.
 >Just remember to identify each drive as a separate identity.
 >
 >Look for the SCSI identification bridge pins on the drives. They are
 >usually identified by the letters "A0, A1 and A2" or just as "0, 1, and
 >2" printed on the circuit board. "0" equals ID-1, "1" equals ID-2, and
 >"2" equals ID-4.

NO, they don't - it's binary.

The first pin is the first bit of the binary number and so on. There are
three sets of jumpers for narrow SCSI and an extra pin for wide SCSI. DO
NOT USE ID 7 AT ALL (it is reserved for host bus adapters, such as the
one on your motherboard).

         Jumpers
IDs     8 4 2 1
  0      : : : :
  1      : : : I
  2      : : I :
  3      : : I I
  4      : I : :
  5      : I : I
  6      : I I : (put your main boot disk here)
  7      : I I I (don't do it!)
  8      I : : : (wide buses only from here on down)
  9      I : : I
10      I : I :
11      I : I I
12      I I : :
13      I I : I
14      I I I :
15      I I I I

Hope the above helps.


Regards,

Simonm.

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-----End of original message from Simon Muir-----

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Simon D. Muir                 +44 -117-962-9761
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