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Mad With Macintosh

 

Mac Plus ROM Bugs Explained

 

 

From: GMulhollan@aol.com
Date: Sat, 27 Nov 1999 20:56:20 EST
Subject: Re: Booting a Plus from the HD
To: classic-post@hitznet.com

>From: Chris Vale <pav123@erols.com>
>To: classic-post@hitznet.com
>Subject: Booting a Plus from the HD
>
>I'm trying to get a Mac Plus to boot from a newly installed 250 Meg hard
>drive (installed in the kind of enclosure that the Mac sits on top of)
>which came from another Mac.  System 7.1 is on the hard drive, but it
>gives a question mark at startup and wants to boot from diskette.  When
>its finished starting up, it doesn't 'see' the hard drive.

There is a bug in some of the early versions of the Mac Plus ROM as detailed
in Apple Tech Note HW 11.
Quoting from this document:
"1st version (Lonely Hearts, checksum 4D 1E EE E1):
Bug in the SCSI driver; won't boot if external drive is turned off. We only
produced about one and a half  months worth of these.
2nd version (Lonely Heifers, checksum 4D 1E EA E1):
Fixed boot bug. This version is the vast majority of beige Macintosh Pluses.
3rd version (Loud Harmonicas, checksum 4D 1F 81 72):
Fixed bug for drives that return Unit Attention on power up or reset.
Basically took the SCSI bus Reset command out of the boot sequence loop, so it will only reset once during boot
sequence. This version shipped with the platinum Macintosh Pluses."
This leads to the 'unit attention' bug whereby a SCSI drive that issues a
unit attention after powering up or a bus reset is read as a fail so the
Plus issues a SCSI reset and the loop continues.

You can check the ROM version with, among others,  MacEnvy an early free
control panel.

If you can mount the drive after booting, with say SCSI Probe, it is likely
you have the funky ROMs.

The solution:
1. Turn off unit attention in a modern drive with either software driver
control or hardware jumper
2. Find a Plus with the latter version of ROMs
3. Cut the 'sense' line in the SCSI cable (I did this to 'fix' an early
Maxtor external drive; I actually cut the internal cable inside the housing
and it has booted just fine ever since; WARNING; this is not recommended
unless you are desperate).

Good luck,
GM

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