Apple's Technical Note "DV14: SCSI Bugs"
http://developer.apple.com/legacy/library/#technotes/dv/dv_14.html#//apple_ref/doc/uid/DTS10002404
has an interesting tidbit about bugs in SCSI Manager.
In pre-Platinum Mac Pluses, the ROM SCSI Manager code has a bug which will prevent the Plus from seeing hard drives which implement the "Unit Attention" condition.
So, if you're having trouble getting a newer SCSI drive to work on a pre-Platinum Plus, and the drive works perfectly on an SE or some such, try to find a formatting utility which will allow you to disable the Unit Attention option, assuming that the drive hardware allows enable/disable of Unit Attention.
From Tech Note DV14
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In the process of looking for a bootable SCSI device, the boot code issues a SCSI bus reset before each attempt to read block 0 from a device. If the read fails for any reason, the boot code goes on to the next device. SCSI devices which implement the Unit Attention condition as defined by the Revision 17B SCSI standard will fail to boot in this case. The read will fail because the drive is attempting to report the Unit Attention condition for the first command it receives after the SCSI bus reset. The boot code does not read the sense bytes and does not retry the failed command; it simply resets the SCSI bus and goes on to the next device.
If no other device is bootable, the boot code will eventually cycle back to the same SCSI device ID, reset the bus (causing Unit Attention in the drive again), and try to read block 0 (which fails for the same reason).
The `new' Macintosh Plus ROMs that are included in the platinum Macintosh Plus have only one change. The change was to simply do a single SCSI Bus Reset after power up instead of a Reset each time through the SCSI boot loop. This was done to allow Unit Attention drives to be bootable.
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http://developer.apple.com/legacy/library/#technotes/dv/dv_14.html#//apple_ref/doc/uid/DTS10002404
has an interesting tidbit about bugs in SCSI Manager.
In pre-Platinum Mac Pluses, the ROM SCSI Manager code has a bug which will prevent the Plus from seeing hard drives which implement the "Unit Attention" condition.
So, if you're having trouble getting a newer SCSI drive to work on a pre-Platinum Plus, and the drive works perfectly on an SE or some such, try to find a formatting utility which will allow you to disable the Unit Attention option, assuming that the drive hardware allows enable/disable of Unit Attention.
From Tech Note DV14
======================================
In the process of looking for a bootable SCSI device, the boot code issues a SCSI bus reset before each attempt to read block 0 from a device. If the read fails for any reason, the boot code goes on to the next device. SCSI devices which implement the Unit Attention condition as defined by the Revision 17B SCSI standard will fail to boot in this case. The read will fail because the drive is attempting to report the Unit Attention condition for the first command it receives after the SCSI bus reset. The boot code does not read the sense bytes and does not retry the failed command; it simply resets the SCSI bus and goes on to the next device.
If no other device is bootable, the boot code will eventually cycle back to the same SCSI device ID, reset the bus (causing Unit Attention in the drive again), and try to read block 0 (which fails for the same reason).
The `new' Macintosh Plus ROMs that are included in the platinum Macintosh Plus have only one change. The change was to simply do a single SCSI Bus Reset after power up instead of a Reset each time through the SCSI boot loop. This was done to allow Unit Attention drives to be bootable.
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