km@cadre.dsl.PITTSBURGH.EDU (Ken Mitchum) (11/12/88)
The need to disable the "Unit Attention" feature on SCSI drives is well known to those adding their own SCSI drives and software to old MacPlus machines. No mention is ever made of the desirability of disabling the feature on drive intended for use in a MacII or newer MacPlus. While testing a Seagate ST277N on various machines, I discovered the following, which may interest those using more than one SCSI drive on a MacII: If two SCSI drives are connected to a MacII, one with Unit Attention and one withoout it, the drive without Unit Attention will apparantly always be booted from, regardless of the SCSI device numbers, even though both drives will appear as volumes on the desktop. What appears to happen is that on first pass through the SCSI devices, the Mac notices the drive with Unit Attention as busy, and goes on to the other drive. When Unit Attention is disabled on both drives, the drive with highest SCSI device number is booted from, as predicted. Since there appears to be no penalty in disabling the Unit Attention feature, it would appear to be desirable to do this on all SCSI drives that are capable of this. For those who expressed an interest in using the ST277N with SF&I, I will try to make the necessary resources available, or will post the necessary information to the net. Ken Mitchum KY3B Decision Systems Labs University of Pittsburgh km@cadre.dsl.pittsburgh.edu