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NJLen

Q: ? Disable Unit Attention on second SCSI Drive?

I installed a second internal SCSI hard drive on my old Power Mac 9600. I set the SCSI ID# at 2 and the termination to off. Should I put a jumper on J-7 "Disable Unit Attention"? This second drive is a refurbed Apple (IBM UltraStar). Both drives are running OS 8.6. Also, can you explain what this jumper does? The Hitachi/IBM website definition for the JP-7 jumper is over my head. Thanx Len

imac, Mac OS X (10.4.8)

Posted on Nov 3, 2008 5:45 PM

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Q: ? Disable Unit Attention on second SCSI Drive?

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  • by Grant Bennet-Alder,Solvedanswer

    Grant Bennet-Alder Grant Bennet-Alder Nov 4, 2008 10:11 AM in response to NJLen
    Level 9 (60,931 points)
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    Nov 4, 2008 10:11 AM in response to NJLen
    There was a diabolical problem with early Macs (Mac PLus era) and Unit Attention. When the Boot ROM software got a Unit Attention interrupt, it responded by resetting the SCSI Bus. This caused a Unit Attention Interrupt. Some others had problems with this, too, so drive manufacturers put in the strap to Disable that interrupt.

    You may Disable Unit Attention with impunity. It is only ever used in extreme error conditions, and status bits can be used to determine the same information.

    What is extremely important is that only the Last drive on the SCSI Bus have Termination Enabled. ID numbers are used extremely rarely to decide which drive goes first when both make a request at exactly the same time. It is a good practice to set the drives to provide Termination Power to the SCSI Bus to support other devices that may require that power.