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Date:      Wed, 16 Apr 1997 15:07:36 -0600
From:      Brett Glass <brett@lariat.org>
To:        pete@sms.fi, kallio@cc.jyu.fi
Cc:        hardware@FreeBSD.ORG, hackers@FreeBSD.ORG
Subject:   Re: Repeated UNIT ATTENTION, what is it, is this serious?
Message-ID:  <3.0.32.19970416150727.006ccadc@lariat.org>

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In SCSI-speak, a "unit attention" condition means that a device was reset,
was powered up, or had a serious error. The host is notified of the condition
not when it occurs but only when it tries to send a command to the device --
which might be quite some time later. The command is rejected, but usually
succeeds when it's retried. 

This is a hack that was added to SCSI because there was no easy way for a
peripheral to notify the host right away when an error occurred.

The problem could be anything from a parity error to overlapping SCSI IDs
to bad termination to soft errors on the disk or tape.

--Brett




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