X3T9.2/88-115

To:         X3T9.2 Membership

From:       Jim McGrath, Quantum

Subject:    SCSI-2 Implications for SCSI-3 Reconfiguration

Date:       [October 10, 1988]

-----------------------------------------------------------------------------


The following items are issues that should be resolved in the SCSI-2
standard in order to allow SCSI-2 devices to cohabit the same bus as
SCSI-3 devices undergoing auto-reconfiguration.  Note that these
changes are not sufficient to allow the SCSI-2 devices to participate
in reconfiguration, but rather to prevent them from interferring with
the SCSI-3 devices.

    1.  POR Ready Time - a time limit must be placed on the time from
        power on reset to the time when the device shall respond to a
        SELECTION by asserting BSY.  This is to insure that when
        configuration is being done by SCSI-3 devices that all SCSI-2
        devices are already powered up and ready to respond to a
        SELECTION.  It should be on the order of under 1 minute.

    2   Bus Reset Recovery Time - a time limit must be placed on the time
        from RST going false to the time when the device shall
        respond to a SELECTION by asserting BSY.  This is to insure
        that when configuration is being done by SCSI-3 devices that
        all SCSI-2 devices are already powered up and ready to
        respond to a SELECTION.  It should be on the order of under 1
        second.

    3   Additional OR-tied Signals - in order to allow for a signaling
        protocol while RST is asserted, at least one additional
        OR-tied signal (i.e.  other than RST and BSY) is required.
        Suggestion for maximum flexibility is to require SEL, I/O,
        and MSG to be OR-tied.

    4   Driving signals during Reset - during the reset condition, no
        SCSI-2 device should be driving any SCSI bus signal (except
        of course for RST if they are resetting the system).


Although any reconfiguration system implies some other limitations,
no other changes to the standard are required.  For instance, soft
reset may pose a problem, but in that case the system should simply
implement the hard reset option.