Tue Aug 16 09:01:31 PDT 1994

With reference to my proposal to require SCSI-3 targets to free the bus if
-ATN is not activated after selection, Gerry Houlder wrote:

  "This change means that SCSI-3 targets cannot function in SCSI-1 or
  SCSI-2 systems that use selection without ATN. This will absolutely
  require initiators to put the drive in "SCSI-2 mode" or "SCSI-3 mode"
  before installing it in the system. ..."

A tradeoff needs to be made between compatibility with very old systems and
data integrity. I may be wrong, but I would guess that there are not
many very old initiators which do not activate the -ATN line, and
these initiators are probably not in systems which contain SCSI-3
targets. There should be a large number of SCSI-3 initiators which use
SCSI-3 targets, however. These SCSI-3 initiators are *required* to set
the LUN field to zero because it is reserved. (Please see SAM section
2.4.) Therefore if the targets accept commands without an IDENTIFY
message, they expose themselves to data integrity problems because they
will route all commands from SCSI-3 initiators to LUN 0 whenever the -ATN
wire is broken or it's driver is defective.

Another recent comment asked why the -ATN line is singled out for
protection. It is singled out because it is unprotected and it can cause
loss of customer data. The data lines, which obviously can cause loss of
customer data, have been protected by parity since SCSI-1. With a higher
percentage of multiple LUN devices becoming available, it's time to protect
against -ATN failures also since these failures may be equally

                                        Giles Frazier
                                        IBM Austin
                                        gfrazier at

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