vail at prepress.pps.com
Tue Aug 16 13:28:58 PDT 1994
Date: Tue, 16 Aug 94 11:01:31 CDT
From: GFRAZIER at ausvm6.vnet.ibm.com
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
I write firmware for SCSI targets and initiators. Since most of my
applications are in closed special purpose systems I simplify things
and don't use SELECT with ATN for most commands. With SCSI-3 I can
update my firmware easily but I hate new protocols that break older
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
If an INITIATOR SELECTs with ATN the next phase should be MESSAGE OUT.
Anything else and the transaction is broken by a phase mismatch.
Wouldn't this detect a broken ATN line?
A mandatory MESSAGE OUT following a SELECTION would only serve to
ignore the ATN line and hide bad hardware until later.
Or am I missing something still?
PrePRESS Johnathan Vail vail at prepress.pps.com (508) 663-7435
---------+ 900 Middlesex Turnpike Billerica Massachussetts 01821
solutions MEMBER: League for Programming Freedom (league at prep.ai.mit.edu)
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