FW: SCAM and Parity and Targets w/o ID Strings

Jeff Williams JWILLIAM at wdroc.wdc.com
Fri Dec 15 07:32:00 PST 1995

* From the SCSI Reflector, posted by:
* <Jeff Williams <JWILLIAM at wdroc.wdc.com>>


Two questions have come up.

1)  Does the initiator of the SCAM selection have to assert proper parity on 

the SCSI bus during the SCAM selection?  I read the protocol to say that no
parity is on the bus.  From SIP r15a Section B.4.1, "It [initiator of SCAM
selection] shall release all DATA BUS signals and assert the MSG
signal,...".  Is there any other interpretation?

2)  What happens when there are two or more targets on the bus which have
not spun up when the SCAM isolations have begun and their unique IDs are on
the drive?  In this case, all of the ID strings for same model targets will
have identical isolation phases and will all be assigned the same ID by the
two host bus adapters  I have tested.  Shouldn't the protocol take drives
without IDs and ignore them if the SNA bit is clear?  The annex states
that the target shall stall the isolation phase if it cannot get an ID
string.  What if the target is not configured to auto spin up?  Don't we
have a chicken and egg problem here?  I can't spin up until I get a Start
Unit command and I can't get a command until I spin up...

Jeff Williams
Jeffrey L. Williams                                   Office: (507) 286-7589
Western Digital Rochester                                Fax: (507) 286-7528
Enterprise Storage Group                      e-mail: jwilliam at wdroc.wdc.com

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