Narrow SCSI - Expectations of Target's
Hugh Curley
hcurley at indra.com
Sat Oct 11 14:05:45 PDT 1997
* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Hugh Curley <hcurley at indra.com>
*
Henry,
Wide and narrow are negotiated for data transfer phases only. For =
selection, you see all the bits you are capable of seeing. If you are =
wide, you default to narrow on reset only for the data phases, you are =
still "wide" for arbitration and selection phases. Therefore, your case =
2 comes closest to answering the question, but the target is not in =
"pseudo-wide mode", it is operating as it should.
Hugh Curley
----------
From: Henry Wong
Sent: Thursday, October 09, 1997 3:53 PM
To: hwong at plum.asic.qntm.com; T10 at Symbios.COM
Subject: Narrow SCSI - Expectations of Target's=20
* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* Henry Wong <hwong at asic.qntm.com>
*
Being in ASICs I seldom get out of the 1's & 0's world.
I would like to hear what is the correct or prevelant way=20
to handle the following 'system' scenario (which is
out of my league and handled by the firmware folks).=20
I would appreciate any & all feedback from both host
adapter & target device folks!!=20
=20
BASIC SCENARIO:
1) A 'physically' WIDE Initiator with an ID=3D15
2) A 'physically' WIDE Target with an ID=3D1
3) System does not have SCAM & ID's are=20
hard jumpered...let's keep this simple=20
5) SCSI System is WIDE with Narrow only
devices mixed in ..but the Target doesn't=20
know this yet. =20
6) Target defaults to Narrow mode (let's
say after power-up) since it doesn't
know if the Bus if physically Narrow
or Wide yet=20
7) The Initiator trys to select the Target
=20
=3D=3D> Now which of the three cases below is "correct"?
=3D> Case1: The target is only monitoring the lower
SCSI Bus bits since it has reverted to=20
Narrow and is not aware if the System is
a Wide or Narrow Bus. It responds as if
it "thinks" it's in a SASI system because
it see's only one ID (it's own ID) on the
Bus. Yes.. the target knows it's not a=20
real SASI system since it gets the Identify
/ATN phase. The target basically handles
the command w/o Disconnect. After Cmd=20
cmplt, switches to Wide (until negotiated
otherwise).
=3D> Case2: The target being physically Wide always
monitors the high SCSI Bus bits even if=20
it has reverted to Narrow mode...thus
will respond to the Initiator as if=20
it were Wide and business as usual as
a "psuedo-wide device"...versus.. a=20
Narrow device in a "real" Narrow bus.
=3D> Case3: Have the target ignore the Selection=20
all together since it doesn't know if
it must assume it is Narrow because it
may be on a Narrow Bus and that it should
not expect to be seeing a SASI system.
Also, knowing it won't know if it is a
SASI system until it responded to the=20
Selection. =20
=20
Perhaps "the" basic question is should the Target (knowing
it's SCSI connector is physically Wide... ie. 68-pin=20
or 80-pin SCA-2) always observe and react accordingly
to what is present on the High SCSI Bus byte. In=20
the case of a Selection, always monitor bits 15-8+parity
for proper ID & good parity before responding <irrespective
of whether or not the Target is defaulting to Narrow
Bus transfer mode>?? This also implies that the SCSI=20
system MUST terminate the High Bus if it intends to use
and adapt a physically Wide device for a Narrow-only
SCSI system. Also, the High Bus Byte is ONLY ignored=20
when the Target PCB is physically Narrow (ie 50-pin=20
connectored)??
Thanks!
Regards, Henry =20
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