Narrow SCSI - Expectations of Target's

Hugh Curley hcurley at
Sat Oct 11 14:05:45 PDT 1997

* From the T10 (formerly SCSI) Reflector (t10 at, posted by:
* Hugh Curley <hcurley at>

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; T10 at Symbios.COM
Subject: 	Narrow SCSI - Expectations of Target's=20

* From the T10 (formerly SCSI) Reflector (t10 at, posted by:
* Henry Wong <hwong at>
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
  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
=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
 =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
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

Regards, Henry       =20

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