Narrow SCSI - Expectations of Target's

[Henry Wong]

* 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 
to handle the following 'system' scenario (which is
out of my league and handled by the firmware folks). 

I would appreciate any & all feedback from both host
adapter & target device folks!! 
  
BASIC SCENARIO:
  1) A 'physically' WIDE Initiator with an ID=15
  2) A 'physically' WIDE Target with an ID=1
  3) System does not have SCAM & ID's are 
     hard jumpered...let's keep this simple 
  5) SCSI System is WIDE with Narrow only
     devices mixed in ..but the Target doesn't 
     know this yet.  
  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 
  7) The Initiator trys to select the Target
   
==> Now which of the three cases below is "correct"?
 => Case1: The target is only monitoring the lower
           SCSI Bus bits since it has reverted to 
           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 
           real SASI system since it gets the Identify
           /ATN phase.  The target basically handles
           the command w/o Disconnect.  After Cmd 
           cmplt, switches to Wide (until negotiated
           otherwise).
 => Case2: The target being physically Wide always
           monitors the high SCSI Bus bits even if 
           it has reverted to Narrow mode...thus
           will respond to the Initiator as if 
           it were Wide and business as usual as
           a "psuedo-wide device"...versus.. a 
           Narrow device in a "real" Narrow bus.
 => Case3: Have the target ignore the Selection 
           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 
           Selection.   
                
Perhaps "the" basic question is should the Target (knowing
it's SCSI connector is physically Wide... ie. 68-pin 
or 80-pin SCA-2) always observe and react accordingly
to what is present on the High SCSI Bus byte.  In 
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 
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 
when the Target PCB is physically Narrow (ie 50-pin 
connectored)??

Thanks!
Regards, Henry        

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