QAS and non-packetized mode

Robert Elliott relliott at hobbit.eng.hou.compaq.com
Fri May 26 08:46:54 PDT 2000


* From the T10 Reflector (t10 at t10.org), posted by:
* relliott at hobbit.eng.hou.compaq.com (Robert Elliott)
*
Given that the Parallel SCSI working group seems to want to keep 
the patented QAS algorithm in SPI-4, I suggest we fix it so it 
can be used without packetized protocol.  It's a significant 
burden for tape drives and enclosure management devices to 
implement packetized.  They might be able to implement QAS 
alone, however, helping them coexist on a bus with disk drives.

I think the main technical reason for limiting QAS to packetized
mode was to simplify message snooping.  Snooping devices need
to know when to interpret 0x55 on the data bus as QAS_REQUEST 
rather than part of some other message.  It was simpler to
detect that in packetized mode when the task management functions
were handled in the packets rather than in message phases.
To relax this rule, QAS devices should snoop all message bytes, 
counting extended message lengths and letting the QAS_REQUEST 
occur at any time.

-- 
Rob Elliott      UNIX mailto:relliott at hobbit.eng.hou.compaq.com    
Houston, TX        PC mailto:Robert.Elliott at compaq.com
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