SPI-2 Issues - for Plenary vote in September

Jim Mcgrath jmcgrath at QNTM.COM
Fri Aug 29 09:55:24 PDT 1997


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* jmcgrath at qntm.com (Jim Mcgrath)
*
     Gene,
     
     I presume Paul was just giving formal notice and may not be
     directly advocating a plennary change in September.  However,
     many companies (including both of ours) will have LVD products
     out soon, so we cannot sit on this for long.
     
     In talking to Dick Uber, it appears that the original
     specification was made without any bottom up analysis or
     top down justification other than "less difference is
     better" (which is probably true in a limited sense).
     Given the chip and terminator tolerances Paul discussed,
     the fact that much of this difference is dynamic (i.e.
     it cannot be compensated for by adding capacitance in
     the PCB layout), 0.5 does seem unreachable.
     
     I think the real question is what should be the new,
     higher value.  If anyone has the resources, it would be
     excellent if data could be gathered on the difference
     in received signal between a bus with 14 intermediate
     devices with no capacitance difference and 14 devices
     with varying values of difference (0.5, 1, 2, 3, pF).
     Note that you only need one LVD drive for this test,
     since the intermediate capacitance loads can be sourced
     from a connector plug, not a drive unit.
     
     Jim
     


______________________________ Reply Separator _________________________________
Subject: Re: SPI-2 Issues - for Plenary vote in September
Author:  Gene Milligan <Gene_Milligan at notes.seagate.com> at SMTP2
Date:    8/28/97 8:04 AM


* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by: 
* Gene Milligan <Gene_Milligan at notes.seagate.com>
*
The issues Paul raised are not necessarily items for Plenary vote in September 
but they are clearly issues for working group discussion. The SCSI-2 terminator 
allowance is already a proposal being worked by Dean. 
     
The discussion on the capacitance requirements needs to include the tolerance 
versus margin tradeoffs not just what is easy. I look forward to the 
information that the tolerance was set too tight originally due to lack of 
analysis.
     
Gene
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