Ultra SCSI capacitance - ag

dsteele at ccmgate1bb.FTCollinsCO.NCR.COM dsteele at ccmgate1bb.FTCollinsCO.NCR.COM
Mon Jun 13 08:20:14 PDT 1994

I have to take issue with the statement that there is no data to support 
reducing device capacitance.  Data on this has presented more than a year ago.  
We have seen over and over again that device capacitance is THE MAJOR 
contributor to the the bus reflections that cause the REQ/ACK double clocking 
problem.  This is explainable with basic transmission line physics.  I am not 
sure how practical it is to require 20 pF.  However, it is clear that it 
desirable if we can achieve it.

Dave Steele

                      Subject:                              Time:  5:39 PM
  OFFICE MEMO         Ultra SCSI capacitance - again        Date:  6/9/94

I read Larry's minutes of our last meeting, and was shocked to see the device
capacitance once again changed to 20 pF from 25 pF.  I realize that I was
stepping in and out of the room, but I did not notice that change.

Once again, Quantum's official position is that no data we have seen compels
us to go with 20 pF - 1.5m cables and 8 devices work fine, 3 m and 4 devices
also work fine.  In practice, desktop systems using Ultra SCSI drives will
probably have old SCSI devices at 25 pF on them anyway, and we are convinced
that 20 pF will cost our desktop customers money that they are not willing to
spend for no reason.

I am getting really pissed off at revisiting this issue, especially since we
see this urge to 20 pF as not being supported by any technical data we have


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