Ultra SCSI capacitance

Jim McGrath jmcgrath at qntm.com
Wed Jun 15 07:31:43 PDT 1994

        Reply to:   RE>>Ultra SCSI capacitance - ag

    I have to take issue with the statement that there is no data to
    support reducing device capacitance.

Bill Ham has been the only one presenting empirical data during the Ultra
SCSI discussions, and nothing he has presented implies we need 20 pF.  Even
in the SPI days nothing was presented that documented this as a need (as
opposed to a desire) - indeed, we settled on 25 pF in SPI.

While I would agree that lower capacitance is a desirable characteristic, so
is shorter cables and fewer loads.  The perfect SCSI bus environment would be
one where there is no cable and one device - but this is hardly a useful one.
 I have contended that it is NOT practical to require 20 pF for two reasons:

1) any system that requires all devices on the bus to be 20pF will not work
in the real world, since many systems will mix Ultra SCSI (at 20 pF) and SPI
(at 25 pF) devices on the same bus.  Indeed, the SPI 25 pF devices could
easily out number the Ultra SCSI devices initially.  Since the damage is
caused by the load placed on the entire bus by all devices, this does not
solve the problem.  Any attempt to outlaw mixing man matching 20 pF and 25 pF
devices simply cannot work in practice - the market will ignore anything like
that in the standard.

2) More importantly, we have concluded that 20 pF at the device is not
practical for low cost SCSI devices.  Quantum is an expert here - we make
more low cost SCSI devices than anyone else in the world.  Our customers are
requiring Ultra SCSI, but are not willing (i.e. cannot possibly afford) to
pay extra for the devices.  We have ben very consistent in our position here,
and believe we would be backed up by other high volume, low cost SCSI device

Ultimately my objections are similar to Norm's on cable length - this is a
market requirement that bounds the effective use of SCSI on the desktop.  And
the desktop is still key to SCSI, since it still drives the volume (unless
you guys want to hoist up the flag of surrender to IDE right now and like
with a low volume interface).


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