Spec issue on MSE leakage current in SPI-3

Daniel_F_Smith at notes.seagate.com Daniel_F_Smith at notes.seagate.com
Thu Apr 29 14:56:22 PDT 1999


* From the T10 Reflector (t10 at symbios.com), posted by:
* Daniel_F_Smith at notes.seagate.com
*
The problem I see is the chasm between a 5V device going higher than a
nominal TTL level on the bus and an LVD device's ability to control its
input current because of 5V tolerant circuitry that's trying to protect its
silicon.  If the LVD device must sink current to protect itself from older
devices that insist on sourcing more current than the spec or is practical
(I realize 'practical' is a very relative term), then the spec should allow
this type of current.  Otherwise, we don't have a chance of specifying a
multimode bus that will work for all people.  There's a practical limit to
this type of circuit design.  How realistic are we going to be?  Maybe its
time to retire the older devices that keep us from moving forward.  (I just
heard a big "Amen" from the gallery.)





"Bill Galloway" <BillG at breatech.com> on 04/29/99 01:51:47 PM

Please respond to BillG at breatech.com

To:   Daniel F Smith, t10 at Symbios.COM
cc:

Subject:  RE: Spec issue on MSE leakage current in SPI-3




* From the T10 Reflector (t10 at symbios.com), posted by:
* "Bill Galloway" <BillG at breatech.com>
*
>I don't know where 4.1V came from but it is obviously higher than the
>3.63V which any 3.3V device could possibly expect to generate on the bus.
>Besides, if LVD devices are current mode devices, why are we concerned
>about the bus being driven to levels that are HIGHER than the Vdd of the
>driving devices?

Dan,

What your Vdd happens to be is not important.  If a bus supports multiple
vendors and or multiple generations of devices the only thing that matters
is
the highest voltage of any device on the bus. There are 5V devices still in
common use today.  I do understand the design problems that this causes for
the
silicon but one major benefit of SCSI is the simultaneous support of
multiple
device generations.  Maybe someday we can outlaw old devices on new busses
but
for now I do not see how.

Bill Galloway
BREA Technologies, Inc.
P: (281) 530-3063
F: (281) 988-0358
BillG at breatech.com

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