DiffSense Timing

Bill Galloway BillG at breatech.com
Fri Nov 20 08:50:21 PST 1998

* From the T10 (formerly SCSI) Reflector (t10 at symbios.com), posted by:
* "Bill Galloway" <BillG at breatech.com>

I am not all that excited by the 300ms maximum.  I would like to see both
numbers reduced.  I came up with 300ms because Paul Aloisi told me that this
number had been proposed in the past.  The 3 to 1 range is probably
appropriate because some of these delays may be implemented with R/C
circuits which are not very precise.  I thought it was more important to
agree on a maximum than fight too hard to lower all of the numbers. (I am
game if you are)

>>     Finally I just want to point out that the standard would need to be
>>clear when the 300 ms (which is OK by me) starts. For hot plugging the
>>change, if any, occurs 100 ms after the DIFFSENS change is sensed. But for
>>the device that is plugged in some time is consumed while it enables
>>resets the resetables, does a sanity check to determine it is eligible to
>>participate, primed and eager waits a first 100 ms, then makes an initial
>>DIFFSENS determination and considers DIFFSENS for 100 ms to see that it
>>stays in one mode (what about the alleged noise during that 100 ms) and
>>finally does its SCSI thing. As a minimum this takes 200 ms plus a skosch.
>>Depending upon how the 300 ms is measured the minimum time for a worse
>>device could be 600 ms plus a skosch.

I do not believe this 300ms maximum applies to the hot plugged device.  This
device is already constrained to be high-impedance until it is ready for
operation.  You could argue that it has the 10 second power on to selection
time to transition from high impedance to the correct mode.

I only want the maximum to apply to any device or terminator that is
sourcing or sinking current from the bus.  The high impedance devices are
not hurting anything.

>>     Well 300 ms is OK by me if the host treats it simply as the real
>>mandatory requirement for reset to selection and that the SCSI devices are
>>designing with their own DIFFSENS maximum filter time to meet the reset to
>>selection time.

While I would like the reset to selection time to be mandatory, it is not
required to fix this problem.  If a device changes its driver mode within
the 300ms the host can talk to all of the well behaved devices on the bus.
It still may not be able to talk to this very slow device but the problem is
no worse than any other reset.

Bill Galloway
BREA Technologies, Inc.
(281) 988-0358
BillG at breatech.com

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