LVD SCSI Question

George Penokie GOP at
Tue Oct 15 05:53:16 PDT 1996

* From the SCSI Reflector (scsi at, posted by:
* "George Penokie" <GOP at RCHVMP3.VNET.IBM.COM>
In SCSI-2 and SPI the differential dirver protection is a simple circuit
that outputs directly into the differential drivers enable pin.  The
wording and the circuit (SPI only) are very clear as to how the protection
works.  It is basicly an on/off switch and when off the device will just
not be able to access the bus.

Now enter LVD where the on/off switch become an on/on/off switch and there
is no longer a clear indication as to what to do with the three resulting

One can only assume the hardware would have logic that would continuously
monitor the three signals as follows.
-The HVD signal would cause all the drivers to be disabled in the
same fashion as in SCSI-2 and SPI.
-The LVD signal would cause the LVD drivers to be enabled and the single
ended driver to be disabled.
-The single ended signal would cause a the single ended drivers to be enabled
and the LVD drivers to be disabled.

So far so good. One could argure that, based on the descriptions in SPI,
that is the only logical conclusion.

But there is more: To support LVD and single ended operations the software
must be involved.  In SCSI-2 and SPI how the software handled the mixing
was not importance because the drivers where turned off so the device
could not receive or send anything on the bus.  This not the case for
LVD, however, where the device must operated properly in both a
single ended mode and a LVD mode.  In other words the devices software
must immediatly know when there is a mode change so it can transfer
information correcly on the bus.

The problem is the current description in SPI-2 could lead you to think
these mode changes will only occur after a reset or power on.  This is
NOT the case.  A device could be hot-plugged without a corresponded
reset or POR occurring. If this occured and the new device was single ended
the drivers would switch but the software would not and you could end up
transfering fast-40 data on a single ended bus (not good).

I belived that SPI-2 should make it clear that the DIFFSENS line
can change at any time and the devices hardware and software must immediately
respond to any of those changes.

Any comments???

Bye for now,
George Penokie
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