LVD - Multi Mode

Paul D. Aloisi 603-429-8687 ALOISI at
Wed Oct 16 04:13:54 PDT 1996

* From the SCSI Reflector (scsi at, posted by:
* "Paul D. Aloisi 603-429-8687 aloisi at" <ALOISI at UICC.COM>
George pointed out on the reflector that the bus can change from
LVD to SE when a devices is hot plugged into a running system.

We have talked about this a lot in the SPI-2 working group. The
multimode tansceivers and terminators are designed to switch after
a time delay. Any activity on the bus when this occurs will get
an error. The line + of all the signals is grounded by the device 
as well as the diff sense line. There are signals from our multimode
terminator to tell the controllers what mode the terminator is
in LVD or SE -- HVD the terminator shutsdown.

If the bus is configuration by be to the LVD rules which may mean
that Fast-20 SE may not work. Error checking with backoff speeds need
to be worked into the controllers. The user should be flagged that
the bus has changed to SE.

Hot plugging SE onto a multimode bus is nasty to say the least. The
switch over to SE by all the devices is in 100s of milliseconds to
prevent ground noise from ever triggering a change. A reset or 500
millisecond delay should be implemented, then unless it is a very
short bus - <0.5 meters Fast-20 is the maximum speed, but in reality
on a long bus this may be async is the highest speed that can be
run on the bus.

There should be a warning in the multimode controller manuals or help
files aobut SE devices hot plugged into a multimode system.

Thank you,
Paul Aloisi
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