Ultra SCSI-2 (This title is too seriously funny: let's fix it..)

Tak Asami asami at dt.wdc.com
Tue Oct 3 10:27:21 PDT 1995


Lee Cleveland of IBM wrote:
> Is there any more documentation that describes a POWER up sequence where all
> devices assume singled ended and at some point switch to differential
> based on the system letting everybody know?

No, not really, as far as I know...

My understanding is that terminators define the diffsens line voltage level
to inform all devices on the bus that it is a LVDS bus.  If not, the terminator
has to change itself from "ideal diode" circuit to the "active" termination
circuit for single ended.  Therefore, you immediately know if you are on 
LVDS bus or not when you turn on.

It is not clear to me if multiple devices on the bus can drive diffsens line
to force it to differential or single ended.  If it cannot (I don't think 
it can), the terminator decide what type of bus it is, so a single
ended bus stays SE and LVDS bus stays LVDS as long as they live.

Well, someone correct me if I'm wrong, but if I am right, then why is it
so important to have that "backward compatibility?"  Originally, I thought
it was a good idea, but more I think about it, it really doesn't save you
very little.

Only potential is *IF* chip vendors put out the chips with do-either-one
drivers *at the same price* as the single-ended only chip (which cannot
exist by definition), then drive vendors can put out the device that can
hang on either SE or LVDS, but they are not expected of changing the 
personality.

Well, think again.  For Fast-40 wide bus, your burst rate is 80MB/sec.
Your on-board buffer must be build to allow that kind of bandwidth, but
it won't be utilized for single ended case, since it is not likely that
you can drive Fast-40 single ended (or will we?).  That's a major cost
compared to a mere protocol chip.  For a "lower cost" SE only drive, that's
the first thing to go.

I guess I am not complaining, but want to understand exactly what are we
driving to....  I'm sorry if I am boiling over the subject that were already
discussed in the past meetings (which I must have missed).  But I do 
appreciate if someone can explain it to me....
=========================================================================
                           Tak Asami
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_|/ _| WESTERN DIGITAL     (714) 932-7621 : Voice
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