LVDS and terminators

Skip Jones sk_jones at
Fri Oct 6 14:29:49 PDT 1995


If I understand you correctly, you are referring to the true "dream machine"! 
It is exactly what I proposed at our SPI meeting at the Newporter last March. 
(If you can call it a "proposal".  I have been told that it was more like a
typical hairbrain idea that spews from marketing mouths!).  Ideally, the same
bus could operate both LVDS bewteen LVDS nexus' and switch dynamically to se
when required.  There is no doubt that such a notion is by far the most
appealing from a marketing point of view.

Unfortunately, I am but a slave to the engineering community of our industry.
Their technical puissance collectively banded together at that meeting and
afterwards to reduce my notion to nothing more than the babblings of an idiot! 
In other words, from an engineering point of view, I'm told it's, "Not
feasible".  If there is some way that it could be feasible, it certainally is
the right way to go!

Perhaps someone out there could re-shed some light on why it is that the bus
cannot actively support dual modes.  

Again, the idea is that the bus would default to se if there were any se
devices attached (let's say for now that this would be determined by the
DIFFSENS line).  Then a message negotiation could take place to determine which
of the attached units are capable of switching to LVDS.  SE operation prevails
when xferring data with the SE device/s, while LVDS is activated when xferring
with LVDS-capable devices.  

I assume that there is some electrical problem that the SE device introduces or
senses when the LVDS xfers are occurring. Or maybe it has something to do with
none-data xfer phases, where everyone on the bus has to deal with arbitration
between se and LVDS and the initiator has to reckon with resel from both types.
 But rather than make assumptions, I await the advise of those of you more
astute in the field of xmission theory.

Troubleshooters, please take your best shot!

Skip Jones
Marketing Manager, QLogic
From: Tak Asami on Thu, Oct 5, 1995 7:11 PM
Subject: RE: LVDS and terminators
To: scsi at

Hi Skip:

Thank you for your commentary.

Well, I am not trying to be "wrapped up in the cost of technology", 
rather, I had a hidden agenda.

I openly asked the question, "what's the big deal about auto sensing
of the LVDS"?  Jim's reply was so that he can have an inventory of one
product that he can sell to either (SE or LVDS) market.
Well then, perhaps you didn't need the dynamic sensing of the LVDS.

Truth is, I wanted it, but didn't know if it was possible with the way
it is being defined in the draft document.

I was wondering what's limiting us from switching back and forth between 
SE and LVDS.  So a HBA can negotiate with each target to go LVDS during 
the data transfer, just like you negotiate synchronous transfer.  I think 
that's more useful, and we almost have all the hardware to do it.  
We will need a new message or twisting of an old message (like negotiage 
for 25nsec cycle and go LVDS).
This is only between the initiator and the target with a nexus.  No one
else need apply, and consequently, all arbitration and selection processes
are performed single ended.  Now that's a backward compatibility.

What's wrong with this picture?  I have some unanswered questions:
- Can someone implement a terminator that switches back and forth quickly
  between SE and LVDS mode by sensing the diffsens?
- Does switching SE to/from LVDS like that cause any electrical problems?

I think this is more useful and makes sense if the standard is put in 
place to enable this "interactive swithing between SE and LVDS".
But I didn't know if it was technically feasible, and didn't want to
discuss too openly until I have some more sense of it, plus if anyone is
really interested.
                           Tak Asami
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