Serious s2ibis problem?

Hello IBIS folk:

We've been trying to figure out why s2ibis produces a Pull-down (and
Pull-up) curve that is VERY different from our self-produced
tables (from SPICE), in particular at voltages from Vcc to 2*Vcc.
This led me to the FAQ under the /man directory of S2IBIS. Here,
I found:

  QUESTION (6): "How does s2ibis generate tables?"

  ANSWER:
        s2ibis uses the following scheme for determining what tables
        need to be generated for a particular pin:
        0) For POWER, GND, and NC ...
        
        --snip--

         3) For each I/O or 3-state pin...

        --snip again--

-> pulldown(-5->0):subtract the associated ground clamp table from
                the pulldown table

-> pulldown(0->Vcc): just use the pull-down table as simulated

-> pulldown(Vcc->2*Vcc):extrapolate using the point at
        Vcc and the point five points before Vcc in the pulldown table

A similar explanation is provided for pulldown table generation.
My questions are: WHY DO THEY DO OPT FOR EXTRAPOLATION WHEN THEY HAVE
                ALL THE DATA NESSESSARY IN THE SPICE OUTPUT FILES?
                WHY ISN'T THE TRI-STATE DATA SUBTRACTED FROM THE
                ENTIRE VOLTAGE RANGE?

After reviewing all of the files that are created (and left) by
s2ibis, I found that it creates V/I tables (***from -Vcc to 2*Vcc***)
for an I/O in: 1)Tri-state (un-enabled) conditions, 2)Pull-up conditions,
and 3)Pull-down conditions.

In searching through all of the IBIS documentation I could find, the best
explanation for how to produce proper Pull-down and Pull-up curves (for
either SPICE sim data or real data) was in the V2.1 Spec., under
"Other Notes:" of the [Pulldown], [Pullup], [GND Clamp], [Power Clamp]
Keyword section:

... The clamp curves of an input or I/O buffer can be measured
directly with a curve tracer, with the I/O buffer 3-stated.
However, sweeping enabled buffers results in curves that are
the sum of the clamping curves and the output structures.
Based on the assumption outlined above (which makes sense), the pull-up
and the pull-down curves of an IBIS model must represent the DIFFERENCE
 of the 3-stated and the enabled buffers curves. (Note... ...)
This requirement enables the simulator to sum the curves, without the
danger of double counting, and arrive at an accurate model in both the
3-stated and enabled conditions. ...

This explanation seems good for producing V1.1 tables as well (and
perhaps just didn't make it into documentation until the V2.1 Spec).

So:
Why does s2ibis not subtract the ***Tri-state (un-enabled)*** data
from the entire range of both pull-down and pull-up data to derive the
corresponding [Pulldown] and [Pullup] tables? Instead, it appears that it
takes EXTRA steps to produce seemingly innacurate tables... Is this a
serious mistake with s2ibis, or are am I missunderstanding something.
Thanks ahead for all feedback.

Regards,
-Scott Schlachter
 Design Engineering
 Actel Corporation
 Sunnyvale, CA
Received on Tue Nov 5 16:36:18 1996