Here's how s2ibis2 sets up the default sweep ranges.

There are four reference values to define:

    pullup_ref:     Defined by the [Pullup Reference] keyword.  If this
                    keyword is not specified, defined by the 
                    [Voltage Range] keyword.

    pulldown_ref:   Defined by the [Pulldown Reference] keyword.  If
                    this keyword is not specified, pulldown_ref defaults
                    to 0 V.

    pwr_clamp_ref:  Defined by the [POWER Clamp Reference] keyword.  If
                    this keyword is not specified, defined by the 
                    [Voltage Range] keyword.

    gnd_clamp_ref:  Defined by the [GND Clamp Reference] keyword.  If
                    this keyword is not specified, gnd_clamp_ref
                    defaults to 0 V.


IF the model is not an ECL-type model, sweep ranges are defined as:

For output and I/O models:

    typ_range = pullup_ref - pulldown_ref  (This has a maximum of 5 V)

    pullup(down)_span = (gnd_clamp_ref - typ_range) to (pullup_ref + typ_range)

    gnd clamp span = (gnd_clamp_ref - typ_range) to (pwr_clamp_ref)

    power clamp span = (pwr_clamp_ref) to (pwr_clamp_ref + typ_range)


For input only models:

    typ_range = pwr_clamp_ref - gnd_clamp_ref  (This has a maximum of 5 V)

    gnd clamp span = (gnd_clamp_ref - typ_range) to (pwr_clamp_ref)

    power clamp span = (pwr_clamp_ref) to (pwr_clamp_ref + typ_range)



IF the model is an ECL-type model:

For all (input, output, I/O) models:

    pullup(down) span = (pullup_ref - 2.2 V) to (pullup_ref)

    gnd clamp span = (gnd_clamp_ref - 2.2 V) to (pwr_clamp_ref)

    power clamp span = (pwr_clamp_ref) to (pwr_clamp_ref + 2.2 V)

(Many thanks to Bob Ross at Interconnectix, Inc. for his helpful
suggestions in this area.)
