R_load for linear ramp

Hi all:

In reading the "IBIS Forum I/O Buffer Modeling Cookbook" as well as the
IBIS v2.1 spec. on the [Ramp] keyword, the R_load subparameter is said
to be a preferred 50 Ohm load. The Cookbook states that "if the device
does not have enough drive capability to make a "significant" output
transition than a higher value of load resistance may be used." My
question is - what constitutes a "significant" output transition? The
particular 3.3V I/O I am trying to model (and I am very new to IBIS) can
only drive around a 1.5V transition with a 50 Ohm load. Increasing
R_load increases the output transition and significantly changes the
[Ramp] dV/dt_r and dV/dt_f ratio's. Knowing nothing about the IBIS
model users concerns (that is - what constitutes a good IBIS model to
the user), does it matter as long as the R_load is specified in the
model?

Regards,
Robert Goodrich
Motorola, SPS
Received on Tue Nov 24 12:44:02 1998