Bi-polar power supplies in IBIS

IBIS-users,

Is it possible, using IBIS, to model devices that use bi-polar power supplies?
For example data-transmission devices, like RS-232, that use a +15 V and -15 V
supply?

If so, how is the pull-down modeled? Is the Pull-down VI table -Vcc referenced
instead of ground referenced?

Thanks.

-- 
Regards,
Stephen M. Nolan