[IBIS-Users] [dV/dt_r] and [dV/dt_f]

Hi,
Table 3.1 of page 19 of IBIS cookbook for IBIS Version 4.0 recommends 4 V-T waveforms:
1R + 1F driving 50 Ohms to Vcc
1R + 1F driving 50 Ohms to GND
Note that from this set of 4 waveforms, I can derive 2 dV/dt_r and 2 dV/dt_f
How do I decide which ones would be going into IBIS keywords [dV/dt_r] and [dV/dt_f] ?

Thank you in advance,
Nam

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