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Thanks for asking about IBIS! The [Ramp] keyword refers to the (given) amount of time a driver uses to transition through a (given) voltage range, into a defined load. The values of time and voltage are given explicitly, as a ratio of dV to dt. Separate ratios are given for rising and falling transitions.
dV/dt_f 1.8V/0.5n NA NA
dV/dt_f 1.8V/0.6n NA NA
means that, for the typical corner, the buffer will transition through a range of 1.8 V in 0.5 ns for a rising edge driven into a 50 ohm load to ground. The buffer will cover the same voltage range on a falling edge in 0.6 ns for the same load connected to Vcc.
Note that the voltage is 20-80% of the total swing into the given load. In other words, if the example buffer buffer above has a Vcc of 5 V, and its 20-80% swing into the load is 1.8 V, its actual full swing into the load is 3 V or from 5 V down to 2 V. The Ramp voltage is always a 20-80% value.