Estimating Actual Output Timing Without Board Simulation (WP217) - This document can help designers obtain more accurate I/O timing data without the need for board-level IBIS or SPICE simulations. Until recently, Xilinx specified outputs into a lumped capacitive load. However, since rise and fall times force board interconnect to be considered transmission lines, a lumped capacitive load is no longer relevant (see the TechXclusives document on this for more detail).