I think one of the main differences is that PCI input pads will always having clamping diodes. All of the PCI signals (other than the PCI clk) rely on reflected vs incident wave switching. The PCI loads are expected to be unterminated so as to present a high impedance and fully reflect an incoming signal. The clamping diode limits the amount of overshoot.
radix may be right. you can find those in PCI standrant. if you want learn the detail your can reference the pad's circuit schematic which is often come with the PAD lib. in my mind TSMC and SMC contain it.