Some I/O standards can be used only in unidirectional mode, while some can be used in bidirectional mode or unidirectional mode.
Some I/O standards have attributes to control drive strength and slew rate, as well as the presence of weak pull-up or pull-down and weak-keeper circuits (not intended for use as parallel termination). Drive strength and slew rate can be used to tune an interface for adequate speed while not overdriving the signals. Weak pull-ups, weak pull-downs, and weak keepers can be used to ensure a known or steady level on a floating or 3-stated signal. The SelectIO Resources chapter of the UltraScale Architecture SelectIO Resources User Guide (UG571) [Ref 10] describes which standards support these attributes. Refer to this user guide for more information.
LVCMOS, when set to 6 mA DRIVE and FAST slew, has an approximate output impedance close to 50W, allowing it to be used as a crude approximation of a controlled-impedance driver. The impedance match of the weak driver to the transmission line is only approximate and varies with voltage and temperature. LVDCI and HSLVDCI, true controlled-impedance drivers, are adaptive, maintain a much closer impedance match, and remain constant over voltage and temperature.