three-state logic or four-valued logic). This is the basis for bus-systems in computers, among many other uses. The high-impedance state of a given node in a circuit cannot be verified by a voltage measurement alone. A pull-up resistor (or pull-down resistor) can be used as a medium-impedance source to try to pull the wire to a high (or low) voltage level. If the node is not in a high-impedance state, extra current from the resistor will not significantly affect its voltage level. References Digital circuits