In electronic logic circuits, a pull-up resistor is a resistor connected between a signal conductor and a positive power supply voltage to ensure that the signal will be a valid logic level if external devices are disconnected or high-impedance is introduced. They may also be used at the interface between two different types of logic devices, possibly operating at different logic levels and power supply voltages. A pull-down resistor works in the same way but is connected to ground. It holds the logic signal at a low logic level when no other active device is connected.