Pull-up resistor

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-up resistor pulls the voltage of the signal it is connected to towards its voltage source level. When the other components associated with the signal are inactive, the voltage supplied by the pull up prevails and brings the signal up to a logical high level. When another component on the line goes active, it overrides the pull-up resistor. The pull-up resistor ensures that the wire is at a defined logic level even if no active devices are connected to it.

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.

A pull-up resistor may be used when interfacing logic gates to inputs. For example, an input signal may be pulled by a resistor, then a switch or jumper strap can be used to connect that input to ground. This can be used for configuration information, to select options or for troubleshooting of a device.

Pull-up resistors may be used at logic outputs where the logic device cannot source current such as open-collector TTL logic devices. Such outputs are used for driving external devices, for a wired-OR function in combinational logic, or for a simple way of driving a logic bus with multiple devices connected to it. For example, the circuit shown on the right uses 0V logic level inputs to actuate a relay. If the input is left unconnected, pull-down resistor R1 ensures that the input is pulled down to a logic low. The 7407 TTL device, an open collector buffer, simply outputs whatever it receives as input, but as an open collector device, the output is left effectively unconnected when outputting a "1". Pull-up resistor R2 thus pulls the output all the way up to 12 V when the buffer outputs a "1", providing enough voltage to turn the power MOSFET all the way on and actuate the relay.

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