Pull-Up / Pull-Down Resistor
It is the resistor that weakly ties an input to the supply voltage (pull-up) or to chassis (pull-down), preventing a digital input from being left undefined at the moment no source is driving it. The typical value is 4.7–10 kΩ; that value is strong enough to suppress noise and weak enough for the real signal to flip the input easily.
While a button contact is open, the input pin is left electrically floating. A floating input can change state by itself by picking up the field radiated by a motor cable running nearby; this is why the system thinks it has received a command every time the motor starts.
- Pull-up: The input is held at logic 1 while idle; the button produces 0 by pulling the line to chassis. It is the arrangement compatible with dry contacts and NPN sensors.
- Pull-down: The input is at logic 0 while idle; the source produces 1 by pulling the line to the supply. It is used with PNP sensors.
The balance in choosing the value is this: a very high resistance such as 100 kΩ cannot hold the input on a long and noisy cable; a very low resistance such as 470 Ω on the other hand consumes current continuously and loads the driver output needlessly. In a 12/24 V panel environment it is safer to lean towards 4,7 kΩ.
Adding a 10–100 nF capacitor in parallel with the input forms, together with the resistor, a simple filter that swallows short interference pulses.
Context where this term is used: DC Motor Driver Selection Guide