Schematic diagrams showing pull-up (PU) and pull-down (PD) resistors. When the switch is open, the PU/PD pulls the digital input voltage to Vcc or Vss, respectively. When the switch is closed, the digital input is connected to a low-impedance driving source, which may be a logic low or high level irrespective of whether the circuit has a PU or PD.
(echo:: @ ᯤ)