English: Schematic diagrams showing pull-up (PU) and pull-down (PD) resistors. The switching device is depicted as, but is not necessarily, a mechanical switch; for example it could be an open-collector or open-drain transistor, tri-state logic buffer, option jumper, or connector to external circuitry. Also, although the switch is shown connected to a power supply rail (Vcc or Vss), in many cases it is actually an active driver which can drive the digital input node to either logic state, irrespective of whether the circuit has a PU or PD.