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In electronic logic circuits, a pull-up resistor (PU) or pull-down resistor (PD) is a resistor used to ensure a known state for a signal. [1] It is typically used in combination with components such as switches and transistors , which physically interrupt the connection of subsequent components to ground or to V CC .
A pull-up or pull-down resistor provides a voltage for a circuit when it is otherwise disconnected (such as when a button is not pushed down or a transistor is not active). A pull-up resistor connects the circuit to a high positive voltage (if the circuit requires a high positive default voltage) and a pull-down resistor connects the circuit to ...
See also: Diode logic § Active-high AND logic gate Open-collector buffers connected as wired AND.. The wired AND connection is a form of AND gate.When using open collector or similar outputs (which can be identified by the ⎐ symbol in schematics), wired AND only requires a pull up resistor on the shared output wire.
All cathodes are connected to the output, which has a pull-down resistor. If any input is high, its diode will be forward-biased and conduct current, and thus pull the output voltage high [b]. If all inputs are low, all diodes will be reverse-biased and so none will conduct current. The pull-down resistor will quickly pull the output voltage low.
The purpose is to reduce the overall power demand compared to using both a strong pull-up and a strong pull-down. [10] A pure open-drain driver, by comparison, has no pull-up strength except for leakage current: all the pull-up action is on the external termination resistor.
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If all the input voltages are low (logical "0"), the transistor is cut-off. The pull-down resistor R 1 biases the transistor to the appropriate on-off threshold. The output is inverted since the collector-emitter voltage of transistor Q 1 is taken as output, and is high when the inputs are low. Thus, the analog resistive network and the analog ...
Common circuit diagram symbols (US ANSI symbols) An electronic symbol is a pictogram used to represent various electrical and electronic devices or functions, such as wires, batteries, resistors, and transistors, in a schematic diagram of an electrical or electronic circuit. These symbols are largely standardized internationally today, but may ...