**OR Logic Gate**

OR logic gates are implemented by parallel connected *normally open* switches. So, in diode OR logic gates, the input voltage sources are connected to diode *anodes*. Diode cathodes are joined to the output (node 1 in the figure), which is connected through the pull-down resistor R1 to ground.

**Input logical one.** If the voltage of a particular input voltage source is high (input logical *1*), the according diode is forward biased and this diode switch is closed. The input source passes current through the diode and creates high voltage drop across the resistor R1 (output logical *1*). The rest of diodes connected to low input voltage (input logical *0s*) are backward biased and their input sources (grounds) are disconnected from the output.

**Input logical zeros.** If all the input voltages are low (input logical *0*), the voltage drops across the diodes are zero. These diode switches are open and the input sources (grounds) are disconnected from the output. No current flows through the resistor. The output voltage is low (output logical *0*) and the output resistance is R1.

If two diode OR logic gates are cascaded, they behave as *current-sourcing* logic gates: if the first gate produces high output voltage, the second gate consumes current from the first one. If the first gate produces low output voltage, the second gate does not inject current into the output of the first one. A diode OR gate does not use its own power supply. The input sources with high voltage (logical *1*) supply the load through the forward-biased diodes.

Read more about this topic: Diode Logic, Diode Logic Gate Versions

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