page_title: Diode logic
section_title: OR logic gate
hierarchical_section_title: Diode logic / Diode logic gates / OR logic gate
caption_attribution_description: English: Diode AND with Ideal Diode
context_page_description: Diode logic, or diode-resistor logic, is the construction of Boolean logic gates from diodes. Diode logic was used extensively in the construction of early computers, where semiconductor diodes could replace bulky and costly active vacuum tube elements. The most common use for diode logic is in diode–transistor logic integrated circuits that, in addition to diodes, include inverter logic for power gain and signal restoration.
  While diode logic has the advantage of simplicity, the lack of an amplifying stage in each gate limits its application. Not all logical functions can be implemented in diode logic alone; only the non-inverting logical AND and logical OR functions can be realized by diode gates. If several diode logic gates are cascaded, the voltage levels at each stage are significantly changed, so diode logic is normally limited to a single stage, though, in special designs, two-stage systems are sometimes achieved.
context_section_description: The image to the right shows a diode OR circuit.  The diode symbol is an arrow showing the forward low impedance direction of current flow.  All diodes have inputs on their anodes and their cathodes are connected together to drive the output.  R is connected from the output to some negative voltage (-6 volts) to provide bias current for the diodes.
  If all inputs A and B and C are at 0 volts (logic level 0), current flowing through  R will pull the output voltage down until the diodes clamp the output. Since these diodes are treated as ideal, the output is clamped to 0 volts, which is logic level 0. If any input switches to a positive voltage (logic 1), current flowing through the now forward-biased diode will pull the output voltage up, providing a positive voltage at the output, a logic 1. Any positive voltage will represent a logic 1 state; the summing of currents through multiple diodes does not change the logic level. The other diodes are reverse biased and conduct no current. 
  If any input A OR B OR C is 1, the output will be 1.  Only if all inputs, A and B and C are 0 will the output be 0.  This is the definition of a logic OR.  The truth table on the right of the image shows the output for all combinations of inputs. 
  This can be written as:
  A OR B OR C = OUTPUT
  or
  A+B+C=OUTPUT
  In Boolean algebra the plus sign (+) is used to denote OR.
  R can return to any negative voltage.  If R is connected to 0 volts it will have no drive current available to drive the next circuit; practical diodes need a bias current.  In a practical circuit, all signal levels, the value of R and its return voltage are chosen by the circuit designer to meet the design requirements.
caption: A diode OR gate with positive logic truth table
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