Diode logic or Diode-resistor logic, is a logic family that makes use of diodes to construct logic gates for Boolean logic circuits. It has been superseded by transistor logic in most applications. The main drawbacks of diode logic are that it cannot be used to build a NOT gate and signals degrade after only a few layers of gates causing loss of noise immunity and unreliable logic operation.
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Gate construction
In the following constructions, we use these voltage conventions:
- Voltage less than or equal to zero (ground voltage) equals logic zero.
- Voltage more positive than ground equals logic one.
OR gate construction
To construct an OR gate with <math>n</math> logic inputs, the following components are required:
- <math>n</math> diodes
- pull-down resistor
<math>n + 2</math> electrical nodes are used (counting ground as a node). The circuit of a two-input gate is shown opposite. The cathode of every diode is connected to the same node (node 1), a resistor connects node (1) to ground (2), and the remaining <math>n</math> nodes are the inputs. The output is node (1). Whenever a logic 1 is present at the anode of a diode, it forward-biases that diode, causing it to conduct. Hence, a logic 1 voltage level is transferred to the output node. If a logic 0 is present at the anode of every diode, they are all unbiased or reverse-biased and the resistor holds node 1 low.
AND gate construction
The following components are required:
- <math>n</math> diodes
- pull-up resistor
- positive voltage source
<math>n+3</math> electrical nodes are used chnnu(counting ground as a node). An example circuit is shown on the right. Resistor R1 connects node 1 to a positive voltage source (2). Nodes 4 and 5 are the inputs. The output is obtained at node 1. If a logic 1 is present at the cathode of every diode, then they are all unbiased (voltage equal to the source voltage) or reverse-biased (voltage higher than source voltage), remaining in a high-impedance state, and node 1 is held high by the power supply through R1. If a logic 0 is present at any diode, it forward-biases that diode and holds node 1 low.
NOT gate construction
It is not possible to construct a NOT gate using diode logic. This gate requires an active device such as a transistor, vacuum tube, or relay to invert the signal.
