Diode logic

Diode logic (DL), or diode-resistor logic (DRL), 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 DTL (Diode Transistor Logic) integrated circuits that include an inverter 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 one-stage is normally used. In special designs two-stage applications are sometimes achieved.

In logic gates, logical functions are performed by parallel or series connected switches (such as relay contacts or insulated gate FETs like CMOS) controlled by logical inputs or parallel resistors or diodes which are passive components. Diode logic is implemented by diodes which exhibit low impedance when forward biased and a very high impedance when reverse biased. There are two kinds of diode logic gates - OR and AND. It is not possible to construct NOT (Invert) diode gates because the NOT or Invert function requires an active component such as a transistor.

For illustration this discussion assumes idealized diodes that conduct in the forward direction with no voltage drop and do not conduct in the reverse direction. Logic design assume two distinct levels of signals that are labeled “1” or “0”. For positive logic the 1 represents the most positive level and 0 for the most negative level. For illustration in this discussion positive logic 1 is represented by +6 volts and 0 volts represents logic 0. In binary logic the exact magnitude of the signal voltage is not critical and it is only necessary that 1 and 0 states be represented by detectably different voltage levels.

In these examples at least one input of every gate must be connected to a voltage level providing the defined logic 1 or logic 0 levels. If all the inputs are disconnected from any driving source the output signal is not confined to the correct voltage range.

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