Concept
Diode logic (or diode-resistor logic) constructs AND and OR logic gates with diodes and resistors. An active device (vacuum tubes in early computers, then transistors in diode–transistor logic) is additionally required to provide logical inversion (NOT) for functional completeness and amplification for voltage level restoration, which diode logic alone can't provide. Since voltage levels weaken with each diode logic stage, multiple stages can't easily be cascaded, limiting diode logic's usefulness. However, diode logic has the advantage of utilizing only cheap passive components. Logic gates evaluate Boolean algebra, typically using electronic switches controlled by logical inputs connected in parallel or series. Diode logic can only implement OR and AND, because inverters (NOT gates) require an active device. Main article: Binary logic uses two distinct logic levels of voltage signals that may be labeled high and low. In this discussion, voltages close to +5 volts are high, and voltages close to 0 volts (ground) are low. The exact magnitude of the voltage is not critical, provided that inputs are driven by strong enough sources so that output voltages lie within detectably different ranges. For active-high or positive logic, high represents logic 1 (true) and low represents logic 0 (false). However, the assignment of logical 1 and logical 0 to high or low is arbitrary and is reversed in active-low or negative logic, where low is logical 1 while high is logical 0. The following diode logic gates work in both active-high or active-low logic, however the logical function they implement is different depending on what voltage level is considered active. Switching between active-high and active-low is commonly used to achieve a more efficient logic design. Forward-biased diodes have low impedance approximating a short circuit with a small voltage drop, while reverse-biased diodes have a very high impedance approximating an open circuit. The diode symbol's arrow shows the forward-biased direction of conventional current flow.