Диодтық логика: ЖӘНЕ және НЕ есіктерін құру

Logical AND and OR with diodes

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.

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.

Each input of a diode logic gate connects through a diode connected to a shared wired logic output. Depending on the voltage level of each input and direction of the diode, each diode may or may not be forward biased. If any are forward biased, the shared output wire will be one small forward voltage drop within the forward biased diode's input. If no diode is forward biased then no diode will provide drive current for the output's load (such as a subsequent logic stage). So the output additionally requires a pull up or pull down resistor connected to a voltage source, so that the output can transition quickly and provide a strong driving current when no diodes are forward biased. Note: the following circuits have two inputs for each gate and thus use two diodes, but can be extended with more diodes to allow for more inputs. At least one input of every gate must be connected to a strong enough high or low voltage source. If all inputs are disconnected from a strong source, the output may not fall within a valid voltage range.

Diodes also have a transient response that might be of concern. The capacitance between anode and cathode is inversely proportional to the reverse voltage, growing as it approaches 0 volts and into forward bias. There is also a recovery concern: a diode's current will not decrease immediately when switching from forward biased to reverse biased, because discharging its stored charge takes a finite amount of time (trr or reverse recovery time). In a diode OR gate, if two or more of the inputs are high and one switches to low, recovery issues will cause a short term dip in the output voltage or increase current in the diodes that remain high. If a diode–transistor logic gate drives a transistor inverter of similar construction, the transistor will have a similar base collector capacitance that is amplified by the transistor gain, so that it will be too slow to pass the glitch. But when the diode is much slower, recovery will become a concern:In one unusual design, small selenium diode discs were used with germanium transistors. The recovery time of the very slow selenium diodes caused a glitch on the inverter output. It was fixed by placing a selenium diode across the base emitter junction of the transistor making it think it was a selenium transistor (if there could ever be one).

Historically, diode logic was used extensively in the construction of early computers, since semiconductor diodes could replace bulky and costly active vacuum tubes. The invention of the transistor allowed transistors to replace tubes as the active element in diode–transistor logic. Since early transistors were not reliable, the D 17B missile guidance computer, for instance, primarily used diode logic and only used transistors when necessary. Transistors quickly advanced to replace diode logic almost entirely. However, diode logic still finds some modern uses.

Low impedance push–pull outputs of conventional ICs shouldn't directly be connected to external circuitry, as they may create a short circuit between power and ground. Such outputs, however, may be used as inputs to passive AND or OR diode logic gates. This avoids the costs of adding active logic gates. However, diode logic will degrade voltage levels and result in poor noise rejection, so designers should be aware of the interfaced logic family's voltage ranges and limitations, to prevent failures.

The humorously named "Micky Mouse Logic" described in Don Lancaster's CMOS Cookbook suggests using diodes as a multi tool for augmenting the limited capabilities of regular CMOS 4000 series ICs, for instance by using a diode OR gate to add extra inputs on a flip flop, or a diode AND gate to configure a divide by N counter. A variant approach suggests keeping a supply of 1N914 diodes with inverting Schmitt trigger ICs to provide hysteresis and functional completeness.

An active low OR diode logic gate is formed by a keypad containing diodes at each switch, all connected to a shared pull up resistor. When no switch is closed, the pull up keeps the output high. But when the switch for any key connects to ground, the output goes low. This OR result can be used as an interrupt signal to indicate that any key has been pressed. Then a microcontroller can wake from power saving standby and scan the key matrix to determine which key specifically was pressed.

During the 1960s the use of tunnel diodes in logic circuits was an active research topic. When compared to transistor logic gates of the time, the tunnel diode offered much higher speeds. Unlike other diode types, the tunnel diode offered the possibility of amplification of signals at each stage. The operating principles of a tunnel diode logic rely on biasing of the tunnel diode and supply of current from inputs over a threshold current, to switch the diode between two states. Consequently, tunnel diode logic circuits required a means to reset the diode after each logical operation. However, a simple tunnel diode gate offered little isolation between inputs and outputs and had low fan in and fan out. More complex gates, with additional tunnel diodes and bias power supplies, overcame some of these limitations. Advances in discrete and integrated circuit transistor speed and the more nearly unilateral nature of transistor amplifiers overtook the tunnel diode gate, resulting in it no longer being used in modern computers.