Logic gate
The primary way of building logic gates uses diodes or transistors acting as electronic switches.
The analytical engine devised by Charles Babbage in 1837 used mechanical logic gates based on gears.
[9] In an 1886 letter, Charles Sanders Peirce described how logical operations could be carried out by electrical switching circuits.
Walther Bothe, inventor of the coincidence circuit,[11] got part of the 1954 Nobel Prize in physics, for the first modern electronic AND gate in 1924.
From 1934 to 1936, NEC engineer Akira Nakashima, Claude Shannon and Victor Shestakov introduced switching circuit theory in a series of papers showing that two-valued Boolean algebra, which they discovered independently, can describe the operation of switching circuits.
[12][13][14][15] Using this property of electrical switches to implement logic is the fundamental concept that underlies all electronic digital computers.
[18] Both types were later combined and adapted into complementary MOS (CMOS) logic by Chih-Tang Sah and Frank Wanlass at Fairchild Semiconductor in 1963.
The "distinctive shape" set, based on traditional schematics, is used for simple drawings and derives from United States Military Standard MIL-STD-806 of the 1950s and 1960s.
The mutual goal of IEEE Std 91-1984 and IEC 617-12 was to provide a uniform method of describing the complex logic functions of digital circuits with schematic symbols.
[21] These are, however, shown in ANSI/IEEE Std 91 (and 91a) with this note: "The distinctive-shape symbol is, according to IEC Publication 617, Part 12, not preferred, but is not considered to be in contradiction to that standard."
In the 1980s, schematics were the predominant method to design both circuit boards and custom ICs known as gate arrays.
Today custom ICs and the field-programmable gate array are typically designed with Hardware Description Languages (HDL) such as Verilog or VHDL.
A De Morgan symbol can show more clearly a gate's primary logical purpose and the polarity of its nodes that are considered in the "signaled" (active, on) state.
Consider the simplified case where a two-input NAND gate is used to drive a motor when either of its inputs are brought low by a switch.
Unlike a regular NAND symbol, which suggests AND logic, the De Morgan version, a two negative-input OR gate, correctly shows that OR is of interest.
A functionally complete logic system may be composed of relays, valves (vacuum tubes), or transistors.
The switch circuit creates a continuous metallic path for current to flow (in either direction) between its input and its output.
Systems with varying degrees of complexity can be built without great concern of the designer for the internal workings of the gates, provided the limitations of each integrated circuit are considered.
For higher speed and better density, the resistors used in RTL were replaced by diodes resulting in diode–transistor logic (DTL).
To reduce power consumption still further, most contemporary chip implementations of digital systems now use CMOS logic.
CMOS uses complementary (both n-channel and p-channel) MOSFET devices to achieve a high speed with low power dissipation.
A group of three-states driving a line with a suitable control circuit is basically equivalent to a multiplexer, which may be physically distributed over separate devices or plug-in cards.
