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Logic Gate

A logic gate is an arrangement of switches and can be constructed using mechanical components, relays, vacuum tubes or transistors. A logic gate is a physical device implementing a Boolean function such as the logical AND, OR and NOT operations. A logic gate performs a logical operation on one or more inputs, and produces a distinct single output. The implementation for each Boolean operation requires a different design of the respective logic gate. Thus an AND gate has to be build in a different way from an OR gate. Some operators are implemented by breaking them down into a logically equivalent combination of 2 or more other logic gates. Logic gates can be cascaded in the same way that Boolean functions can be composed to produce more complex logic functions.
The output of a logic gate can be feed back as input and thereby other logic gates can be controlled. This makes the digital machine perform a 'sequence' of operations. The simplest sequential system is probably a flip-flop, a mechanism that represents a binary digit or "bit". Logic gates today are primarily implemented using diodes or transistors acting as electronic switches. Microprocessors are composed of logic circuits, which may contain more than 100 million gates.

Konrad Zuse built the Z1 using mechanical logic gates. After building the Z1 Helmut Schreyer advised Konrad Zuse to use vacuum tubes for the Z2 in favor of mechanical means. Zuse decided to use relays instead because electronic components were too expensive for him at that time. The Z2, Z3 and Z4 therefore utilized relay logic gates. The Z22 was the first computer developed by Konrad Zuse's company that used vacuum tube logic gates. With the Z23 they were substituted with transistor logic gates which were employed in all following computer models such as the Z31.

cf. Wikipedia