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Transistor-transistor logic

Transistor-transistor logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJT), diodes and resistors. TTL gained almost universal acceptance after Texas Instruments had greatly facilitated the construction of digital systems with their TTL 74 family of integrated circuits introduced in 1962. That family included logic gates (such as the 7400 quad NAND), flip-flops (such as the 7474 twin D-type flops), counters (such as the 74160 decade counter), binary adders and other simple subfunctions all of which were implemented as TTL circuits. All TTL circuits operate with a 5V power supply. A TTL signal is defined as "low" or L when between 0V and 0.8V with respect to the ground terminal, and "high" or H when between 2V and 5V.

Improved variations have then been developed and made available in the years to follow: The 74L family is a relatively low-power, but slower version of the 74 family. The 74H family is a high threshold version of the 74 family, designed for use in noisy industrial environments. Both of these variants of the 74 family were obsoleted by later versions and CMOS logic families. The 74S family uses more power than the 74, but is faster. The 74LS family of ICs is a lower-power version of the 74S family, with slightly higher speed but lower power than the original 74 family. The 74F family was introduced by Fairchild Semiconductor and adopted by other manufacturers. It is faster than the 74, 74LS and 74S families.

TTL consumes more power than CMOS logic, but used to be faster. TTL was largely relegated to glue logic applications, such as fast bus drivers on a motherboard, for instance, once CMOS technology had developed to a point that made it possible to economically integrate much more complex circuits on a single chip than with TTL technology. The final blow came in the mid 1990s when the long-time standard supply of 5V could no longer be maintained for reasons of energy efficiency and to accommodate new generations of high performance CMOS circuits.

See also: resistor-transistor logic[?] (RTL), diode-transistor logic[?] (DTL), emitter coupled logic (ECL).



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