Transistor-transistor logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJT), diodes and resistors; it is notable for being the base for the first widespread semiconductor integrated circuit (IC) technology. TTL gained almost universal acceptance after Texas Instruments had greatly facilitated the construction of digital systems with their 1962 introduction of the 7400 series of ICs.
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.
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.