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Transistor–transistor logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJT) and resistors. It is called transistor–transistor logic because both the logic gating function (e.g., AND) and the amplifying function are performed by transistors (contrast this with RTL and DTL). It represents a logic 0 for 0V and 1 for 5V.
TTL is not really used commercially due to the high power requirements. CMOS is the ideal family for low power applications. None of the other technologies has any better low power performance. That's why most devices now use CMOS or NMOS transistors. TTL and ECL is still used for historical compatibility and availability and in places where speed is essential. No other technology has ever been developed that can compete with this triad.
TTL (chiefly in the form of the 7400 series of IC's) became an early standard for compatibility in commercial electronics during the 1960's and 70's. It used a low supply voltage (5v), and set V levels for binary low and high.
Soon there were the inevitable advances (Schottky, Low power Schottky, H, F, etc.).
It became common to print tables comparing their characteristics against TTL, showing why they were a better choice than TTL.
It wasn't long before the tables had several columns, one for each family of IC.
And even though the more modern series use less power, and perform better, TTL was never dropped from the first column.