The Free On-line Dictionary of Computing (30 December 2018):

Transistor-Transistor Logic

   (TTL) A common semiconductor technology for building discrete
   digital logic integrated circuits.  It originated from Texas
   Instruments in 1965.

   There have been several series of TTL logic:

    7400:	10 ns propagation time, 10 mW/gate power consumption,
   	obsolete;

    74L00:	Low power: higher resistances, less dissipation
   	(1 mW), longer propagation time (30 ns);

    74H00: High power: lower resistances, more dissipation: less
   	sensitivity for noise;

    74S00: Schottky-clamped: faster switching (3 ns, 19 mW) by
   	using Schottky diodes to prevent the transistors from
   	saturation;

    74LS00: Low power, Schottky-clamped (10 ns, 2 mW);

    74AS00: Advanced Schottky: faster switching, less
   	 dissipation, (1.5 ns, 10 mW);

    74ALS00: Advanced Low power Schottky (4 ns, 1.3 mW).

   For each 74xxx family there is a corresponding 54xxx family.
   The 74 series are specified for operation at 0 - 70 C whereas
   the 54 (military) series can operate at -55 - 125 C

   See also CMOS, ECL.