Source : Free On-Line Dictionary of Computing

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}.