Transistor–transistor logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJTs) and resistors. It is called transistor–transistor logic because transistors perform both the logic function (e.g., AND) and the amplifying function (compare with resistor–transistor logic (RTL) and diode–transistor logic (DTL)).
TTL integrated circuits (ICs) were widely used in applications such as computers, industrial controls, test equipment and instrumentation, consumer electronics, and synthesizers. The designation TTL is sometimes used to mean TTL-compatible logic levels, even when not associated directly with TTL integrated circuits, for example as a label on the inputs and outputs of electronic instruments.[1]