Resistor-transistor logic

Resistor-transistor logic (RTL) is a class of digital circuits built from bipolar junction transistors (BJT), and resistors; it is the earliest transistorized digital logic used. Its main limitation was its limited fan-in, usually 3 inputs being the limit before it completely lost usable noise immunity.

See also: diode-transistor logic (DTL), transistor-transistor logic (TTL), emitter coupled logic (ECL).