Resistor–transistor logic

RTL integrated circuits were used in the Apollo Guidance Computer, whose design began in 1961 and which first flew in 1966.

The logical operation OR is performed by applying consecutively the two arithmetic operations addition and comparison (the input resistor network acts as a parallel voltage summer with equally weighted inputs and the following common-emitter transistor stage as a voltage comparator with a threshold about 0.7 V).

The base resistances and the number of the inputs are chosen (limited) so that only one logical "1" is sufficient to create base-emitter voltage exceeding the threshold and, as a result, saturating the transistor.

It consists of a set of parallel-connected transistor switches driven by the logic inputs (see the figure on the right).

To ensure stability and predictable output of the bipolar transistors their base-inputs (Vb or base-terminal voltage) is biased.

[5] The disadvantage of RTL is its high power dissipation when the transistor is switched on, by current flowing in the collector and base resistors.

Engineers and technicians use "RCTL" (resistor-capacitor-transistor logic) to designate gates equipped with "speed-up capacitors".

[citation needed] Using a high collector supply voltage and diode clamping decreased collector-base and wiring capacitance charging time.

These diode networks known as various Baker clamps reduced the voltage applied to the base as the collector approached saturation.

Schematic of a one-transistor RTL NOR gate.
Schematic of a multi-transistor RTL NOR gate, as used in the Apollo Guidance Computer integrated circuits. [ 4 ]
Photograph of the dual 3-input NOR gate chip used to build the Apollo Guidance Computer . Connections (clockwise from top center) ground, inputs (3), output, power (V cc ), output, inputs (3). The six transistors (two groups of three) are in the center. The thin wires from the terminals to the transistors are resistors.
Flatpack RTL NOR gate integrated circuits in the Apollo guidance computer