Resistor–transistor logic

Resistor–transistor logic (RTL) is a class of digital circuits built using a resistor to bias the output from a transistor, which may be a bipolar junction transistor (BJT) or field-effect transistor (FET). RTL is the earliest class of transistorized digital logic circuit used; other classes include diode–transistor logic (DTL) and transistor–transistor logic (TTL).

Function

In Transistor Component Circuits IBM informs us, “The logical function is performed by the input resistor network and the invert (a.k.a. “NOT”) function is accomplished by the common emitter transistor configuration.”^[1]

Advantages

The obvious advantage of RTL was its low cost of construction, in the era when transistors were more expensive than resistors. In all but the earliest integrated circuit technologies, the cost advantage is on transistors, so RTL quickly gave up this advantage.

Limitations

The obvious disadvantage of RTL is its high current dissipation when the transistor conducts to overdrive the output biasing resistor. This requires that more current be supplied to and heat be removed from RTL circuits. In contrast, TTL circuits (including CMOS) minimize both of these requirements.

Lancaster says that integrated circuit RTL NOR gates (which have one transistor per input) may be constructed with "any reasonable number" of logic inputs, and gives an example of an 8-input NOR gate.^[2]

A standard integrated circuit RTL NOR gate can drive up to 3 other similar gates. Alternatively, it has enough output to drive up to 2 standard integrated circuit RTL "buffers", each of which can drive up to 25 other standard RTL NOR gates.^[2]

Speeding up RTL

Various companies applied the following speed-up methods to discrete RTL.

Transistor switching speed has increased steadily from the first transistorized computers through the present. The GE Transistor Manual (7th ed., p.181, or 3rd ed., p.97 or intermediate editions) recommends gaining speed by using higher-frequency transistors, or capacitors, or a diode from base to collector to present saturation.^[3]

Placing a capacitor in parallel with each input resistor decreases the time needed for a driving stage to back bias a driven stage's base-emitter junction. Engineers and technicians use “RCTL” (resistor capacitor transistor logic) to designate gates equipped with “speed-up capacitors.” The Lincoln Laboratory TX-0 computer's circuits included some RCTL.^[4]

Using a high collector supply voltage and diode clamping decreased collector-base and wiring capacitance charging time. This arrangement required diode clamping the collector to the design logic level. This method was also applied to discrete DTL (diode-transistor logic).^[5]

Another method used a diode and a resistor, a germanium and a silicon diode, or three diodes in an arrangement that reduced the voltage applied to the base as the collector approached saturation. Because the transistor went less deeply into saturation, the transistor accumulated fewer stored change carriers. Therefore, less time was required to clear stored charge during transistor turn off.^[3] Because DTL usually required a diode in series with the transistor's base terminal, this method applied more directly to DTL.

None of these methods found their way into major integrated logic families—at least not in direct integration of the discrete implementations.

Books

IBM Form 223-6889-Transistor Component Circuits ^[1]

A detailed treatment with applications is found in Lancaster.^[2]

Design information for discrete RTL gate and flip-flop circuits can be found in the GE Transistor Manual, third through seventh editions.^[3]

Fadiman, J.R, TX0 Computer Circuitry, MIT Lincoln Laboratory, 1956.^[4]

The Digital Logic Handbook describes the B series logic which included RCTL and DTL and could be purchased as individual cards.^[5]

References

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See also

• Diode–transistor logic (DTL)
• Transistor–transistor logic (TTL)
• Emitter-coupled logic (ECL)
• Integrated injection logic (I2L)de:Widerstands-Transistor-Logik

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