RTL Logic
Part of Boolean Logic and Gates
Resistor-Transistor Logic — the earliest transistor logic family, buildable from the simplest possible components with minimal fabrication requirements.
Why This Matters
RTL (Resistor-Transistor Logic) was the first widely used transistor-based logic family, used in the early 1960s and in some of the first computers to use transistors rather than vacuum tubes. The Minuteman missile guidance computer used RTL ICs. The earliest Apollo Guidance Computer prototypes used RTL.
For a civilization rebuilding computing from scratch, RTL is important for a different reason: it is the easiest logic family to build from discrete components. The basic NOR gate requires just one resistor and one transistor per input. With a small transistor fabrication capability, RTL is the first working digital logic you can produce.
RTL has real limitations — low fan-out, poor noise margins, slow speed — but it is functional, understandable, and requires the least component variety of any logic family. Building a working RTL circuit is the simplest path from “understanding gates abstractly” to “running digital logic on a bench.”
The RTL NOR Gate
The fundamental RTL gate is the NOR gate. For a 2-input RTL NOR:
Components:
- Resistors Rb1 and Rb2 (typically 450–680 Ω) from inputs A and B to transistor bases
- Transistors Q1 and Q2 (NPN, 2N2222 or equivalent), emitters to GND, collectors to common output node
- Pull-up resistor Rc (typically 640 Ω) from VCC (+3.6V for original RTL or +5V for modernized) to the common collector node
If either input A or B is HIGH (≥2.0V), the corresponding transistor saturates, pulling the output node to near GND. Only if both inputs are LOW (<0.8V) do both transistors stay off, and Rc pulls the output HIGH.
This is NOR: output HIGH only when all inputs are LOW.
For an N-input NOR, add N transistors in parallel sharing the same collector node, each with its own base resistor from the respective input.
The RTL Inverter
An inverter is a 1-input NOR: a single transistor with one base resistor and one pull-up.
Input HIGH → transistor on → output LOW. Input LOW → transistor off → output HIGH (pulled up by Rc).
This is identical to the transistor inverter described in the NOT gate article, with the specific resistor values characteristic of RTL. RTL standardizes on particular resistor values that balance speed, power dissipation, and fan-out within the family.
Logic Levels and Supply Voltage
Original RTL (e.g., Fairchild µL900 series) used +3.6V supply:
- Logic 1: 2.0–3.6V (typically 3.1V loaded)
- Logic 0: 0–0.2V (transistor saturation ≈ 0.1V)
- VIH = 1.5V, VIL = 0.8V
- NMH ≈ 0.5V, NML ≈ 0.6V
Noise margins are modest — RTL is not suitable for very noisy environments.
For discrete construction using +5V:
- Increase Rc to 1.5 kΩ to maintain similar logic levels proportionally
- Increase Rb to 1.0 kΩ
- NPN transistors with low Vce_sat and hFE > 40 work well
The small supply voltage of 3.6V is not mandatory for discrete RTL. Using +5V with proportionally scaled resistors gives better compatibility with modern ICs.
Fan-Out Limitations
RTL has low fan-out, typically 3 to 5 in the original family. The limitation comes from current: when the driving gate output is HIGH, Rc must supply current to all driven gate base resistors. Each RTL input draws approximately VCC/Rb = 5V/640Ω ≈ 8 mA from the preceding gate’s pull-up resistor. The output voltage HIGH level falls as more loads are connected.
With 5 loads drawing 8 mA each = 40 mA through Rc = 640 Ω: voltage drop = 40 mA × 640 Ω = 25.6 V — clearly impossible in a 5V system. This calculation shows why original RTL operated at low voltages with higher base resistors.
For practical 5V discrete RTL with Rb = 5 kΩ: each load draws 1 mA, 5 loads draw 5 mA through Rc = 2.2 kΩ: voltage drop = 11V. Still too much for 5 loads at full current. Fan-out of 3–4 is more realistic.
To increase fan-out, add an emitter follower buffer: a transistor with emitter as output and no Rc. The emitter follower provides current gain without voltage inversion. Place a buffer at any node driving more loads than the standard fan-out allows.
Building a Simple Logic System
A 1-bit SR latch from two RTL NOR gates:
Gate 1 (Q output): NOR(R, Q_bar) → Q Gate 2 (Q_bar output): NOR(S, Q) → Q_bar
Wire the output of Gate 1 to one input of Gate 2, and the output of Gate 2 to one input of Gate 1 (feedback). Connect switches to the free inputs (S and R). Add LED indicators on both Q and Q_bar.
Total components: 4 transistors, 4 base resistors (Rb), 2 pull-up resistors (Rc), 2 switches, 2 LEDs, 2 current-limiting resistors.
Test:
- Set S=1, R=0: Q should latch HIGH (LED on), Q_bar LOW
- Release S (both inputs LOW): Q stays HIGH (latch holds state)
- Set R=1, S=0: Q should reset to LOW, Q_bar HIGH
- Release R: Q stays LOW (latch holds state)
This is a working 1-bit memory cell — the first experience of a circuit that remembers its last state.
RTL vs. DTL
RTL progresses naturally to DTL:
- RTL: input resistors directly drive transistor bases
- DTL: diodes at inputs provide isolation and better noise margins
The evolution from RTL to DTL improved noise margins from ~0.5V to ~1.0V and increased fan-out. DTL then evolved to TTL (replacing input diodes with multi-emitter transistors) for still faster switching.
RTL is the starting point. Its simplicity makes it the right first step for anyone building digital logic from scratch, even if the goal is eventually to graduate to DTL or TTL.