⚙️ Evolution of Implementation on Digital Logic Notebook

RTL

Mon, 01 Jan 0001 00:00:00 +0000

RTL#

Resistor-Transistor Logic was the first practical way to build logic gates inside an integrated circuit. Its name is its schematic: resistors at the inputs, a transistor doing the switching. It is obsolete now, but it is where the family tree starts, and every weakness of RTL is the reason the next family exists.

How It’s Built#

The basic RTL gate is a NOR. Each input drives the base of its own transistor through a resistor; all the transistors share a single collector resistor to the supply. If any input is high, its transistor conducts and pulls the shared output low — low-when-any-input-is-high is exactly NOR. Adding transistors in parallel adds inputs; that is the entire trick.

DTL

Mon, 01 Jan 0001 00:00:00 +0000

DTL#

Diode-Transistor Logic answered RTL’s weaknesses by changing what does the logic at the input: diodes instead of resistors. It is also obsolete, but it is the most instructive stop in the lineage, because understanding DTL is understanding exactly what problem TTL then solved.

How It’s Built#

DTL splits the work in two. A network of diodes at the input performs the combinational logic — several diodes into a common node form an AND — and a transistor stage behind it inverts and restores the signal to a clean level. Diode-AND followed by transistor-inversion is a NAND, the characteristic DTL gate.

TTL

Mon, 01 Jan 0001 00:00:00 +0000

TTL#

Transistor-Transistor Logic was the family that made integrated logic ubiquitous. For roughly two decades it was simply what “a logic chip” meant, and although CMOS has replaced it in almost every new design, TTL’s conventions — its part numbers and its voltage levels — are so deeply embedded that they still govern logic built today.

How It’s Built#

TTL took DTL and replaced its two weak spots. The string of input diodes became a single multi-emitter transistor: each emitter is an input, and the transistor actively sweeps charge out of the following stage, switching far faster than passive diodes could. The passive pull-up became a totem-pole output — a stacked pair of transistors that actively drives the output both high and low, giving fast edges and real drive strength. The signature gate is the 7400 quad two-input NAND.

CMOS

Mon, 01 Jan 0001 00:00:00 +0000

CMOS#

Complementary Metal-Oxide-Semiconductor logic is the milestone the whole lineage was building toward. Every other family on this list is either an ancestor of CMOS or a variety of it, and every integrated circuit in a modern device — processor, memory, FPGA, microcontroller — is CMOS underneath. It won on the one axis that ultimately mattered most: power.

How It’s Built#

A CMOS gate uses two complementary networks between the output and the rails: a PMOS network that can pull the output high and an NMOS network that can pull it low, arranged so that in any stable input state exactly one network conducts. Because there is never a path from supply to ground in a settled state, a CMOS gate draws essentially no static current — it dissipates power only briefly while switching, charging and discharging capacitance.

HC / HCT Families

Mon, 01 Jan 0001 00:00:00 +0000

HC / HCT Families#

The 74HC and 74HCT families are where CMOS became the practical default for board-level logic. They pair CMOS power consumption with speed comparable to the popular 74LS TTL of their day, wrapped in the familiar 74xx part numbers — which is why, for a great deal of discrete digital design, “HC” is simply the logic reached for first.

High-Speed CMOS#

74HC (“High-speed CMOS”) is silicon-gate CMOS built to be fast enough to replace LS-TTL while keeping CMOS’s near-zero static power, its wide 2–6 V supply range, and its high, rail-to-rail noise margin (thresholds near half the supply). Functionally it carries the entire 74xx catalog forward: a 74HC00 is the CMOS quad NAND, a 74HC595 the CMOS shift register, and so on.

LVC / AHC

Mon, 01 Jan 0001 00:00:00 +0000

LVC / AHC#

As supply voltages fell — 5 V giving way to 3.3 V, then 1.8 V and below, for both power savings and speed — logic families followed them down. LVC, AHC, and their many siblings are today’s discrete logic: the parts actually specified in current designs, where HC/HCT is increasingly the legacy 5 V choice.

Today’s Low-Voltage CMOS#

These are all CMOS, refined for lower voltages, higher speed, and mixed-voltage systems:

Tiny Logic (74LVC1Gxx)

Mon, 01 Jan 0001 00:00:00 +0000

Tiny Logic (74LVC1Gxx)#

Tiny logic is the endpoint of discrete-logic miniaturization: one gate per package. Where the classic 7400 put four NAND gates in a 14-pin package, a part like the 74LVC1G00 is a single two-input NAND in a five-pin package smaller than a grain of rice. It is the most modern link in this chapter, and it exists because of what happened to logic everywhere else in the notebook.