How to Read a Logic Diagram

A practical primer on reading gate symbols, signal lines, bubbles, and truth tables so schematics become legible at a glance.

Why a primer helps

A logic diagram is a compact language. Once you recognize a handful of symbols and conventions, a schematic reads like a sentence: inputs on the left, logic in the middle, outputs on the right. This page teaches the vocabulary.

The distinctive shapes

Kronos motion — cta read papers

Each gate has a fixed silhouette. The AND shape is flat-backed with a rounded nose. The OR shape has a curved back and a pointed nose. A triangle is a buffer. A small circle, called a bubble, means inversion: an AND with an output bubble is a NAND, an OR with a bubble is a NOR, and a triangle with a bubble is a NOT gate.

Lines and junctions

Wires carry one bit each. A solid dot at a crossing means the wires connect; no dot means they merely cross. A bus is a thick line or a slash with a number, meaning several bits travel together. Signal flow is conventionally left to right unless an arrow says otherwise.

Active-low and bars

A bubble on an input means the gate responds to a 0 on that line, called active-low. A bar over a name, like RESET with an overline, marks the same idea in text: the signal does its job when low.

Reading a small circuit

Name every input and trace each wire to the gates it feeds.
Evaluate gates nearest the inputs first, writing intermediate values on the wires.
Follow bubbles carefully; each one flips the sense of the signal.
Continue toward the output, one gate layer at a time.

With practice this becomes automatic, and a page of gates resolves into the Boolean function it computes.