"The bus doesn't work" is not a bug report a logic analyzer can answer by itself, and the gap between clipping on four wires and actually learning something is where most of a session goes. Lab Equipment and What It Answers makes the case for reaching for the analyzer first and works one I²C bug through it end to end; this page assumes you have already made that choice and is about running the capture well enough that the answer it gives you is true.
Below the level of PCIe and USB, most chip-to-chip communication on a circuit board — a sensor
A UART has no clock wire. That single fact generates every interesting property of the peripheral and every way it fails. SPI and I²C both ship a clock alongside the data, so the receiver is told exactly when to look; a UART receiver is told nothing. It sees a falling edge, starts its own counter, and from that moment guesses where the bit centres are using an oscillator the transmitter has never met. Everything below — the divider arithmetic, the oversampling modes, the tolerance budget, the overrun flag — is machinery built around that one guess.