Duplex describes which directions data can flow and when. Simplex is one-directional only, data goes one way, like a sensor that only broadcasts. Half-duplex is bidirectional but only one direction at a time over a shared medium, the line must be turned around between talking and listening; examples are I2C (SDA is shared), RS-485, and CAN. Full-duplex is bidirectional simultaneously, using separate paths for each direction, SPI with its separate MOSI and MISO lines, and UART with separate TX and RX. The practical implications: half-duplex needs a protocol to decide who drives the line and when (addressing, arbitration, or turnaround timing) and can't send and receive at once, while full-duplex needs the extra wires but lets both ends transmit concurrently, which matters for throughput and for protocols like SPI that shift data both ways on every clock.
Communication Protocols · Interview question
What does duplex mean, and give an example of each kind?
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From the lesson
Serial vs Parallel, Sync vs Async
The two axes that classify every bus: serial vs parallel (one wire or many) and synchronous vs asynchronous (shared clock or agreed-upon rate), and where UART, SPI, I2C, and CAN land.