A directional coupler separates the two signal directions on a shared line for measurement and logging.

A directional coupler is a measurement or separation element that can be used to separate the two signal directions running simultaneously on a single-pair or full-duplex line. This is especially useful with 100BASE-T1 and similar Automotive Ethernet PHYs because master and slave signals are transmitted simultaneously on the same wire pair. For physical-layer debugging and high-fidelity logging, the directional coupler is therefore a key tool.