High-power diplexers can be used in ECRH systems as power or beam combiners (BC), slow and fast directional switches (FADIS) to toggle the power from continuously operating gyrotrons between two launchers, and discriminators of low-power ECE signals from high-power ECRH in launchers used for in-line ECE. In the paper, design options for resonant diplexers are presented, and detailed low-power investigations on transmission characteristics and insertion losses are discussed. Two types, a purely quasi-optical and a compact waveguide-compatible diplexer, respectively, have been tested with high-power in the 140 GHz ECRH system for the stellarator W7-X. Fast switching, arbitrary distribution of the gyrotron power to two outputs, as well as power combination could be demonstrated with the first prototype. In recent experiments with a compact design, a mirror drive for tracking of the resonator to the gyrotron frequency was implemented; first long-pulse tests on transmission characteristics are shown. Finally, plans for implementation of this diplexer in the ECRH at ASDEX Upgrade are described.