Abstract: An optical communications system includes an optical bus and a plurality of terminal devices coupled to the bus and a reference signal generator for propagating a reference signal through the bus for use by all terminal devices in selecting one of a predetermined number of frequency distinct communications channels. The reference signal is frequency swept in a recurring manner between two fixed frequencies and is introduced onto the system bus, along with a system synchronization signal, for propagation to bus tap resonators associated with the terminal devices in the system. Each terminal device detects the moment at which the swept reference frequency passes through one of the resonant modes of its bus tap resonator and determines the elapsed time thereof from the system synchronization signal.