In a ring communication system comprising selectively connectable data stations, each station has connecting circuitry including a phase-locked loop arrangement (111) which is frozen at its current frequency and phase in response to a signal interruption caused by a station insertion or removal operation. Control logic (113) generates a freeze control signal in dependence of the signal energy status on input lines (75). This control signal activates gates (149,151) and switches (155,157) to interrupt change control signals of the phase-locked loop circuitry and to keep an oscillator control voltage at a given value. The control logic also provides an auxiliary data signal during the freeze interval which is applied to the ring to keep stations downstream in synchronism. By this method and arrangement, undesired reactions and excursions of the phase-locked loop circuitry in response to a signal energy outage are prevented, and synchronization can be rapidly regained after station insertion or removal.