Digital signal hardware protection switching

Protection switching of digital signal transmitting and receiving hardware is realized in a "hitless" manner. The hitless switch is obtained by employing hardware switching units (112(1),112(0)) which can individually determine the instant to switch from an active state to a standby state and vice versa. A switch is initialized by supplying a switch request signal (SWRQ) from a control unit (110) to a hardware switching unit 112(1) in the active state. The active hardware switching unit, upon detecting a prescribed bit condition, for example, a predetermined bit state, of the digital signal being supplied as an output therefrom, supplies a predetermined synchronization pulse (SYNCN) to the standby hardware switching unit (112(0)). In response to the synchronization pulse, the standby hardware switching unit (112(0)) forces the bit condition of a bit of the digital signal to be supplied as an output therefrom resulting from a current input bit to be in the same bit condition as that detected in the active hardware unit. Output circuits in both the active hardware switching unit and standby hardware switching unit are simultaneously turned OFF and ON, respectively, in the individual units by observing the digital signal (TDATA) being supplied as an input to both units. Upon detection of a predetermined bit condition of the input digital signal individually by each of the hardware switching units, the active unit output circuit is turned OFF and the standby unit output circuit is turned ON at the same instant. The instant of turning OFF and ON of the output circuits is determined individually in each hardware switching unit.