Abstract: A digital signal constituted by a stream of bits is so encoded that its running digital sum (RDS) is confined to a range defined by predetermined lower and upper permissible limits, i.e., absent transmission error. Instead of using a forwards-backwards digital counter to monitor the RDS, with its associated expense and relatively high power consumption when high transmission rates are involved, use is made of an analog integrator which integrates the digital signal to be monitored and generates an analog RDS signal. A digit-error signal is generated when the analog RDS signal attempts to exceed a predetermined analog value corresponding to the upper permissible limit of the RDS or to fall below a predetermined analog value corresponding to the lower permissible limit of the RDS.

Abstract: A succession of emergency-phone stations distributed along the length of a communications cable laid alongside a highway is powered from a remote central station. The phone stations are provided with lamps to form a blinking-lamp signalling system warning of upcoming traffic hazards. Each phone station's blinking system includes a code-evaluator circuit stage which interprets code signals transmitted along the power-supply circuit of the communications cable, the code signals identifying which stations are to have their lamps blink and in accordance with which blinking schedules. Each station is provided with a bistable activation stage which responds to the first occurrence of a transmitted-voltage boost by disconnecting from power the code-evaluator circuit stage of all flashing stations, and all circuitry of all non-flashing stations, but which reconnects to power upon the second occurrence of such voltage boost.