Fail-safe CATV system center facility

Claim 1

In a CATV system in which a central facility is coupled through cables to a number of terminal units and two-way data communication is effected therebetween, said central facility comprising: normal down data signal transmitting means; abnormality detecting means connected to said data signal transmitting means for detecting a down data signal abnormality in a down data signal produced by said down signal data tramsmitting means, said abnormality detecting means including an FM detector responsive to the down data signal, an AM detector, responsive to an information carrier signal and AND gate means having first and second inputs coupled, respectively, to outputs of said FM detector and said AM detector, an output of a AND gate means being coupled to control a switching position of said switching means; back-up transmitting means for producing a clock signal by modulating a carrier signal to form a frequency shift keyed signal having the same frequency as that of said down data signal; and said switching means for stopping, when said abnormality detecting means detects that said down data signal is in an abnormal condition, transmission of said down data signal and coupling said clock signal fromm said back-up transmitting means to said cables, wherein when said down data signal is abnormal, clock signals for down data signals are transmitted to said terminal units by said back-up transmitting means.