Intrinsically safe telephone

Claim 1

An intrinsically safe telephone for use in a hazardous environment comprising: (a) a single line instrument for providing a dialing output and an audio output signal to a telephone network and for receiving an audio input signal from that network, said single line instrument being situated in said hazardous environment, said single line instrument including a transmitter means for generating a carrier signal, said audio output signal being modulated on said carrier signal; (b) a transmission path coupling said single line instrument to a safe environment; (c) electrical isolation means situated in said safe environment and coupled between said transmission path and said network for controlling power to said single line instrument, said electrical isolation means including (i) a telephone line interface for reducing power to said single line instrument, for reducing the power of incoming ringing signals from the network and for isolating the single line instrument from the network, said telephone line interface including receiver means responsive to said carrier signal for providing a demodulated audio output signal to said network; a sidetone network responsive to said demodulated audio output signal for providing a phase adjusted feedback signal over said transmission path to said single line instrument; an off-hook detector responsive to changes in loop current in said transmission path; a network interface responsive to a ringing input signal form the network, said ringing input signal having a defined ring interval; a ring generator for applying a local periodic ring signal to the transmission path during that interval, said local periodic ring signal including active portions interrupted by relatively quiescent portions over each period; and control means responsive to the off-hook detector for generating an off-hook signal when a change in loop current has been detected only during the quiescent portions of said periodic ring signal or when a change in loop current has been detected and no ringing input signal has been detected by the network interface, said ring signal generator being responsive to said off-hook signal; (ii) a shunt diode barrier for controlling the maximum power to the single line instrument; and (d) a local power interface coupled to said telephone line interface for supplying power to said telephone line interface from a source other than said telephone network.