Apparatus for utilizing an a.c. power line to couple a remote terminal to a central computer in a communication system

Claim 1

In a system for communicating between a central computer and a plurality of remote inquiry terminals having a communication data channel connecting said central computer and a distribution substation, and a telephone party line coupled between said substation and said plurality of remote inquiry terminals, wherein an alternating current power line for transmission of electrical power at frequency f.sub.0, is located adjacent to at least one of said remote inquiry terminals, the improvement comprising: a means for utilizing said alternating current power line for coupling one or more of said remote inquiry terminals to said telephone party line, said means for utilizing said alternating current power line comprising: a party line modem including: a party line coupling means for coupling signals from said party line to said line modem and from said line modem to said party line, a controlled oscillator generating a carrier signal nominally at a frequency f.sub.1, f.sub.1 being substantially different from the frequency f.sub.0 of the current on said alternating current power line, said oscillator, providing as an output said carrier signal at frequency f.sub.1 as modulated by signals received through said party line coupler from said party line, a power line coupling means coupling said party line modem to said power line, said power line coupling means only passing signals substantially at frequency f.sub.1 from said modem to said power line and only passing signals substantially at a frequency f.sub.2 from said power line to said party line modem, wherein frequency f.sub.2 differs substantially from frequency f.sub.1 and from the power transmission frequency f.sub.0 of said alternating current power line, said power line coupling means isolating said party line modem from the power current on said alternating current power line, a demodulator connected to said power line coupling means for demodulating said modulated signals substantially at frequency f.sub.2 and for providing output baseband signals, the output of said demodulator being connected to said party line coupler for providing transmission of said output baseband signals from said demodulator on said party line; and at least one terminal modem associated with one of said remote inquiry terminals, said terminal modem including: a terminal coupling means for coupling signals from said terminal modem to the associated one of said inquiry terminals and from the associated one of said inquiry terminals to said terminal modem, a controlled oscillator for generating a carrier signal nominally at said frequency f.sub.2, and connected to said terminal coupling means, said oscillator providing as an output signal said carrier signal nominally at frequency f.sub.2 as modulated by signals transmitted from said associated inquiry terminal through said terminal coupling means, a power line coupling means coupling said terminal modem to said power line, said power line coupling means passing only said signals substantially at frequency f.sub.2 from said terminal modem to said power line and passing only signals substantially at frequency f.sub.1 from said power line to said terminal modem, said power line coupling means isolating said terminal modem from the power current at frequency f.sub.0 on said alternating current power line, a demodulator coupled to said power line coupling means for demodulating said modulated signals substantially at frequency f.sub.1 and for providing output baseband signals, the output of said demodulator being coupled through said terminal coupling means to the associated one of said inquiry terminals.