Abstract: A line/interface circuit for a key telephone system, which system is of the type including a subscriber telephone line manifesting a pair of conductors. The conductors emanate from a central office or PBX and the line/interface circuit provides interconnections between a key subset associated with said subscriber and said central office via said telephone line. The line/interface circuit has optical sensing means for producing a signal indicative of ringing to indicate an incoming call. Means are coupled to the optical sensing means to access the subset with an alternate signal. The subset is coupled for the transmission of voice signals to the telephone lines by means of a transformer to provide isolation. Additional means are provided to enable operation of the subset in a hold condition without releasing a connection between a connected party and the subset.

Abstract: The exclusion system is employed with a central office/private branch exchange (CO/PBX) system and a party line paging and intercom system. A first circuit is coupled to the "A" lead of the party line system and is responsive to a predetermined potential on this "A" lead to couple a tip conductor and a ring conductor of an associated subset of the party line system to the tip and ring conductors of a selected one of the CO/PBX system and the party line system. A second circuit is coupled to the "A" lead of the party line system to manually generate a first exclusion potential for the party line system to exclude subsets not already connected to the party line system. A third circuit is coupled to the "A" lead of the CO/PBX system and is responsive to a given potential on this "A" lead to automatically produce a second exclusion potential on this "A" lead to exclude subsets not already connected to the CO/PBX system.

Abstract: The amplifier includes an audio signal input means and a first pair of integrated circuit audio power amplifiers each having an inverting input and a non-inverting input. The inverting input of one of the first pair of amplifiers is coupled to the input means and the non-inverting input of the other of the first pair of amplifiers is coupled to the input means. A first load has one terminal coupled to the output of one of the first pair of amplifiers and the other terminal coupled to the output of the other of the first pair of amplifiers. A second pair of integrated circuit audio power amplifiers each having an inverting input and a non-inverting input. The inverting input of one of the second pair of amplifiers is coupled to the input means and the non-inverting input of the other of the second pair of amplifiers is coupled to the input means.