Apparatus for a key telephone system for enabling intercom substations to access trunk lines

A scanning system for selecting one of a plurality of circuit means in a first electrical condition, where each such circuit means may be in either a first or in a second electrical condition, comprising:

• electronic counter means, said electronic counter means having a reset input, a trigger input and a plurality of separate output terminals corresponding in number to the circuits to be scanned and with each output associated with a corresponding one of said circuits;
• said counter means being operable from a reset condition responsive to a high pulse or pulses at said trigger input for providing a voltage high at the corresponding one of the plurality of output terminals represented by the accumulated total of high pulses applied to its said trigger input;
• a plurality of NAND gates, each of said NAND gates including an output and two inputs and operable to provide a voltage low at said output only during the presence of a voltage high at each of said two inputs;
• gate means coupled between the outputs of said NAND gates and said trigger input of said counter means responsive to a change in any of said NAND gates from voltage high to low for providing a corresponding voltage high pulse to said trigger input of said counter means;
• whereby said counter accumulates an additional high pulse and provides a voltage high at the respective output representative of the accumulated count of trigger pulses;
• a plurality of detector means corresponding in number to said plurality of circuit means with one detector means being associated with a corresponding one of said circuit means, each said detector means having an output coupled to a first input of an associated one of said NAND gates and an input coupled to said associated circuit means for producing a voltage high at said output responsive to said circuit being in the second electrical condition;
• means connecting each output terminal of said counter means, except the output terminal representative of the maximum count capacity of said counter means, to a second input of a corresponding one of said NAND gates whereby any high at any one output terminal of said counter means is applied to a corresponding NAND gate;
• first activating means coupled to said trigger input of said counter means for providing at least one high pulse to said trigger input of said counter;
• whereby said counter means once triggered by a first pulse sequentially moves a high from one output terminal to the next output terminal in sequence until a circuit is located that is in the first electrical condition.