Private automatic business exchange

Claim 1

In an interface exchange between one public switched telephone line and multiple telephone handsets and/or auto-dial/auto-answer modem-equipped devices, the combination comprised of: a. a ring detector that responds to the public switched telephone network ring signal of 40 to 150 V and 16 to 68 Hz; b. a relay for going off-hook and signaling the public switched telephone network that the exchange is off-hook by completing the dc loop of the public switched telephone network; c. a first transformer to electrically isolate the interface exchange from the public switched telephone network with respect to ground and dc loop currents of the public switched telephone network; d. a second transformer to connect the signal from the exchange to the telephone handsets and/or auto-dial/auto-answer modem-equipped devices as may be connected to the output circuits of the exchange and provide for a dc loop current to any of the telephone handsets and/or auto-dial/auto-answer modem-equipped devices on demand; e. a dual-tone multi-frequency tone detector for detection of dual-tone multi-frequency tones; f. a microprocessor circuit programmed to respond to the output pulses of the dual-tone multi-frequency tone detector and drive the indicated relays to connect the public switched telephone network to the indicated port of multiple ports, and to respond to the photo coupler outputs associated with the loop currents that appear at each port when a telephone handset or auto-dial/auto-answer modem-equipped device goes off-hook, and to respond to establish priority between use of the exchange by received or locally originated calls, and to respond to a call initiated at one of the ports, seal the line and operate relays to go off-hook and obtain a dial tone from the public switched telephone network for the calling telephone handset or auto-dial/auto-answer modem-equipped device to recognize before manual or automatic dialing begins, and to provide a two second delay time after the ring detector response before activating a relay connecting the exchange audio to the public switched telephone network; g. an optical coupler in the output circuit of each of multiple ports to sense the loop current drawn by that port's telephone handset or auto-dial/auto-answer modem-equipped device going off-hook and to signal the said microprocessor; h. a dual-tone multi-frequency tone receiver; with circuitry programmed to drive selected relays to connect the public switched telephone network to one of multiple ports; i. a circuit to generate a ring-back tone to indicate to the public switched telephone network that the indicated telephone handset or auto-dial/auto-answer modem-equipped device is being rung; j. a ring generator under the control of the said microprocessor circuitry to ring the selected port and its telephone handset or auto-dial/auto-answer modem-equipped device; k. a tone detector circuit that inhibits operation of the exchange if signals greater than -9 dBm are sensed at the ports; l. a logic network to drive display lamps to indicate the active port even though the program priority has designated another port which can be connected to the public switched telephone line if the port's telephone handset or auto-dial/auto-answer modem-equipped device goes off-hook.