At&t Bell Laboratories

Bell Telephone Laboratories

At&t Technologies, Inc.

At&t Technologies and At&t Bell Laboratories

Joseph J Nahas

Paul E Crouch

Howard Ng

Dale Harvey Nelson

Brian Albert Wittman

Paul C Davis

Raymond G Jackson

DeWitt G Ong

In an electronic coin telephone set, a fraud prevention arrangement prevents fraudulent coin tones originating at a telephone transmitter from being provided to a central office over a coin subscriber loop. The arrangement comprises a first and second transmission channel for alternately connecting the transmitter to the central office and also a fraud filter disposed in the second transmission channel. The fraud filter attenuates a selected frequency originating at the transmitter that is indicative of coin deposits. When coin deposits are not required by the central office, the transmitter is connected to the central office through the first transmission channel. When coin deposits are required by the central office, however, the arrangement connects the transmitter to the central office through the second transmission channel thereby avoiding the fraudulent generating of coin tones through the telephone transmitter.

Fraud prevention in an electronic coin telephone set

This invention pertains to a microcomputer controlled coin telephone set which prevents coin call fraud and performs all of the coin discrimination functions. The microcomputer also checks the operational readiness of the telephone set to prevent the acceptance of coins into a hopper for late collection by a coin box unless or until the set is fully functional.

A microcomputer controlled coin telephone set prevents coin call fraud and also checks the operational readiness of the telephone set to prevent the acceptance of coins into a hopper for later collection by a coin box unless or until the set is fully functional. Capable of being interrogated remotely, a local memory is used for storing coin deposit information and generating coin deposit signals in order of deposit to a central office. In those instances where the line power drops below that necessary for full operation as a coin telephone, low power sensing circuits convert the telephone to the POTS mode which allows speech and dialing capabilities. Similarly, any type of chute failure, or a full hopper causes the telephone to revert to the POTS mode. While in the POTS mode, any coins deposited in the telephone are returned to the customer.

Controller for a coin telephone set

A digital-to-analog signal converter (1000) provides an analog output signal by subtracting bit currents reflecting a digital input signal from a constant current source (1004, 1018-1022) at a current summer (1003), with the difference current being applied to a constant load. The subtracting of the current, rather than switching the current, avoids the modulation of an associated power source since the total current demand remains the same. In that low power operation is possible, the converter is particularly suited for use in an arrangement where only a low level of operating current is available from a power source.

Digital-to-analog converter

A keypad logic interface circuit (200) provides a switch closure output signal representative of a selected one of a plurality of digits on a crosspoint contact array keypad (100). The selected one of the plurality of digits provides the switch closure signal by diverting a source current (225) from a first to a second direction. The circuit encodes each one of four column outputs from the keypad into a two-bit code. An output signal from two or more columns at the same time causes the circuit to produce an inhibit output signal reflecting a multiple button-push condition. The circuit also encodes each of four row outputs from the keypad into a two-bit code and produces an inhibit output signal when multiple row output signals are received.

Keypad logic interface circuit

In a multifrequency generator providing tone signaling for a telephone, the common switching functions are performed by solid state switching circuitry (1200). Improved signal transfer between the telephone line and transmit and receive transducer of the telephone is provided by arranging the solid-state transmit and receive switches (1201, 1401) in a gain configuration to increase the level of the speech signal. The tone signals are digitally synthesized and use an integrated R-C oscillator (600) as a reference. Button down and button up debouncing of a contact closure signal from a single contact per button keypad is provided.

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Wyomissing Hills, PA, United States of America

Joseph J Nahas