Modem security device

Abstract

A modem security access circuit device, which is coupled between a dial-up telephone line and an automatic-answering modem, includes a line matching transformer circuit which trips an incoming ringing signal for a predetermined time interval. During the predetermined interval the caller, who receives no indication of the presence of the modem, must send an access code consisting of a preselected sequence of up to ten digits from a DTMF tone pad, or else the line matching transformer circuit disconnects from the telephone line. The device also includes a decoding circuit coupled to the line matching transformer circuit for receiving the access code. If the correct access code is received, the decoding circuit activates a ring supply generator which provides an internal ring signal to activate the modem. The activated modem is then coupled to the telephone line through the line matching transformer circuit and a carrier signal is sent to the caller. In addition, the device includes a lock-out circuit which disables the decoding circuit for a preselected time interval if the caller sends a predetermined number of invalid digits. Furthermore, the device includes a counter circuit for counting the number of invalid digits received and an LED display for visual read-out of the invalid digit count.