Modem with improved escape sequence mechanism to prevent escape in

Abstract

An FSK modem for coupling a standard data port (50) to a telephone line (45) in which the signal path between the phone line and the data port passes through a processor (55). When the modem is in a transparent mode of operation, for transmitting data, a timer (190) detects the passage of a predetermined period of time after the most recent data input signal input to a data port (50). If the next sequence of data signals corresponds to an escape character, the program (112) controlling the modem waits to see if a second occurrence of the predetermined period of time follows provision of the escape character. If this occurs, the modem switches to a command mode of operation in which it ceases to modulate and transmit signals and waits for commands to be entered at the data port. The same frequency detection apparatus (83, 192, 180) is used to both demodulate FSK encoded signals and detect the presence of a telephone ringing signal on the line. A pair of impedances (161, 162) provide alternate paths either through an input filtering network (167) or bypassing the network (135, 69) in a manner which allows the ring signals to pass directly to the input (69) of the frequency detector while assuring that only filtered FSK signals reach the input. Also shown is the use of the same frequency synthesizing apparatus (86, 253, 259) for generating FSK output signals and high group DTMF signals with appropriate switchable attenuation (88) provided in this signal path.