MODEM WITH IMPROVED ESCAPE SEQUENCE MECHANISM

摘要

MODEM WITH IMPROVED ESCAPE SEQUENCE MECHANISMAbstract of the DisclosureAn FSK modem for coupling a standard data port (50)to a telephone line (45) in which the signal path between thephone line and the data port passes through a processor (55).The same frequency detection apparatus (83, 192, 180) is usedto both demodulate FSK encoded signals and detect the presenceof a telephone ringing signal on the line. When the modemis in a transparent mode of operation, for transmitting data,a timer (190) detects the passage of a predetermined periodof time after the most recent data input signal input to adata port (50). If the next sequence of data signalscorresponds to an escape character, the program (112)controlling the modem waits to see if a second occurrence ofthe predetermined period of time follows provision of the escapecharacter. If this occurs, the modem switches to a commandmode of operation in which it ceases to modulate and transmitsignals and waits for commands to be entered at the data port.A pair of impedances (161, 162) provide alternate paths eitherthrough an input filtering network (167) or bypassing thenetwork (135, 69) in a manner which allows the ring signalsto pass directly to the input (69) of the frequency detectorwhile assuring that only filtered FSK signals reach the input.Also shown is the use of the same frequency synthesizingapparatus (86, 253, 259) for generating FSK output signalsand high group DTMF signals with appropriate switchableattenuation (88) provided in this signal path.