Multi-mode modem for automatically switching between data transmit and control command modes

A modem having at least two modes, one of said at least two modes being a control command accept mode for accepting an input signal as a control command and performing prescribed control processing based on said control command, and another of at least two said modes being a data transmit mode for accepting an input signal as transmit data and modulating said transmit data for output, the modem comprising:

• a modulation circuit for receiving said input signal and modulating a carrier signal with said input signal;
• a control circuit for interpreting a command input signal;
• identification signal processing means directly connected to said modulation means and said control circuit, including first register identification means, second register identification means, control means, mode switching means and identification signal deleting means,
• said first register identification means receiving said input signal and outputting a first signal to said control means, said first signal being at a first level when a specific identification signal is included in said input signal,
• said second register identification means, during said data transmit mode, receiving said input signal and producing a second to said control means, said second signal being at the first level when said specific identification signal is included in said input signal, and being at the second level when said specific identification signal is not included in said input signal,
• said second register identification means, during said control command accept mode, holding said specific identification signal and producing said second signal at the first level during said control command accept mode;
• and
• control means for receiving said first signal and said second signal, said control means producing a deleting signal when said first signal and said second signal are both at the first level, said control means producing a control command signal when said first signal is at the second level and said second signal is at the first level,
• said mode switching means receiving a control signal from said control means,
• when said control signal from said control means is said control command signal during said data transmit mode, said mode switching means switching said data transmit mode to said control command accept mode and outputting said input signal to said control circuit,
• when said control signal from said control means is said control command signal during said control command accept mode, said mode switching means holding said control command accept mode and outputting said input signal to said control circuit,
• when said control signal from said control means is not said control command signal during said data transmit mode, said mode switching means switching said control command accept mode to said data transmit mode and outputting said input signal from said first register identification means to said second register identification means, and
• when said control signal from said control means is not said control command signal during said data transmit mode, said mode switching means holding said data transmit mode and outputting said input signal from said first register identification means to said second register identification means, and
• said identification signal deleting means receiving said control signal produced by said control means, and when said control signal from said control means is said deleting signal, said identification signal deleting means deleting said input signal from said second register identification means, and when said control signal produced by said control means is not said deleting signal, said identification signal deleting means outputting said input signal from said second register identification means to said modulation circuit.