Communication system and transmission unit employed in same

A communication system, comprising a transmission unit that repeatedly transmits a transmission signal to a communication line, a first terminal that communicates by the transmission signal, and a second terminal that communicates by a superimposition signal that is superimposed on the transmission signal,

• said transmission unit, said first terminal and said second terminal being connected to said communication line,
• wherein the transmission signal is a time-division signal that is divided, frame by frame, into a plurality of regions in a time axis direction, the plurality of regions including:
• an interrupt segment for detecting presence or absence of an interrupt signal that is generated by said first terminal;
• a transmission segment for transmitting data to said first terminal;
• and
• a reply segment that is a time slot for receiving response data from said first terminal, and
• wherein said transmission unit includes:
• a response requesting unit configured to transmit, when detecting the interrupt signal in the interrupt segment, response requesting data to said first terminal in the transmission segment with the transmission signal;
• a response receiving unit configured to receive the response data that is transmitted in the reply segment with the transmission signal, in response to the response requesting data, by said first terminal that has generated the interrupt signal;
• a segment securing unit configured to transmit, in the transmission segment with the transmission signal, securing data for securing the reply segment for superimposition of the superimposition signal, by prohibiting transmission of the response data in the reply segment, with specific response data as a trigger;
• and
• a superimposition communication unit configured to transmit, to said second terminal, data in the reply segment secured through the securing data, with the superimposition signal.