The present invention relates to a communication system, a transmission device, a reception device, a communication method, a program, and a communication cable, whereby high-speed communication can be executed while maintaining compatibility. In the event that an HDMI® source 71 and an HDMI® sink 72 execute two-way IP communication using a CEC line 84 and a signal line 141, a switching control unit 121 controls a switch 133 to select a partial signal making up the differential signal from a conversion unit 131 at the time of transmitting data, and controls the switch 133 to select a partial signal making up a differential signal from a receiver 82 at the time of transmitting data, and in the case of executing two-way communication using the CEC line 84 alone, the switching control unit 121 controls the switch 133 to select the CEC signal from the HDMI® source 71 or receiver 82 with the switch 133. The present invention may be applied to HDMI®, for example.