Abstract: A communications system for use in a coaxial cable data transmission system such as in a hotel/motel system between a central processor and a room and the room and appliances within the room wherein frequency shift key modulation of a high frequency signal is used for communication between the central processor and the rooms and amplitude shift key modulation of a low frequency signal is utilized for communication within the room between the in-room controller and appliances within the room. In addition, the system also utilizes a serial interface input/output card in the central processor which detects the quality of the signal coming from each room in regard to frequency, signal level and deviation and stores those signals for maintenance purposes. Also, the system provides for long distance interrogation of the central processor over telephone lines so that the hotel/motel operating system may be reviewed from a remote distance.