Data transmission system, and monitoring system

In a data transmission system, a control device and a plurality of terminal devices are connected to each other through transmission path. A pulse signal composed of a data string is transmitted between the control device and the terminal devices. The data string has such a structure, including a mode region, an address storage region, a command region and a return data region which are disposed between a start bit and a stop bit; the stop bit could be replaced by an expanded bit which has a wave shape different from that of the stop bit; the mode region stores the the information for indicating that the data string is used for collecting the data or for controlling the terminal device; the address storage region stores the information for appointing the termial device; the command region stores the information for indicating the command content of the terminal device; and the return data region stores the return data. When the stop bit is replaced by the expanded bit, an expanding return data region is added