A data communication system for the communication between two different data transmission systems, the SDT system and the RDT system. The SDT system includes a host CPU having an output terminal for repeatedly producing first data in a predetermined sequence of time slots by a number of interruptions taken in each time slot, a bus structure extending from the output terminal of the host CPU for transmitting the data, and a plurality of slave CPUs, each having an input terminal connected to the bus structure for receiving the first data. The RDT system for the data transmission between at least first and second CPUs includes a RAM for storing a command to the second CPU. The first CPU is operative to write the command into the RAM. The first command writing takes place at a first predetermined pulse period in a predetermined sequence of pulses. The second CPU reads the command from the RAM at a second predetermined pulse period. The host CPU in the SDT system is used in common with the first CPU in the RDT system. Also, the sequence of time slots have at least one time slot which is sufficiently long to include the sum of one cycle of the predetermined sequence of pulses and one pulse period for carrying out the data transmission by the second data transmission system within one time slot.