PURPOSE: To improve the adaptability of a sequence controller by allowing one unit to link plural systems to which data are to be transmitted and the sequence controller together.

CONSTITUTION: A data transmission unit U couples the control microprocessor 10 of a sequence controller with two different kind of systems 12 and 14 to which data are to be transmitted, and controls the transmission and reception of control data between them. Transmitted data from the CPU10 to the systems 12 and 14 are written in a transmitting image memory 22 through a gate 24 by the access of the CPU10. Then, a transmitting CPU6 accesses the memory 22 through a gate 27 to read the written data, and supplies them to a serial input and output controller 28. Through transmitting adapters 30 and 32, they are transmitted in prescribed transmission format to the systems 12 and 14.