The present invention relates to a multiplex data transmission system, and more particularly to a data transmission system which is suitable for use as a compact wiring system providing multiplex transmission in an automobile. 
An automobile is equipped with a number of electric devices and parts such as lamps, motors, sensors and actuators, and the number of those electric devices and parts can be expected to increase as the automobile is provided with more electronic features. 
Thus, if those large number of electric devices are individually wired, the wiring would become very complex and of large scale, and serious problems, such as increases in cost, weight and a space and mutual interferences is signal transmission will occur. 
One of the methods to resolve the above problems is simplification of wiring by a multiplex data transmission system which enables the transmission of a large number of signals with a small number of wirings.  This is described in a Japanese patent application 17535/82 (corresponding to U.S. patent application Ser.  No.  464,212). 
In the disclosed transmission system, signals are transmitted between a plurality of terminal stations (LCU) and a central station (CCU) through a common transmission line.  Each LCU has one or more detection means or control means, or combinations thereof.  The CCU has a digital computer for controlling the signal transmission between the CCU and the LCUs.  This computer is used not only for the signal transmission but also other processings.  The content of the processings differs from system to system.  Examples of such processings are generation of the signals to be transmitted and diagnosis of failure in the system.  Accordingly, a system which efficiently utilizes the computer of the CCU to achieve such functions is required.