One of the authors previously presented a new concept for the application of multiplexing technology to the automobile (SAE 880589). That concept proposes dividing the wiring harnesses into separate systems, each of which is multiplexed on an individual basis. To implement multiplexed communications in vehicle systems based on this concept, new ICs have been developed that meet the various requirements for production model use.These ICs are designed for time-division multiplexing using a pulse count scheme and are provided with the following features to assure reliable communications against wiring harness trouble. The data bus uses active high logic and a self-clocking technique is employed for the data bits which are encoded using pulse-width modulation. Marks indicating the master synchronization interval are defined as a “low” level of long duration. The fail-safe functions provided include removal of momentary noise using an input filter, error detection by means of a pulse number check and detection of pulse dulling or clock frequency irregularities by a check bit verification technique. One communications line can accommodate transmission of up to 25 switch signals.