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Multiplex electrics

Over the past ten years or so, electronic systems have proliferated in all aspects of vehicle design, including mechanical systems (engines, automatic gearboxes, suspension, ABS, etc.), safety systems (airbags, transponders, etc.) and passenger comfort systems (air conditioning, radio, etc.).  At the same time, a need has developed for on-board systems to be able to communicate with each other. This has made the use of multiplex electrics essential.

Introduced on a trial basis with the XM in 1994, multiplex electrics were first applied on the Xsara Picasso. The C5 takes the concept one stage further with four separate and interdependent networks in which twenty modules and the built-in systems interface (BSI) are linked by harnesses referred to as “buses”.

Multiplexing brings the following major advantages :

  • Greater reliability and simplification of the electrical system, through the reduction in the number of connecting wires (-30 % compared with an individually wired system).

  • Greater user functionality, through permanent dialogue between all the units concerned and the BSI.

The built-in systems interface (BSI) controls the entire system through software running on a microprocessor. This unit centralises incoming data, decodes it and executes the corresponding orders by sending out messages in binary format. These messages are coded so that they can only be read by the unit to which they are addressed. The unit also provides a bridge between the various networks and transmits diagnostic information from other units.
In addition, the built-in systems interface reduces energy consumption by running all the multiplexed controller units in low-power mode whenever possible.  The bus carries data between the built-in systems interface and the other modules in both directions over a single communication link.

The twenty modules (electronic control units) are interconnected by the following four networks :

  • The Controller Area Network (CAN) carries short messages at high speed (e.g. engine speed and vehicle speed data for engine, automatic gearbox, suspension and ABS / ASR modules).

  • The passenger comfort Vehicle Area Network (VAN) transmits long messages at medium speed for the instrument cluster, display, radio, CD autochanger, air conditioning system, navigation system and parking assistance system. 

  • The bodywork VAN (consisting of two distinct networks) controls reliable operation of eight modules : the steering column controls, lighting and signalling indicators, airbags, left and right electric windows, alarm, sunroof and particle filter.

The built-in systems interface offers a host of multiplex functions with many advantages in terms of safety and comfort.

© 2000 Julian Marsh/Citroënët/SA Automobiles Citroën