Multiplex network

Circuit layouts already universally adopted in computers finally made their way into contemporary cars. Although their functioning might be frighteningly complex for people used to traditional circuits, they actually make the cabling very simple and the addition of component interactions possible in ways never experienced before.

Conventionally, cars used individual wires connecting the various elements—steadily increasing in number—on board. The huge amount of wires, connectors, wiring harnesses were a constant source of connection problems. The various circuits were largely independent (sharing only the feed and the ground), although some components had to interact (for instance, fog lights should work only when the headlights are switched on), necessitating connections between the various components (usually using some kind of a switching logic, relays for simpler tasks and small electronic modules for more complicated ones).

As various subsystems (engine management, suspension, ABS, etc.) came from different manufacturers, some functions were even built in parallel. Several subsystems might rely on the signal sent by a coolant temperature or a vehicle speed sensor but it was simpler for the manufacturers to fit two or three such sensors into various places, using every one of them only by their respective subsystem, than to find ways to share the sensors, introducing interconnecting wires and the danger of one failing subsystem to influence the others.

The multiplex wiring first seen on late XMs and later used on newer models like the Xsara Picasso or the C5 introduces a radically different concept: just like in the computer used to read this book, there is a central backbone circuit called bus which goes around the whole car—actually, there are four of them, a Controller Area Network (CAN) and three Vehicle Area Networks (VANs), dealing with different areas: the CAN is only responsible for the connection between the central unit and the engine, gearbox and suspension computers, the VANs for the rest of the systems: the first serves the safety systems like the airbag, the second the various doors (including the sunroof) and the anti-theft system, the third everything else: the instrumentation and the comfort gadgets.

Hydractive Summary

Although every aspect of the functioning of the Hydractive systems was described in the previous chapters, considering the number or factors influencing the suspension and the amount of rules and decisions made by the computer, it is not easy to grasp the actual behavior of the car, including the differences in the various Hydractive generations. To make it easier, we summarize how the various Hydractive systems work in real life.

Hydractive 3

The new C5 has a new suspension system, doing away with many solutions used on Citroëns for several decades, yet offering the same or even better comfort than before. Recent developments in electronics and computics made it possible to delegate many functions previously solved by mechanical-hydraulical components to electronic units.

This third generation suspension system retains the same basic functioning as the previous systems. It also comes in two flavors: a simpler Hydractive 3 reminiscent of the original hydropneumatic suspension of the DS–GS–BX–CX and a slightly more complicated Hydractive 3+, building upon the former Hydractive I and II (actually, Hydractive 3 is not hydractive in the sense we used this term before, its only special activity is to adjust the road clearance depending on speed and road condition).

Although the basic functioning is practically the same, the actual layout underwent significant changes. Most importantly, the previously mechanically operated height correctors became electronically controlled hydraulic units. And all hydraulic units except for the spheres—which were redesigned to give unlimited life expectancy—are now housed in a single unit, the Built-in Hydroelectronic Interface (BHI). This compact unit has three main parts: