Multiplex Network - CITROEN DS series Technical Manual

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The Citroën Guide

Multiplex network

Circuit layouts already universally adopted in com-
puters finally made their way into contemporary
cars. Although their functioning might be fright-
eningly complex for people used to traditional cir-
cuits, they actually make the cabling very simple
and the addition of component interactions possi-
ble 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 har-
nesses 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 be-
tween the various components (usually using some kind of
a switching logic, relays for simpler tasks and small elec-
tronic modules for more complicated ones).
As various subsystems (engine management, suspen-
sion, 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 manufactur-
ers to fit two or three such sensors into various places, us-
ing every one of them only by their respective subsystem,
than to find ways to share the sensors, introducing intercon-
necting wires and the danger of one failing subsystem to in-
fluence the others.
Buses in the C5
Suspension ECU
Autobox ECU
Engine ECU
fuse box
Electrical Systems: Multiplex network
ABS ECU
Sterring wheel
and column
switches
Airbag ECU
BSI
The multiplex wiring first seen on late XMs and later
used on newer models like the Xsara Picasso or the C5 intro-
duces 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 con-
nection between the central unit and the engine, gearbox
and suspension computers, the VANs for the rest of the sys-
tems: 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 instrumenta-
tion and the comfort gadgets.
The bus—in contrast to the traditional wiring harnesses
hosting many individual wires running side by side to serve
different components—is a common channel of informa-
tion flow for all components connecting to it. It uses only
two wires which all associated components connect to in
parallel (in addition to this, the devices are connected to the
ground as usual; the two input wires serve as a safety mea-
sure, using them both makes the system resistant to any
outside interference, and the whole system remains func-
tional even if one of the bus wires becomes broken, shorted
to ground or positive feed). There is no special controller or
owner of this bus, each device connecting to it is free to
send or receive messages and commands to the others, at a
rather high speed (approximately ??? messages per sec-
ond).
Door module
Sunroof
Diesel additive
Anti-theft
Aircon
Navigation
Radio/CD
Multifunction
display
Instrument
panel
Door module
Parking
assistance
CAN (engine)
VAN 1 (safety)
VAN 2 (doors)
VAN Comfort
47