CITROEN DS series Technical Manual page 48

U
The Citroën Guide
Each message or command is a sequence of a few num-
bers, specifying:
U the sender and the intended recipient of the message
(every device connecting to the multiplex bus has its
own address, a unique numerical identifier—for in-
stance, the fuel level sensor has the address 4315, the
instrument panel is 0004);
U
whether the recipient should acknowledge the mes-
sage as it processes it;
U the actual data the message transmits;
U some additional values to check the integrity and va-
lidity of the message at the receiving end.
Each major unit sends its own data into the network at pre-
determined intervals, marking the message with its own ad-
dress as a sender (some simpler sensors are connected di-
rectly to a computer which sends the messages relating to
their measured values on their behalf). With our example,
the fuel level sensor sends the amount of fuel it measures,
specifying the central unit (BSI) as the intended recipient.
As soon as the BSI sees this message circulating on the net-
work, it processes it by retrieving the data—the value of
fuel level—from the message and comparing it to the previ-
ously known value. As the amount of fuel is not supposed
to change drastically from one moment to the other, it dis-
cards the new value if it differs too much from the previous
one.
If the new value is acceptable, the BSI emits another mes-
sage of its own, addressed to the instrument panel this
time. As the instrument panel receives this second mes-
sage, it extracts the data representing the amount of fuel
left in the tank and turns this signal into the physical rota-
tion of the gauge needle.
All devices are constantly observing the bus for mes-
sages addressed to them, ignoring the ones sent to other re-
cipients (although there are special broadcast messages
sent to all devices, without specifying a single addressee)—
actually, the instrument panel saw the original message
coming from the level sensor as well but ignored it, it only
acted when the second message, sent by the BSI and ad-
dressed specifically to it, arrived.
All components work in a similar way. Some are simple
enough to send a few simple messages (like sensors or
switches) or to receive only a few ones (like electric window
motors). Others are complex subsystems themselves, like
the suspension, observing the input from a large number of
sensors and performing complex operations. But as they
are all connected to a common bus, the possibility of inter-
action is already there. Whether the headlights light up, the
electric windows close and the wiper starts to work in case
of rain, or whether the passenger side external rear view mir-
ror folds down when engaging reverse gear have all be-
come a simple question of software written for the central
unit. Adding a new feature does not require building a sin-
gle extra wire or connection, just to add a few lines to the
software.
Center of Attention
The four networks all connect to the central unit, the Built-
in Systems Interface (BSI). This control unit manages the
flow of information between the networks (many of the
Electrical Systems: Multiplex network
messages generated in one network has to be relayed to
another, just one example is the suspension computer—
connected to CAN—being interested in messages about
the open or closed position of the doors—communicated
on VAN 2).
In addition to that, the BSI offers an interface to the out-
side world as well, a diagnostic socket which can be used to
check, test and configure the whole system.
The multiplex system switches to an energy-saving low
power mode whenever possible.
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