Hi All

Every so often the subject of XM's wiring and multiplexing crops up. I thought it might be useful to start pulling information on this under a single heading as it seems be something of a black art with virtually nothing in any of the technical books.
I think that as far as XM's are concerned Citroen have used the terms Multiplexing and ECU's to cover some very simple electronic components and wiring. If they used a wire to send two different signals that to them was multiplexing. If the box at the end of the wire had a few simple electronic components to split the signal out that was an ECU. Only a few of the ECU's have the ability to send return status signals and only the main engine ECU the ability to store that status information.
Anyone out there got access to more specific/detailed information on XM multiplexing and ECU's to prove this one way or another?

Regards

XMexc

Hi XMexc,

My knowledge isn't great and I certainly don't have any first hand experience but I'll share what I know.

From my previous investigations into this subject I came to the following conclusions:
The PSA (Peugeot/Citroen/et al conglomerate) implemented a comms bus called CAN (Controller Area Network) in their vehicles from quite an early date. At the same time GM/VAG and it looks like everyone else embarked on OBD (On Board Diagnostics) which was a comms system invented by Bosch. Europe, the Far East and the large US manufacturers implemented OBD but the PSA stuck with CAN (and laterly VAN(Vehicle Area Network)). Now that legislation has been passed even the PSA are forced to use OBD. All new cars sold in the US and the EU must have OBD implemented. I can't confirm for sure but the PSA may have dropped CAN/VAN all together. All of the cheap diagnostic adapters (OBD to USB) and related software on ebay is testimony to the universal nature and spread of the OBD implementation.

The XM was built with the use of discrete ECU's for some of the main systems (Engine Management, Auto Gearbox Management, Air Conditioning, ABS, Intruder Alarm, Pyrotechnic Seatbelts etc etc) These ecu's have their own bus cabling wired out to the multiplug under the steering column. By plugging in a Citroen computer (eg Lexia or Elit) the ecu's can be interrogated one at a time. The Elit/Lexia were able to switch from one set of pins to the other via a built in menu system. Each of the systems communicated with the Elit via a CAN protocol.

At various ages, specific models had a system fitted known by Citroen as multiplexing. My interpretation of this is that all of the previously discrete ecu's were connected on a common bus with addressing so that they could A) talk to each other and talk to a host computer. As it happened, Citroen arranged all of the 'critical' ecu's (eg Engine, Brakes etc) on the CAN bus whereas the non-critical systems were put on a VAN bus. Controller nodes were added to act like hubs or switches in an ethernet system. This allowed all of the on-board ecu's to be interrogated via a single pair of cables.

Multiplexing is Citroen-speak for combining multiple ecu's on a single bus.

The radio RDS information repeated on the dash display gets there by means of a VAN bus between the radio and the display.

If I remember tonight I'll post a copy of the multiplexing wiring schematic once I get home.

Cheers

noz

ps
I appreciate I've used the terms bus and protocol interchangeability but that's only because I haven't a clue what I'm talking about

Hi all,

Here's the Multiplexing section from the Factory Manual.

Sorry for the quality but I had to reduce the resolution to make it small enough to post.

Cheers

noz

Hi Noz

Thanks for that.

Regards

XMexc

Strange, I ever knew multiplexing as a universal automotive technology presented as an innovation in the late 80s/early 90s, not PSA only, and thought that nowadays it's pretty standard all cars are multiplexed, because it saves miles of cabling...?

regards
George

Hi George,

The phrase 'multiplexing' is probably throwing a few people off the scent. In the days of hard wired data collection, multiplexing meant using a single pair of cables to transmit multiple (usually volt-free) information. At either end of the pair of wires there would be a rotary device both being in synchronisation. Each one knew at a certain moment in time that the message (open or closed) referring to the status of some piece of equipment was being sent and the receiver, because it was synchronised, knew to which piece of equipment the message referred. It was really a 'mechanical' form of digital information transmission. Very archaic by modern standards. Why Citroen called theirs 'multiplexing' is beyond me because I don't recognise anything in the CAN/VAN protocol that could be called 'multiplexing'. Maybe it was to put people off the scent to stop them developing their own interrogation tools.

The ability to talk to modern ecu's in cars is as the result of legislation which compels vehicle manufacturers to use a comms protocol which 3rd party manufacturers can get automatic, unhindered access. The protocol chosen for this purpose is OBD.

Are there any other electronically minded people with a clearer view of this subject? I feel distinctly out of my depth talking about it.

Cheers

noz