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[Webby the Bear]

Hi guys,

Doing some revision and some practise exam questions. Most are straight forward, but there's a question on ''understanding multiplexing'' and CAN bus.

Googled both. multiplexing seems to be where different areas of the car have their own control unit to save there being tons of wire going to one ECU. That's fine I can understand that.

but I cant understand this explanation of CAN bus....

''The CAN Bus is an automotive bus developed by Robert Bosch, which has quickly gained acceptance into the automotive and aerospace industries. CAN is a serial bus protocol to connect individual systems and sensors as an alternative to conventional multi-wire looms. It allows automotive components to communicate on a single or dual-wire networked data bus up to 1Mbps''.

  Is there anyone out there that can put this in simple English?

[TheBoy]

can is where areas of the car have their own ecu - eg, no central locking ecu, but each door ecu receives lock/unlock commands from bcm.

multiplexing is just a way of putting multiple signals on one wire...

[Webby the Bear]

oh ok, so I basically got the two confused.

multiple signals on one wire. you mean, for example, an o2 sensor sharing a wire with something that produces the same voltage?

[TheBoy]

multiplex is a generic term, lots of variants - TDM being probably the most common, where each seperate signal is given a set time of carrier (wire/cable or wireless) time, before next device is allowed its piece of time.

[Webby the Bear]

multiplex is a generic term, lots of variants - TDM being probably the most common, where each seperate signal is given a set time of carrier (wire/cable or wireless) time, before next device is allowed its piece of time.

interesting! could you give me an example of a sensor(s) that wopuld this wiring and wiring time?

[Marks DTM Calib]

Sensors won't as it requires some intelligence so think ecus's

[Webby the Bear]

Sensors won't as it requires some intelligence so think ecus's

ok, so say BCM would share wires with the ABS ECU due to their close proximity to each other. For example? sorry if ive misunderstood

[05omegav6]

It's well suited to longer cable runs as less wiring is needed...

[Andy H]

Think about it as an electronic postal system. If a controller wants to know something it writes a request and sticks it in an envelope with the address of the sensor it wants the answer from. The sensor receives the request, prepares an answer which it addresses to the controller that sent the request.

Some things need to be sent more urgently than others (first class and second class). Some sensors are more intelligent than others and can send messages without being asked.

I think CAN stands for Controller Area Network.

[05omegav6]

First fitted to the W140 S Class in 1991, which is why they're relatively cheap to buy...

If an Omega scuttle drain gets blocked you get wet carpets, if the Merc one gets blocked you end up with a 17ft garden ornament (some boffin thought best to put one of the major ecus under the passenger footwell so if the water doesn't kill it, people standing on it will )

[Webby the Bear]

thanks guys that makes sense. are these requests made at certain times? set certain times that is?

[05omegav6]

thanks guys that makes sense. are these requests made at certain times? set certain times that is?

Depends on the request... things like brake lights/indicators or anything switch controlled by the driver would be instant on demand, other things such as oil or coolant level or ABS activation would need a number of conditions to be met before anything happened...

[Webby the Bear]

ok thanks taxi.

so as an example....

ECT sends high coolant temp reading..... ECU then sends a request to expansion tank sensor for coolant level.... then sets light should the response be a specific one...

am I getting the gist?

[05omegav6]

I think so... I was thinking more along the lines that...

You throw the car round a tight roundabout, it won't put the (oil or) coolant light on until the level is low for X amount of time (unlike the Omega )

[Kevin Wood]

....
''The CAN Bus is an automotive bus developed by Robert Bosch, which has quickly gained acceptance...

Quickly? PMSL. I remember building a CAN controlled light cluster at school as an A level electronics project (and that was a long time ago).