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What is a multiplexed CAN Bussed system?

CAN stands for Controlled Area Network.

Originally developed in the early 1980’s by the Robert Bosch corporation, for automotive applications.

The use of Can bussed systems allows the manufacturers of automotive vehicles to seriously reduce the cost of construction by incorporating CAN technology.  In place of yards of wiring harnesses, a single pair of wires became sufficient as the transmission medium and because CAN’s are now used in such huge volumes, all the components are extremely affordable. Its high transfer rate and high transmitter reliability, as well as the ability to recognise data error, makes it ideal in electrical noisy environments, where the earth or grounding can be a problem.

How does it work?

The idea behind CAN is simple.  Instead of connecting everything to a central control unit through a main wiring harness, each component in the network has its own processing and communication capabilities, with one data channel connecting all units.

In automotive CAN applications, the instrument panel, power windows, body accessories and even many sensors and actuators all have their own individual mini-control unit.  All the units, called ‘nodes’, communicate with each other through a single pair of wires.  These are twisted together to ensure minimum interference and is known as the ‘data bus’.

All the data on the bus is addressed to a specific node or group of nodes by function, not location.  The relevant nodes respond to a particular pre-programmed protocol.  For example, the vehicle speed sensor will put its data on the bus, and every other control unit that needs vehicle speed information will receive it directly without having to go to the PCM (Pulse Code Modulator ) for a VSS ( Vehicle Speed Sensor ) signal.

Because there are multiple nodes sharing a single bus, every date package includes a priority code.  This will ensure, for instance, that if the ABS control unit and the lighting unit both send a message to the PCM, the critical ABS system message gets to go first.

CAN technology will significantly reduce the number of connector pins and wires in the vehicle. With extra ‘blank’ connectors built into the network loop, or with a splice kit for adding connectors, nodes can be added on the production line (or even aftermarket) without running a new wiring harness. However, designers have built in more than one network on the vehicle, linked by ‘gateways’ where appropriate. The mission-critical nodes will be linked on a high-speed Class C network that can operate in virtual real-time.  Less critical items, such as the instrument panel, lighting systems will use a slower Class B network, and  ‘occasionally used’  body accessories, such as power windows are likely to use a Class A network.  

*Activated driver assistance systems inform the driver about all relevant data being transferred around the vehicle - the corresponding display is then activated either in the instrument display or on a centre panel display. Computers for engine and transmission control or for lighting system management monitor the vehicle using the data transmitted via the CAN bus. The systems no longer operate in isolation alongside one another, but can now quickly react to the information from one another.  The CAN bus is an optimal solution for managing all vehicle electrical and electronic functions.  

As a result, effective fault monitoring and diagnosis of the systems are integrated into the CAN bus, defects can be localized more quickly and even temporarily compensated for by corresponding secondary systems. For example, a faulty tail light function can be temporarily replaced by a brake light bulb. The driver would instantly be informed of the failure in order to address the problem.  

*Activated driver assistance system  is hugely relevant for the towing customer.

Particular features such as TSP (Trailer stability program) now being introduced and currently installed in nearly ALL new VAG vehicles, BMW X series and the newly produced GM vehicles.  The alarming thing about these systems, are that they are a incredibly easy feature to install and activate when towing.  With a change to the existing ESP, ( Electronic Stability Program) the vehicle is able to correct a ‘snake’ in a trailer or caravan.  These systems are dormant in the software of the vehicle and brought into operation when the car detects a trailer connected.  

How does the vehicle know a trailer is connected?  

This happens in a number of ways but the most common method is a signal generated by the towing module, which is recognised by the central computer.  The signal is sent when a tow plug is inserted into the socket. Trailer Detection can also activate other features associated with towing.

For systems such as TSP to operate it is essential to have a tow bar wiring kit installation which communicates directly on the CAN bus.  These WILL NOT function if a by-pass wiring system is installed.