How it works: Controller Area Network

With vehicles becoming more complex due to government and customer requirements for safety, emissions and fuel economy, more and more electronic controls are being fitted to our vehicles.

Computers are now taking care of a lot of the functions the driver traditionally needed to think about; vehicles are becoming safer as the driver is left to concentrate on the road rather than worrying about distracting functions such as headlight control, emergency braking, cruise control et cetera.

By adding more electronic control to the vehicle the need for space and even the addition of weight becomes a problem. More control means additional control units, sensors and actuators and the biggest concern is additional wiring so all of these new units can interact with each other.

Traditionally manufacturers connected electronic devices in vehicles using point-to-point wiring systems. That resulted in bulky wiring harnesses that were heavy and expensive. If you were to wire a modern vehicle with traditional copper wiring (point-to-point) the additional weight would negate any fuel saving the extra control may give.

To overcome these issues communication between control units and some actuators is taken care of with the use of in-vehicle networks that reduce wiring cost, complexity, and weight. Making communications and data sharing between PCs in a modern, computerised office is handled by a Local Area Network (LAN). Whilst this system may work in an office it is still too complicated and cumbersome for the confines of a car.

No can do without CAN
CAN (Controller Area Network), a high-integrity serial bus system for networking intelligent devices, emerged as the in-vehicle network standard during the 1980s. Bosch originally developed CAN in 1985, and the automotive industry quickly adopted the standard, which, in 1993, became officially and internationally recognised as ISO 11898. One of the earliest production-car applications was the E32 generation of BMW 7 Series from 1988 (pictured).

CAN has been adopted in other industries, such as aerospace, transport (such as train and tram systems), and the medical industry.

Let's twist again
A CAN network is a series of smart control units that have the ability to either passively receive information or send data as well. This is known as a Peer to Peer network. Digital information is transmitted over a two-wire network; this is a pair of wires spirally twisted, and naturally called a twisted pair. Each wire is named bus line A (CAN_L) and bus line B (CAN_H), according to its special features. Both bus lines conduct opposite-phase voltage allowing for minimal electrical noise being emitted and reducing reception of electrical noise interference.

With the adoption of CAN the wiring required to communicate between modules is reduced dramatically. For the system to communicate each control unit requires a minimum of 4 wires to operate: power, earth and the two CAN wires. The CAN wires (twisted pair) link all control units on the network to allow the communication to take place. CAN data transmitted on the CAN Hi line is mirror imaged on the CAN Lo line, and for the CAN system to function the CAN Hi voltage must always be higher than the CAN Lo voltage.

There are no master control units; individual control units have access to read-and-write data on the CAN bus when required. When a control unit is ready to transmit data, it checks to see if the bus is busy and then simply writes a CAN message onto the network.

The CAN messages that are transmitted do not contain addresses of either the transmitting control unit or any of the intended receiving control units. Instead, an arbitration ID that is unique throughout the network labels the message. All control units on the CAN network receive the CAN message, and, depending on the arbitration ID of that transmitted message, each control unit on the network decides whether to accept the message.

For example, the vehicle speed signal is required by a number of control units. The Speed signal/message originates from the wheel speed sensors via the anti-lock braking system control unit.

The ABS control unit transmits the message on the CAN, The instrument cluster needs this information to display the vehicle speed to the driver; the automatic transmission may also need the vehicle speed to determine which gear to select; and in some vehicles even the audio system needs the speed signal so the volume can be adjusted automatically.

Any other control unit that is part of the CAN network and doesn't require the vehicle speed information simply ignores the message.

Sent to the back of the bus
Messages on the CAN are prioritised. If multiple control units try to transmit a message on the CAN bus at the same time, the control unit with the highest priority automatically gets bus access. Lower-priority messages must wait until the bus becomes available before trying to transmit again.

Most vehicles have multiple networks that often run at different speeds, depending on what systems they are looking after. For example, the powertrain control units may be on a High speed CAN (Engine control module, ABS, stability control, body control module, et cetera). Other control units may be on a low-speed CAN (instrument cluster, park-assist unit, climate control, et cetera).

Having said this, some information may need to be shared between networks; this is done via a gateway. Depending on the vehicle manufacturer, the control unit that is the gateway between networks may vary, however it is generally the instrument cluster control unit or body control module.

In summary the CAN system is a cost effective way to transfer data quickly around your vehicle's different systems, without compromising space and weight. There are other data transfer systems used in vehicles, one being Local Interconnect Network (LIN). This system is usually used for single-unit data transfer, for example, between an engine control module and an alternator (assuming no other units on the network). These can also be known as private networks.

If you are worried about self-driving cars and their new technology, it may surprise you to know that you have already been testing it for years, and you're getting closer to handing over the controls to a software program!

So maybe there's nothing to worry about after all.