CANBus – What do the signals look like?

3 min read

In previous articles on this topic, we’ve looked at the benefits a CANBus can offer and why modern vehicles are so dependent on CANBus. Instead of controlling devices by individual wires, we can have devices communicate with multiple devices simultaneously using a communications bus. But what does this CANBus communications method look like at an analogue electrical level?

Communicating with wires

A digital comms bus system transmits High (1) and Zero (0) values between two devices. The most straightforward analogy is that of turning a light on and off at a regular interval to represent the ‘bits’ that make up a message. We’ll break down the message content in a later blog, but for now, how do we get the light to switch on and off?

CANBus is a two-wire system with every device simultaneously connected to the ‘Hi’ and ‘Lo’ lines. The wires are typically twisted together to reduce unwanted electrical noise or voltage fluctuations. Anything that receives or transmits data, rotates, or moves will create unwanted voltage spikes called signal noise. The CANBus system is very tolerant of noise which is one of its advantages over other methods in an automotive application. The CANBus is also tolerant of changes in the supply voltage. I doubt a driver would want the engine to turn off because they had turned on the headlights!

Speed limitations of communications networks

There are some messages within any communications network with greater priority than others. However, as more and more signals are included, priority messages take up more and more room on the communications bus. One obvious solution is to send each message faster, but this has implications.

High Speed Communication

The waveform above shows how part of a typical High-Speed CANBus message might look when measured with an oscilloscope. For example, in the ISO11898-2 High-Speed signal, you can see the ‘Hi’ and ‘Lo’ voltages return to the 2.5v reference line to create the logically ‘High’ bit. The voltages then move in an opposed manner to create the Hi and Lo dominant voltage, which is interpreted as a logically ‘Zero’ bit.

Low Speed Communication

The Low-Speed waveform looks considerably different, but the principle is the same. The waveform doesn’t return to the 2.5v reference voltage, which means the speed at which data can be sent is less as it takes longer for a device to determine if the signal is logically High or Zero.

By having the recessive and dominant voltages relative to each other and not a fixed reference point, the absolute voltage of the Hi and Lo lines at a given moment is not critical, which allows for cheaper hardware.

The Basic Pros and Cons of Speed

At its most simple, high-speed buses allow more data to flow. More data allows more signals and ultimately more devices to connect to the network. But it comes at a cost. The faster the signals travel, the more prone to error they become. Therefore, the quality of the components used to send the messages must be higher. And as with all things, higher quality costs money.

Perhaps more critically, different messages need to be sent at different rates. For example, I can not feel an instant change in the air temperature when changing the climate control settings. That change does not need to transmit in a fraction of a millisecond. As a passenger, I probably wouldn’t notice any difference for several seconds. At the same time, I don’t want to wait several seconds for the brakes to react if I’m travelling at 70mph.

So a balance between high and low-speed communications networks is required. Multiple networks and speeds balance the system’s performance against the cost to implement them.

Hardware Fundementals

For any device, there are two parts of the connection to the CANBus. First, a CAN transceiver converts the electrical pulses to logical values (‘0’ and ‘1’). Then the CAN controller manages the conversion of received and transmitted values into messages for the micro-controller.

What do you really need to know about CANBus?

To work with the CANBus, you don’t need to know this low-level waveform arrangement, but what is essential is;

• There are different bus speeds, Low and High speed which have different arrangements and benefits
• The speed of transmission on both bus types run is too great to measure data with a standard multimeter.

I’ve turned years of CANBus experience and a page of outline about the hardware into two lines, but bear with me as we go through the following few articles on CANBus and hopefully, the essential things will become clear.

M

NEXT: Getting on the CANBus
PREV: Why use a comms bus?


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