Controller area networks (CAN)
9.2.2 Controller area networks (CAN)
Bosch has developed the protocol known as CAN or controller area network. This system is claimed to meet practically all requirements with a very small chip surface (easy to manufacture and, there- fore, cheaper). CAN is suitable for transmitting data in the area of drive line components, chassis components and mobile communications. It is a compact system, which will make it practical for use in many areas. Two variations on the physical layer are available which suit different transmis- sion rates. One for data transmission of between 100 K and 1 M baud (bits per second), to be used for rapid control devices. The other will transmit between 10 K and 100 K baud as a low speed bus for simple switching and control operations.
The CAN message signal consists of a sequence of binary digits (bits). Voltage (or light fibre optics) present indicates the value ‘1’; none
Figure 9.4 CAN systems on a vehicle
202 Advanced automotive fault diagnosis transmission of its own message. This is very
For the recessive state the nominal voltage for important in the case of motor vehicle data trans-
the two wires is the same to decrease the power mission.
drawn from the nodes.
All messages are sent to all units and each unit The voltage level on the CAN bus is recessive makes the decision whether the message should
when the bus is idle.
be acted upon or not. This means that further sys- Benefits of in-vehicle networking can be sum- tems can be added to the bus at any time and can
marised as follows.
make use of data on the bus without affecting any of the other systems.
A smaller number of wires is required for each function. This reduces the size and cost
9.2.3 Summary
of the wiring harness as well as its weight. Reliability, serviceability, and installation issues
CAN is a shared broadcast bus which runs at
are improved.
speeds up to 1 Mbit/s. It is based around sending ● General sensor data, such as vehicle speed, messages (or frames) which are of variable length,
engine temperature and air temperature can be between 0 and 8 bytes. Each frame has an identi-
shared. This eliminates the need for redundant fier , which must be unique (i.e. two nodes on the
sensors.
same bus must not send frames with the same ● Functions can be added through software identifier). The interface between the CAN bus
changes unlike existing systems, which require and a CPU is usually called the CAN controller.
an additional module or input/output pins for The Bosch CAN specification does not pre-
each function added.
scribe physical layer specifications. This resulted ● New features can be enabled by networking, in two major physical layer designs. Both commu-
for example, each driver’s preference for ride nicate using a differential voltage on a pair of
firmness, seat position, steering assist effort, wires and are often referred to as a high-speed and
mirror position and radio station presets can
be stored in a memory profile. tecture can change to a single-wire operating
a low-speed physical layer. The low-speed archi-
method (referenced to earth/ground) when one of the two wires is faulty because of a short or open circuit. Because of the nature of the circuitry
9.2.4 CAN diagnostics
required to perform this function, this architecture The integrity of the signal on the controller area is very expensive to implement at bus speeds
network can be checked in two ways. The first above 125 kbit/s. This is why 125 kbit/s is the divi-
way is to examine the signal on a dual channel sion between high-speed and low-speed CAN.
scope connected to the CAN-High and CAN- The two wires operate in differential mode, in
Low lines (Figure 9.5).
other words they carry inverted voltages (to reduce In this display, it is possible to verify that: interference). The levels depend on which standard is being used. The voltage on the two wires, known
● data is being continuously exchanged along as CAN-High and CAN-Low are as follows.
the CAN bus; ● the voltage levels are correct;
Table A ISO 11898 (CAN High Speed) standard
a signal is present on both CAN lines.
Signal Recessive state
Dominant state
CAN uses a differential signal so the signal on
Min Nominal Max Min
Nominal Max
one line should be a coincident mirror image of the data on the other line. The usual reasons
CAN-High 2.0 V 2.5 V
for examining the CAN signals is where a CAN
CAN-Low 2.0 V 2.5 V
fault has been indicated by OBD, or to check the CAN connection to a suspected faulty CAN
Table B ISO 11519 (CAN Low Speed) standard
node. Manufacturer’s data should be referred to for precise waveform parameters.
Signal Recessive state
Dominant state
The CAN data shown in Figure 9.6 is captured
Min Nominal Max Min
Nominal Max
on a much faster timebase and allows the individual state changes to be examined. This enables the mir-
CAN-High 1.6 V 1.75 V
ror image nature of the signals, and the coincidence of the edges to be verified.
CAN-Low 3.1 V 3.25 V
3.4 V 0V 1.0 V
1.15 V
Electrical systems 203
Figure 9.5 CAN signals
Figure 9.6 CAN signals on a fast timebase
The signals are equal and opposite and they are is safety critical, so do not use insulation pierc- of the same amplitude (voltage). The edges are
ing probes!
clean and coincident with each other. This shows The second way of checking the CAN signals that the vehicle data bus (CANbus) is enabling
is to use a suitable reader or scanner. The AutoTap communication between the nodes and the CAN
scanner discussed in Chapter 3 will do this with controller unit. This test effectively verifies the
suitable software.
integrity of the bus at this point in the network. If
a particular node is not responding correctly, the fault is likely to be the node itself. The rest of the
9.3 Lighting
bus should work correctly. It is usually recommended to check the condi-