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Chapter 5 - CAN BUS SYSTEM

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by

Zaimi Zainol

on 7 January 2015

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Transcript of Chapter 5 - CAN BUS SYSTEM

1.0 CAN BASICS
Before automobiles began using CAN Bus system, vehicle electrical operation was completed using a
hardwired
system that typically used:

• 1- Power source
• 2- Conductors
• 3- Loads (lights, radio and wiper motors)
• 4- Circuit protection
• 5- Switching mechanisms

Each of these elements adds additional
weight
to the vehicle.

What is CAN?
CAN is the acronym for
CONTROLLER AREA NETWORK
which often referred to as CAN Bus
CAN
is a messaging system used in vehicles that reduces
wiring
, which also reduces
weight
and
cost
.

CAN facilitates the
sharing of information or commands
and allows for vehicle to have additional features and functions without requiring additional wiring.

Conventional Hard Wiring
2.0 MULTIPLEXING
The result of conventional hardwiring is lots of wires and connectors.

When a signal is sent using a hardwired system, the destination is typically to
one module only
.

This is
NOT the most efficient
way to transmit and receive signals.

Car with one control unit
A solution is accomplished by
linking all of the control modules
together using
Multiplex
technology.

A multiplex wiring system uses a
serial data bus
that connects different computers or control modules together.

Each module can transmit and receive digital codes over the serial data bus, allowing one module to
share information
with other modules.

Car with three control units
Car with three control units and bus system
For example, the signal relating to engine speed may be required by the engine control, transmission control, electronic brake control, and suspension control modules.

Rather than have
separate
engine speed inputs for each module, the serial data bus carries the information, as well as
other information
, to
all
of the control modules.

That control module sends the information, in
binary code
, to the serial data bus.


Drive train CAN Network with three control units
The chip is also prevent the signals from
overlapping
by allowing only one signal to be transmitted at a time.

Signals are via
protocols
,
highest priority goes first
.

If two modules attempt to send a message at the same time, the signal with the
higher priority
code is transmitted
first
.

When one input is being received, the others are disregarded.
The stream of data across the bus is called
serial data
.

It is essentially data that are transferred to and from a computer, one bit at a time.

On many vehicles, serial data can be monitored with a scan tool connected to the vehicle’s DLC (
Data Link Connector
).


A multiplexed system uses a serial data bus to allow communication between various control modules
Each control module has a
chip
that
reads
and
sends
messages on the serial data bus.

The chip of each device
compares
the coded message to its memory list to see whether the information is
relevant
to its own operation.

F A C T
All information on the serial data bus is available for all control modules.
F A C T
Monitoring serial data allows technicians to diagnose the various control modules and to check for DTCs (
Diagnostic Trouble Codes
).
CAN communication enables
efficient
and
reliable

communication
between sensors, actuators, controllers, and other modules in real-time applications.

In conclusion, we can say that a vehicle’s network is just a module talking to other modules to perform a vehicle function effectively.
3.0 ADVANTAGES
The need for
redundant sensors is eliminated
because sensor data, such as vehicle speed and engine temperature, are available on the serial data bus where they can be used by several control modules.

Accessories and vehicle
features can be easily
added to the vehicle through software changes.


Fewer wires
are required for the operation of each system, which means smaller wiring harnesses, lower cost and weight, and improved serviceability, reliability, and installation.

Without multiplexing, it is necessary to add a ground, a power source, and control wires whenever an electronic component is added to the vehicle.

Improved communications between control modules allows for more
accurate recording
and
reporting of faults
, which helps in locating and solving problems.
4.0 CAN-BUS WIRING
CAN wiring between two control units
Several control units with CAN wiring
Twisted wire in CAN-BUS wiring
CAN wiring is often
a pair of wires
that run between at least 2 control units/ modules.

Inside control units are small circuit chips that manage incoming and outgoing communication.

The wires are often called
CAN-H
and
CAN-L
on wiring diagrams since they have slightly
higher
and
lower
voltages from each other.

These wires are
twisted together
to reduce magnetic interference, which can cause false information.


F A C T
Never run the repair wire in such a way that it
bypasses the twisted sections
.

CAN bus wires are likely to be influenced by
noise
if you bypass the twisted wires.
The CAN-L (Low Signal) wire is low voltage and can average around 1.7V.

Whereas the CAN-H (High Signal) wire averages around 3.2V.

Even the
slightest change in voltage
can affect the operation of one or more systems; therefore, all potential for voltage spikes, electrical noise, or induction must be
eliminated
.

Twisting the wires eliminates the possibility of voltage being induced in one wire as current flows through the other.
To eliminate other
potential spikes and noise
, two 120 Ω resistors are connected in parallel across the ends of the main CAN bus wires.

These are called
terminating resistors
.

The location of the resistors varies.

One may be located in the fuse block, and others can be internal to the ECM or PCM and the BCM.
F A C T
If one wire fails the other wire can transfer all CAN communication signals.
Example of twisted wire in CAN-BUS wiring
Example of twisted wire in CAN-BUS wiring
To eliminate potential voltage spikes and noise, two terminating resistors are used
If the bus wire needs
repair
due to an open or short, it
must not
be relocated or untwisted.

The twisting serves an
extremely important purpose
.

After a bus wire has been repaired by soldering, wrap that part of the wire with vinyl tape.

5.0 DIGITAL MESSAGES
There must be a voltage difference between the wires
Binary codes represented by voltage
Information exchange of a message on a CAN-BUS
Exchange information is referred to as
messages
.

Any control unit can send or receive messages.

A message contains
physical values
such as the
engine speed
(rpm).

The engine speed In this case, is represented as a
binary value
.

For example: The engine speed of 1800 rpm is represented as 00010101 in binary notation.
F A C T
Station = Module = Node
Before sending, the binary value is converted into a
serial bit stream
.

The bit stream is sent over the
TX line
(transmit line) to the transceiver.

The transceiver
converts
the bit stream into
voltage values
which are then sent over the bus line one by one.

In the reception process, voltage values are converted back into a bit stream by the transceiver and sent over the
RX line
(receive line) to the control units.

The control units then convert the serial binary values back into messages.

For example: The value 00010101 is converted back to the engine speed 1800 rpm.


However, the control units/modules and signals (messages) are
ranked
by
protocol layers
according to
safety
or the
systems
that they control.

For example, suppose the
ABS system
receives a message from the wheel sensors to say that the wheels have
locked
.

The ABS control module sends a
message
, through the CAN Bus, to the Engine Control Module (ECM) to
reduce throttle angle
(acceleration).

At this point, the ABS control module has
priority
to send messages.
F A C T
High Speed CAN messages are digital signals created by
offsetting
the voltage
between
two wires.

This
binary code
of 0’s and 1’s make up the digital message.

Broadcasting principle, one sends, everyone receives
Another example; consider the vehicle does has an impact event or
accident
.

At this point the airbag
(SRS) control module
sends out a crash signal.

Now, the SRS unit has
priority
, as it is safety critical.
A message sent can be received by
any control unit
. This principle is also called a broadcast message.

The broadcasting process ensures that all control units connected to the bus have the
same
information status.

The message transfer on the CAN bus is based on
four different formats
.

Data frame
: A transmitted message
contains data
that is provided by the transmitting station (data source).

Remote frame
: Station can
request the data
they need from the data source. The data source responds by sending the relevant data frame.

Error frame
: If a station
detects a fault
, it communicates with other stations using error frame.

Overload frame
: This can be used to
create a delay
between a preceding and subsequent data frame or remote frame. The transmitting station reports that it
cannot currently process
another frame.


How can controller
handle the error
if it detects a fault?

If a station detects a fault or format error, it
interrupts
the current transmission by sending an
error frame
.

If the message sender detects that its message has been interrupted by an error frame, it
stops transmitting
and make
another attempt a
t a later time.

This action
prevents
other stations from accepting the
erroneous
message and thereby ensures the consistency of data across the entire system.
6.0 PROTOCOLS
The
differences
in protocol are based on the
speed
and the technique used.

The SAE has classified the different protocols by their speed and operation.

The total network in most vehicles is comprised of
two
or
three
CAN buses.
Each of these networks operates at
different speeds
.

The different CAN buses are identified by a prefix or suffix.

For example, a low speed bus is called
CAN B
or MS-CAN.

Likewise, a high-speed bus is called
CAN C
or HS-CAN.

Manufacturers are not consistent with these labels, so there are a variety of them.
These networks are
integrated
through the use of a
gateway
.

A
gateway module
allows for data
exchange between
the different buses.

It
translates
a message on one bus and
transfers
that message to another bus
without changing
the message.

In many vehicles; the BCM (
Body Control Module
) serves as the gateway for the different buses.

F A C T
A
protocol
is a set of rules or
standard
used between computers or electronic control modules.

Low speed CAN
are typically used for
body functions
, such as:

Interior and exterior lights
Horn
Locks
Windshield wipers
Seats
Window
Sound systems
High speed CAN
is used for
real-time functions
such as:

Engine management
Antilock brake systems
Transmission control
Tire pressure monitoring systems
Vehicle stability systems

CHAPTER 5
DATA BUS SYSTEM
Full transcript