Many vehicles are already fitted with a large number of electronic control and regulation systems.
The complexity of these systems means there is an increasing need for them to exchange data
between each other.However, the increase in data means that it’s no longer sensible to trans-
fer data in the conventional way. In other words, if data were exchanged via individual lines,
the sheer complexity of the additional sensors,wiring and connections would make this system
scarcely manageable, not to mention the potential error sources that this would introduce.
Another advantage is that fewer sensors have to be installed, as their analogue signals are digit-
ised in the dedicated control unit and made available to other control units via a ‘data bus’.
Protocol
Before a data bus system is developed a communication protocol is defined.The protocol agrees
the following:
1. Medium used to transmit information – copper wire, twisted pair of wires or fibre optic
cable.
2. Speed of transmission – different systems on the vehicle require information quickly while
other systems do not. Audio system (slow speed rate) compared to anti-locking braking
(high speed rate). Also referred to as class of system – Class A low speed up to 10Kbps,
Class B medium speed up to 125Kbps and Class C high speed up to 1Mbps.
3. Method of addressing information (lengths of signals sent).
4. Signals – digital, analogue, voltage, current or frequency manipulated.
5. Error detection and treatment.
The protocol allows all control modules to communicate in the same language.The ‘protocol’
is provided in the form of software in each control unit.
Data bus
A data bus is the circuit and interface used to allow the information to be available.The mod-
ules are in technical terms referred to as ‘nodes’ in the data bus system and are configured to
form a network. The nodes have integrated circuits in them and software to allow them to
communicate using the protocol via the bus system.The system becomes more complex when
different systems, speed rates and protocols are used on a vehicle. The data bus can also be
used to transmit information or actual commands from one control unit to another, for example
in order to operate its actuators (e.g. servomotor, relay).The DLC (Diagnostic Link Connector)
is also connected to the data bus and the vehicle’s systems can be checked with modern diag-
nostic systems (see section 3.10 for Tech 2 and WDS).
Network
In a bus system, several nodes (control units) are connected with each other via either copper
cables or in some systems via fibre optic cables.This is called a network.The nodes communi-
cate with each other through this network and exchange information and data in digital form.
Digital transmission
If you take a specific length of time and divide it into small time windows, it is possible to
switch on (1) or off (0) a defined voltage (e.g. 5 volts) on an electric cable for the duration of
such a time window, i.e. you can create two switching states (Fig. 3.121). A time window as
mentioned above is called a bit (binary digit).As there are only two possible statements for a
bit, 8 bits have been grouped together to form a block,which is known as a byte.A byte has 2#
(2 to the power of 8) 256 possible statements (2X2X2X2X2X2X2X2X256). So,
with 2 switching states and 8 bits (1 byte) there are 256 possible statements. If more are required,
several bytes can be grouped together into one dataframe.
FIGURE 3.121
3.122
Networking principles
To enable data to be exchanged between the control units, the control units must be connected
to each other electrically.Each control unit can send and receive data on one cable (Fig. 3.122);
this is known as a serial exchange, i.e. the individual bits (binary digits) are sent one after the
other, like marbles rolling through a tube, on the data bus cable and are read in this way too.
The typical transmission speed is around 50 kbit per second.To countercheck the information
sent by a control unit and for the purposes of self-monitoring, a second cable is required; the
data transmitted on the first line is also transmitted on this line at the same time, but is inverted.
If the signal on one of the lines fails, this is detected by the self-diagnosis facility and stored in
the error memory of the control unit as a communications error. If two control units are trans-
mitting at the same time, the significance of the data blocks is stipulated based on the bit com-
bination so that priority can be given to the more important information at the processing stage.
Types of bus system
Types of bus
Various bus systems are used in modern vehicles:
● ISO bus
● SCP bus
figure 3.123
● ACP bus
● CAN bus
Control units that communicate via CAN, SCP and ISO buses can be checked via the DLC
using serial diagnostic equipment; however, the ACP bus cannot be checked in this way.
ISO bus
The ISO (International Organisation for Standardisation) data bus consists of a single commu-
nications line (K cable).This K cable is not used for communication between the control units
but exclusively for diagnosing an individual control unit.On new vehicles, the ISO bus is being
increasingly replaced by the CAN bus. However, the K cable is still present in most current
control units and is used to import and export parameters during vehicle production. If there is
a fault on the cable caused by a power interruption, or a short circuit or short circuit to ground,
the module and diagnostics tester will not be able to communicate with each other any more.
SCP bus
The SCP (Standard Corporate Protocol) databus consists of two twisted wires. It is used exclu-
sively for communication between the PCM and the diagnostics tester. Even if only one of the
two cables is faulty, the PCM and the diagnostics tester will not be able to communicate with
each other any more. All information and data are placed in a frame containing bus control
information and are transmitted as a packet (datablock) using the serial transmission method.
The complete packet consists of data in precisely defined sequences of bits.All nodes, i.e.
control unit connecting points, have equal priority.Therefore, several control units can partici-
pate in performing a function. Both functional and physical addressing are possible. If there
are several simultaneous messages, these are processed in turn according to their significance.
At least one valid response must be returned for each message sent. If this does not happen,
an error is stored in the error memory.
ACP bus
ACP (Audio Control Protocol) is similar to the SCP bus, has a simpler protocol and is exclu-
sively for audio applications and telephone systems.
CAN bus
The CAN (Controller Area Network) bus is also a serial data bus, but has a different protocol to
the SCP bus and is therefore also faster. It operates at a transmission rate of up to 1 megabit/s
(1 million) and comes close to real-time transmission.CAN was originally developed by Robert
Bosch AG as a cost-effective network solution for use in vehicles.The CAN bus is an international
standard and is described in ISO 11898 (high speed CAN) and ISO 11519 (low speed CAN).
The features of the CAN bus CAN is a so-called multi-master bus system which means that all
bus users (control and test equipment) can send and request data. In the CAN system, there is
no addressing of the individual users in the traditional sense. Instead, ‘identifiers’ are assigned to
the data packets to be sent. Each user can send its data on the data bus, the other users decide,
using the identifiers, whether they read out and process this data further or not. The identifier
also carries out prioritisation, i.e. it stipulates the priorities of the data packets, if several bus users
want to send data simultaneously.One of the outstanding properties of the CAN bus is its high
transmission reliability. Each user’s CAN controller records transmission errors. These are
recorded and evaluated statistically so that appropriate action can be taken.This can even result
in the bus user which has produced the error being switched off from the bus.A data packet in
its dataframe can consist of up to 8 bytes.More comprehensive transmission data is sent distrib-
uted across several dataframes. The maximum transmission rate is 1Mbit/s, i.e. up to 1 million
pulses per second. Because of the line resistance, this speed applies to networks with a databus
length of up to 40 metres. For longer distances, the transmission rate has to be reduced: to
125 kbit/s for distances up to 500 metres and to 50 kbit/s for distances up to 1000 metres.
CAN bus variants In vehicle engineering applications, three different CAN buses are used at
present:
● A high-speed CAN bus (HS CAN).
● A mid-speed CAN bus (MS CAN).
● A low-speed CAN bus (LS CAN).
figure 3.124
The bus connections consist of twisted cable pairs.On the HS CAN bus, the transmission rate
is 500 kbit/s.On the MS CAN bus, the transmission rate is 125 kbit/s.On the LS CAN bus, the
transmission rate is 50 kbit/s.
Portal (gateway) On vehicles that operate with different network speeds (e.g. HS CAN and
MS CAN), the information also has to be exchanged between the different bus systems and
transmission speeds.As the two bus networks cannot be connected directly with each other, an
interface is required via which the two networks can communicate with each other.This inter-
face is also called a portal or a gateway.The gateway matches the different transmission speeds
to each other and thereby enables the control units of the different networks to communicate.
Figure 3.124 shows the instrument cluster (HEC) module is the gateway between the MS
CAN and the HS CAN.
1. 120 ohm matching resistor.
2. DLC.
3. Instrument cluster (HEC) as portal (gateway) between HS CAN and MS CAN.
4. Private bus between the ABS, yaw rate – lateral acceleration sensor (YAW) and steering
angle position sensor (SAS).
The EATC (air-conditioning module with automatic temperature control) is on the MS CAN sys-
tem. Not all sensors and actuators are ‘busable’. This means they still operate in a conventional
sense by manipulating an analogue voltage, current or through PWM, saturated voltage etc.
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