COMMUNICATION
DESCRIPTION
The primary on-board communication network between microprocessor-based electronic control modules in this
vehicle is the Controller Area Network (CAN) data bus system. A data bus network minimizes redundant wiring con-
nections; and, at the same time, reduces wire harness complexity, sensor current loads and controller hardware by
allowing each sensing device to be connected to only one module (also referred to as a node). Each node reads,
then broadcasts its sensor data over the bus for use by all other nodes requiring that data. Each node ignores the
messages on the bus that it cannot use.
The CAN bus is a two-wire multiplex system. Multiplexing is any system that enables the transmission of multiple
messages over a single channel or circuit. The CAN bus is used for communication between all vehicle nodes.
However, in addition to the CAN bus network, certain nodes may also be equipped with a dedicated Serial Con-
troller Interface (SCI) or a K-Line serial link bus to provide direct communication between that node and certain
sensor inputs.
There are actually three separate CAN bus systems used in the vehicle. They are designated: the CAN-B, the
CAN-C and the Diagnostic CAN-C. The CAN-B and CAN-C systems provide on-board communication between all
nodes in the vehicle. The CAN-C is the faster of the two systems providing near real-time communication (500
Kbps), but is less fault tolerant than the CAN-B system. The CAN-C is used exclusively for communications
between critical powertrain and chassis nodes. The slower (83.3 Kbps), but more fault tolerant CAN-B system is
used for communications between body and interior nodes. The CAN-B fault tolerance comes from its ability to
revert to a single wire communication mode if there is a fault in the bus wiring.
The added speed of the CAN data bus is many times faster than previous data bus systems. This added speed
facilitates the addition of more electronic control modules or nodes and the incorporation of many new electrical and
electronic features in the vehicle. The Diagnostic CAN-C bus is also capable of 500 Kbps communication, and is
sometimes informally referred to as the CAN-D system to differentiate it from the other high speed CAN-C bus. The
Diagnostic CAN-C is used exclusively for the transmission of diagnostic information between the Front Control Mod-
ule/Central GateWay (FCM or FCMCGW) and a diagnostic scan tool connected to the industry-standard 16-way
Data Link Connector (DLC) located beneath the instrument panel on the driver side of the vehicle.
The FCM is located on the Integrated Power Module (IPM), which is located in the engine compartment near the
battery. The central CAN gateway or hub module integral to the FCM is connected to all three CAN buses. This
gateway physically and electrically isolates the CAN buses from each other and coordinates the bi-directional trans-
fer of messages between them.
OPERATION
The Controller Area Network (CAN) data bus allows all electronic modules or nodes connected to the bus to share
information with each other. Regardless of whether a message originates from a module on the low speed CAN-B
bus or on the high speed CAN-C or CAN-D bus, the message structure and layout is similar, which allows the Front
Control Module/Central GateWay (FCM or FCMCGW) to process and transfer messages between the buses. The
FCM also stores a Diagnostic Trouble Code (DTC) for certain bus network faults.
All modules (also referred to as nodes) transmit and receive messages over one of these buses. Data exchange
between nodes is achieved by serial transmission of encoded data messages. Each node can both send and
receive serial data simultaneously. Bus messages are carried over the data bus in the form of Variable Pulse Width
Modulated (VPWM) signals which, when the high and low voltage pulses are strung together, form a message. Each
node uses arbitration to sort the message priority if two competing messages are attempting to be broadcast at the
same time.
The voltage network used to transmit messages requires biasing and termination. Each module on the bus network
provides its own biasing and termination. Each node terminates the bus through a terminating resistor and a ter-
minating capacitor. There are two types of nodes on the bus. The dominant node terminates the bus through a 1
KW resistor and a 3300 pF capacitor, typically resulting in about a 3300 ohm termination resistance. However, this
resistance value may vary somewhat by application. The FCM (or FCMCGW) is the only dominant node in this
network. A non-dominant (or recessive) node terminates the bus through an 11 KW resistor and a 330 pF capacitor,
typically resulting in about a 10800 ohm termination resistance.
The communication protocol being used for the CAN data bus is a non-proprietary, open standard adopted from the
Bosch CAN Specification 2.0b. The CAN-C is the faster of the two primary buses in the CAN bus system, providing
near real-time communication (500 Kbps).
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ELECTRONIC CONTROL MODULES - SERVICE INFORMATION
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