Raider 2WD V6-3.7L (2008)
Information Bus: Description and Operation
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 connections; 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. In addition, certain vehicles may also be equipped with a Serial Controller
Interface (SCI) or a K-Line serial link bus to provide direct 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 problem 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 Module (FCM) and a diagnostic scan tool connected to the DLC.
All vehicles have a central CAN gateway or hub module integral to the FCM that is connected to all three CAN buses. This gateway physically and
electrically isolates the CAN buses from each other and coordinates the bi-directional transfer of messages between them.
Operation
The CAN 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 lower 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. There are two types of nodes on the bus. On the CAN-C bus a dominant node has a 120 ohm termination resistance while a non-dominant
(or recessive) node has about a 2500 to 3000 ohm (2.5 to 3.0 kilohm) termination resistance. The dominant nodes on the CAN-C bus are the FCM and
Powertrain Control Module (PCM).
The termination resistance of two dominant nodes is combined in parallel to provide a total of about 60 ohms. This resistance value may vary somewhat
by application, depending upon the number of non-dominant nodes on the CAN-C bus. On the CAN-D bus (or Diagnostic CAN-C) all of the 60 ohm
termination resistance is present in the Central GateWay (FCMCGM).
Note: All measurement of termination resistance is done with the vehicle battery disconnected.
Note: Termination resistance of a CAN-B node cannot be verified with a Digital Multi-Meter (DMM) or Digital Volt-Ohm Meter (DVOM). The
transceiver of each CAN-B node connects to termination resistors internally. When the vehicle battery is disconnected, the internal connections of all
CAN-B transceivers are switched open, disconnecting the termination resistors. Therefore, the total bus resistance measured under these condition will
be extremely high or infinite, which does not accurately reflect the actual termination resistance of the CAN-B bus.
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 is the faster of the two primary buses in the hybrid bus system, with the CAN-C bus providing near real-time communication (500 Kbps).
The CAN bus nodes are connected in parallel to the two-wire bus using a twisted pair, where the wires are wrapped around each other to provide
shielding from unwanted electromagnetic induction, thus preventing interference with the relatively low voltage signals being carried through them. The
twisted pairs have between 33 and 50 twists per meter (yard). While the CAN bus is operating (active), one of the bus wires will carry a higher voltage
and is referred to as the CAN High or CAN bus (+) wire, while the other bus wire will carry a lower voltage and is referred to as the CAN Low or CAN
