Commander 4WD V8-4.7L Flex Fuel (2009)
Information Bus: Description and Operation
Communication - Operation
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 lower speed CAN-B bus or on the higher 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 CAN 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. Each digital bit of a CAN bus messages is
carried over the bus as a voltage differential between the two bus circuits which, when 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 ElectroMechanical Instrument Cluster (EMIC) (also known as the Cab Compartment Node/CCN) is the Local Interface Network (LIN) master
module in this vehicle and it gathers information from the compass module, the instrument panel switch bank, the Steering Control Module (SCM), and
the Heated Seat Module (HSM) through the LIN data bus. There is also LIN bus communication between the individual Tire Pressure Monitor (TPM)
transponders and the Wireless Ignition Node (WIN). Both the EMIC and the WIN either act directly upon the information received through the LIN data
bus, relay the information to other nodes in the vehicle using electronic messages placed on the CAN bus, or both.
The voltage network used to transmit messages requires biasing and termination. Each module on the CAN bus network provides its own biasing and
termination. There are two types of nodes used in the CAN bus network. 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 WIN and the 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 (FCMCGW).
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 node transceivers are switched open, disconnecting the termination resistors. Therefore, the total
bus resistance measured under these conditions 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-C is the faster of the two primary buses in the CAN bus system, 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
bus (-) wire. Refer to the CAN Bus Voltages table.
