Electronic Control Modules (Service Information): Communication: Operation

The primary communication network between Electronic Control Units (ECUs) on this vehicle is the Controller Area Network (CAN) data bus system. 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 data bus allows all ECUs connected to the bus to share information with each other. Regardless of whether a message originates from an ECU on the higher speed CAN-Chassis (CAN-C) (500K) bus or on the lower speed CAN-Interior High Speed (CAN-IHS) (125K) bus the message structure and layout is similar, which allows the Body Control Module (BCM) to be a central gateway to process and transfer messages between the CAN-C and CAN-IHS buses. The BCM also stores Diagnostic Trouble Codes (DTCs) for certain bus network faults. These data communication network is known as the Powernet  electronics architecture.

These networks and their subnetworks are protected by a Security Gateway Module (SGM).

All ECUs transmit and receive messages over one of these buses. Data exchange between the ECUs is achieved by serial transmission of encoded data messages (a form of transmission in which data bits are sent sequentially, one at a time, over a single line). Each ECU can both send and receive serial data simultaneously. Each data bit of a CAN bus message is carried over the bus as a voltage differential between the two bus circuits which, when strung together, form a message. Each ECU uses arbitration to sort the message priority if two competing messages are attempting to be broadcast at the same time. Corruption of a single bit within a message will corrupt the entire message. Each message contains a Cyclic Redundancy Check (CRC) which specifies the message size exactly. If the message detected conflicts with the CRC the ECU receiving it will determine the message to be an error and consider that communication has not been possible. Diagnosis of this condition using a lab scope may reveal activity that appears to be bus data messages even if no actual communication is possible. Communication problems that affect the whole bus, as a result of opens and terminal push outs are more likely to occur on data busses that operate at a high speed than a data bus that operates at a lower speed.

When an open circuit or terminal push out occurs one or more ECUs can become isolated from the remainder of the bus. The isolated ECU will attempt to communicate, but will not be able to receive messages or determine arbitration from other ECUs. Each time the isolated ECU attempts to communicate it alters the bus voltage on the intact bus circuit. Without functioning arbitration the isolated ECU alters the bus voltage while other bus messages are being sent thereby corrupting the messages on the remainder of the bus.

The vehicle communication systems may be diagnosed with the Mopar Scope. Refer to COMMUNICATION, NON-DTC BASED DIAGNOSTICS .

CAN BUS FAULTS 

TYPES OF CAN BUS FAULTS
LOSS OF COMMUNICATION	will set by an active receiving/reporting ECU on a CAN Bus network that detects no communication from another ECU on the same CAN Bus network. Insufficient power, ground, bus voltage, or inaccurate vehicle configuration will cause a loss of communication.
IMPLAUSIBLE MESSAGE	will set by an active receiving/reporting ECU, when it determines the data sent from the active transmitting/offending ECU is missing part of the message, or the message is an irrational value over the CAN Bus.
MISSING MESSAGE	will set by an active receiving/reporting ECU, when it determines a data message to be missing partial information when sent from the active transmitting/offending ECU over the CAN Bus network.
BUS OFF	set by an ECU that has experienced approximately 32 transmit errors, this can be caused by ECU internal faults as well as external bus faults like shorts or plugging and unplugging test tools to the diagnostic connector.
PHYSICAL	is only detectable by an ECU that has a transceiver that is able to detect shorts on the bus. If the ECU does not, it generally will set bus off faults due to shorted bus lines.

The CAN bus ECUs 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 bus (+) wire, while the other bus wire will carry a lower voltage and is referred to as the CAN bus (-) wire. Refer to the CAN Bus Voltages table below.

CAN Bus Operational Voltages
CAN-C Bus Circuits	Sleep	Recessive (Bus Idle)	Dominant (Bus Active)	CAN (-) Short to Ground	CAN (+) Short to Ground	CAN (-) Short to Battery	CAN (+) Short to Battery	CAN (+) Short to CAN (-)
CAN (-)	0 V	2.4 - 2.5 V	1.3 - 2.3 V	0 V	0.3 - 0.5V	Battery Voltage	Battery Voltage Less 0.75 V	2.45 V
CAN (+)	0 V	2.4 - 2.5 V	2.6 - 3.5 V	0.02 V	0 V	Battery Voltage Less 0.75 V	Battery Voltage	2.45 V
CAN-IHS Bus Circuits	Key-Off (Bus Asleep)		Key-On (Bus Active)	CAN (-) Short to Ground	CAN (+) Short to Ground	CAN (-) Short to Battery	CAN (+) Short to Battery	CAN (+) Short to CAN (-)
CAN (-)	0.0V		1.3 - 2.3 V	0 V	0.3 - 0.5 V	Battery Voltage	Battery Voltage Less 0.75 V	2.45 V
CAN (+)	0.0 V		2.6 - 3.5 V	0.02 V	0 V	Battery Voltage Less 0.75 V	Battery Voltage	2.45 V
Notes  All measurements taken between ECU ground and CAN terminal with a standard DVOM. DVOM will display average network voltage. Total resistance of CAN networks can be measured with the battery disconnected. The average resistance is approximately 60 Ohms. The termination resistors are integral to the Star Connectors.

The CAN-IHS bus network remains active until all ECUs on that network are ready for sleep. This is determined by the network using tokens in a manner similar to polling. When the last ECU that is active on the network is ready for sleep, and it has already received a token indicating that all other ECUs on the bus are ready for sleep, it broadcasts a bus sleep acknowledgment  message that causes the network to sleep. Once the CAN-IHS bus network is asleep, any ECU on the bus can awaken it by transmitting a message on the network. The BCM will keep either the CAN-IHS or the CAN-C bus awake for a timed interval after it receives a diagnostic message for that bus over the Diagnostic CAN-C bus.

In the CAN system, available options are configured into the BCM at the assembly plant, but additional options can be added in the field using the diagnostic scan tool. The configuration settings are stored in non-volatile memory. The BCM also has two 64-bit registers, which track each of the as-built  and currently responding  ECUs on the CAN-C and CAN-IHS buses. The BCM stores a Diagnostic Trouble Code (DTC) in one of two caches for any detected active or stored faults in the order in which they occur. One cache stores powertrain (P-Code), chassis (C-Code) and body (B-Code) DTCs, while the second cache is dedicated to storing network (U-Code) DTCs.