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 Electronic Ignition Switch/Central GateWay (EIS or EISCGW) (also known as the EZS control unit) to process and transfer messages between the CAN buses. The EIS also stores a Diagnostic Trouble Code (DTC) or a hexadecimal equivalent of a 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 message 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 Engine Control Module (ECM) is a Local Interface Network (LIN) master module in this vehicle and it communicates directly over a LIN bus circuit with the voltage regulator internal to the generator (also known as the alternator). The Tire Pressure Monitor (TPM) module is also a LIN master module in this vehicle and communicates directly over LIN bus circuits with the individual Tire Pressure Monitor (TPM) transponders. Both the PCM and the TPM 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. The optimum termination resistance for the CAN-B bus is 100 ohms, but this value varies somewhat depending upon the number of modules on the bus. Vehicles with fewer modules have higher resistance than those with more modules. Each CAN-B module has two internal termination resistors. These resistors are connected in series between the transceiver termination pins of the module and their respective CAN (+) and CAN (-) circuits. To provide termination and bias, the transceiver connects the CAN (+) resistor to ground and the CAN (-) resistor to a 5-volt source.

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 optimum termination resistance for the CAN-C bus is 60 ohms. This vehicle uses two 120 ohm resistors connected in parallel to provide this resistance. One of the termination resistors is installed in the CAN-C bus voltage distributor, which is in the junction point where the CAN-C circuits connect located at the right cowl side inner panel. The other termination resistor is located in the ECM.

As for Diagnostic CAN-C bus termination, in order to permit CAN bus compatible scan tools to connect to many different bus configurations, mandated regulations do not permit the scan tools to contain any termination resistance. For this reason, the entire termination for the Diagnostic CAN-C resides in the EISCGW. Unlike a typical CAN-C bus where termination resistance is split between dominant CAN-C nodes, the EISCGW provides the full 60 ohms of termination resistance for the Diagnostic CAN-C 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.





In order to minimize the potential effects of Ignition-OFF Draw (IOD), the CAN-B network employs a sleep strategy. However, a network sleep strategy should not be confused with the sleep strategy of the individual nodes on that network, as they may differ. For example: The CAN-C bus network is awake only when the ignition switch is in the ON or START positions; however, the EIS, which is on the CAN-C bus, may still be awake with the ignition switch in the ACCESSORY or UNLOCK positions. The integrated circuitry of an individual node may be capable of processing certain sensor inputs and outputs without the need to utilize network resources.

The CAN-B bus network remains active until all nodes on that network are ready for sleep. This is determined by the network using tokens in a manner similar to polling. When the last node that is active on the network is ready for sleep, and it has already received a token indicating that all other nodes on the bus are ready for sleep, it broadcasts a bus sleep acknowledgment message that causes the network to sleep. Once the CAN-B bus network is asleep, any node on the bus can awaken it by transmitting a message on the network. The EISCGW will keep either the CAN-B 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 appropriate nodes 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 EIS has registers, which track each of the as-built and currently responding nodes on the CAN-B and CAN-C buses. The EISCGW stores a Diagnostic Trouble Code (DTC) or a hexadecimal value attributed to a DTC in a cache for any detected active or stored faults in the order in which they occur. This cache stores powertrain (P-Code), chassis (C-Code), body (B-Code) and network (U-Code) DTCs or hexadecimal value.

If there are intermittent or active faults in the CAN network, a diagnostic scan tool connected to the Diagnostic CAN-C bus through the 16-way Data Link Connector (DLC) may only be able to communicate with the EIS. To aid in CAN network diagnosis, the EIS will provide CAN-B and CAN-C network status information to the scan tool using certain diagnostic signals. In addition, the transceiver in each node on the CAN-C bus will identify a bus off hardware failure, while the transceiver in each node on the CAN-B bus will identify a general bus hardware failure. The transceivers for some CAN-B nodes will also identify certain failures for both CAN-B bus signal wires.