COMMUNICATION

OPERATION

The hybrid bus network allows all electronic modules connected to the bus to share information with each other. Regardless of whether a message originates from a module on the low speed Programmable Communications Interface (PCI) bus or on the high speed Controller Area Network (CAN-C) bus, the message structure and layout is similar, which allows the Body Control Module Central GateWay (also known as the BCMCGW) to process and transfer messages between the buses. The BCMCGW 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 two buses, either the single-wire PCI bus or the two-wire CAN-C bus. 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 terminating 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 BCM (or BCMCGW) 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.

PROGRAMMABLE COMMUNICATIONS INTERFACE DATA BUS

The PCI (or J1850) data bus communication protocol exceeds the Society of Automotive Engineers (SAE) J1850 Standard for Class B Multiplexing. The PCI data bus speed is an average 10.4 Kilobits per second (Kbps).

CONTROLLER AREA NETWORK DATA BUS

The communication protocol being used for the CAN-C 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 hybrid bus system providing near real-time communication (500 Kbps).

The CAN-C 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. 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-C bus voltage table.

The CAN-C bus network is awake only when the ignition switch is in the ON or START positions. However, an individual node on the CAN bus may still be awake with the ignition switch in the ACCESSORY or UNLOCK positions. This is because the integrated circuitry of an individual node may be capable of processing certain sensor inputs and outputs without the need to utilize network resources.