Electronic Control Modules (Service Information): Communication: Operation: Notes

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 (sometimes referred to 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.