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Communication - Operation






OPERATION

The hybrid bus system integrates the Programmable Communications Interface (PCI) data bus with the Controller Area Network (CAN) data bus and allows all electronic modules or nodes connected to either bus to share information with each other. Regardless of whether a message originates from a module on the lower speed PCI or on the higher speed CAN-C bus, the message structure and layout is similar, which allows the Central GateWay (CGW) module to process and transfer messages between the buses.

All modules (also referred to as nodes) transmit and receive messages over one of these 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. PCI 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. On the other hand, 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 in either bus 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 bus messages requires biasing and termination. Each module in the PCI 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 used in the PCI bus network. 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 CGW is the only dominant node in the PCI bus 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.

There are also 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 node has about a 2500 to 3000 ohm (2.5 to 3.0 kilo ohms) termination resistance. The CAN-C bus on this vehicle has two dominant nodes: the Powertrain Control Module (PCM) and the CGW. The termination resistance is combined in parallel to provide a total of about 60 ohms. This resistance value may also vary somewhat by application, depending upon the number of nodes on the bus.

NOTE: All measurement of termination resistance is done with the vehicle battery disconnected.

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 data bus is a non-proprietary, open standard adopted from the Bosch CAN Specification 2.0b. The CAN is the faster of the two primary buses in the hybrid 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-C 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.






The CAN-C bus network is awake only when the ignition switch is in the ON or START positions; however, the CGW, 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 CGW will keep the CAN-C bus awake for a timed interval after it receives a diagnostic message for that bus over the PCI bus.

In the CAN system, available options are configured into the CGW 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 CGW also has two 64-bit registers, which track each of the as-built and currently responding nodes on the PCI and CAN-C buses. The CGW 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.

If there are intermittent or active faults in the CAN network, a diagnostic scan tool connected to the PCI bus through the 16-way Data Link Connector (DLC) may only be able to communicate with the CGW. To aid in CAN network diagnosis, the CGW will provide 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.