Information Bus: Description and Operation
DESCRIPTION
The primary on-board communication network between microprocessor-based electronic control modules in this vehicle is the DaimlerChrysler Programmable Communication Interface (PCI) data bus system in combination with the Controller Area Network (CAN) data bus system, which forms a hybrid bus system. A data bus network minimizes redundant wiring connections; and, at the same time, reduces wire harness complexity, sensor current loads and controller hardware by allowing each sensing device to be connected to only one module (also referred to as a node). Each node reads, then broadcasts its sensor data over the bus for use by all other nodes requiring that data. Each node ignores the messages on the bus that it cannot use.
The CAN bus is a two-wire multiplex system, while the PCI bus is a single-wire multiplex system. Multiplexing is any system that enables the transmission of multiple messages over a single channel or circuit. The CAN-C bus is used for communication between all vehicle powertrain and chassis nodes, including the Powertrain Control Module (PCM), the Controller Antilock Brake (CAB), the Steering Angle Sensor (SAS) and the Electronically Controlled Clutch (ECC) (also known as the All Wheel Drive/AWD) module. The CAN-B bus is used for communication by the Occupant Restraint Controller (ORC) and the Power LiftGate Module (PLGM), while all of the remaining body and interior nodes use the PCI bus.
In addition to the CAN and PCI bus networks, certain nodes may also be equipped with a Local Interface Network (LIN) data bus. The LIN data bus is a single wire low-speed (9.6 Kbps) serial link bus used to provide direct communication between a LIN master module and certain switch or sensor inputs. The premium Tire Pressure Monitor (TPM) trigger modules communicate directly over LIN bus circuits with the Wireless Control Module (WCM), which is a LIN master. Also, the right and left front smart window motors communicate directly over LIN bus circuits with their respective Passenger Door Module (PDM) or Driver Door Module (DDM), which are both LIN masters.
There are actually three separate CAN bus systems used in the vehicle, along with the PCI and LIN bus systems. They are designated: the CAN-B, the CAN-C and the Diagnostic CAN-C. The CAN-B and CAN-C systems provide on-board communication between all of the nodes that are connected to them. The CAN-C is the faster of the systems, providing near real-time communication (500 Kbps), but is less fault tolerant than the CAN-B system. The CAN-C is used typically for communications between more critical nodes, while the slower (83.3 Kbps), but more fault tolerant CAN-B system is used for communications between less critical nodes. The CAN-B fault tolerance comes from its ability to revert to a single wire communication mode if there is a fault in the bus wiring. Slower still, the PCI bus communicates at 10.4 Kbps.
The added speed of the CAN and PCI hybrid data bus network is many times faster than previous data bus systems. This added speed facilitates the addition of more electronic control modules or nodes and the incorporation of many new electrical and electronic features in the vehicle.
The Diagnostic CAN-C bus is also capable of 500 Kbps communication, and is sometimes informally referred to as the CAN-D system to differentiate it from the other high speed CAN-C bus. The Diagnostic CAN-C is used exclusively for the transmission of diagnostic information between the Body Control Module (BCM) and a diagnostic scan tool connected to the industry-standard 16-way Data Link Connector (DLC) located beneath the instrument panel on the driver side of the vehicle. The DLC also has a circuit connected directly to the PCI bus.
All vehicles have a Central GateWay (CGW) or hub module integral to the BCM that is connected to all three CAN buses and the PCI bus. This gateway physically and electrically isolates the CAN and PCI buses from each other and coordinates the bi-directional transfer of messages between them.
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 CAN-B bus or on the higher speed CAN-C or Diagnostic CAN-C bus, the message structure and layout is similar, which allows the Body Control Module Central GateWay (BCM or BCMCGW) 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 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 messages 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 on the PCI 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 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.
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 kilohm) termination resistance. The CAN-C bus on this vehicle has two dominant nodes: the Powertrain Control Module (PCM) and the BCMCGW. 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. On the other hand, the CAN-B bus is unique in that it has only three nodes (the BCMCGW, the Occupant Restraint Controller/ORC and the Power LiftGate Module/PLGM), and all three nodes are dominant.
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.
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, with the CAN-C bus 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 BCM, which is on between the buses, 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 BCM 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 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 nodes on the CAN-B and CAN-C 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.
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 BCM. To aid in CAN network diagnosis, the BCM 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.