Electronic Control Modules (Service Information): Communication: Description

The primary communication network between Electronic Control Units (ECUs) is the Controller Area Network (CAN) data bus system. The Controller Area Network (CAN) data bus allows all ECUs connected to the bus to share information with each other. Regardless of whether a message originates from an ECU on the higher speed CAN-Chassis (CAN-C) (500K) bus or on the lower speed bus, CAN-B (50K) bus, the message structure and layout is similar, which allows the Body Control Module (BCM) to process and transfer messages between the CAN buses. The BCM stores Diagnostic Trouble Codes (DTCs) for certain bus network faults and is the CAN-C Network Manager. The BCM and the Powertrain Control Module (PCM) are the dominating modules containing the terminating resistors on the CAN C.

The CAN-C bus ECUs are connected in series-parallel (daisy chained) having an internal connection that completes the CAN circuit. The CAN is still a series-parallel circuit, however, each module must remain connected in order to complete the circuit. The CAN-B bus ECUs are connected in parallel (stub). Both use a 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. 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.

All ECUs transmit and receive messages over one of these buses. Data exchange between the ECUs is achieved by serial transmission of encoded data messages (a form of transmission in which data bits are sent sequentially, one at a time, over a single line). Each ECU can both send and receive serial data simultaneously. Each data 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 ECU uses arbitration to sort the message priority if two competing messages are attempting to be broadcast at the same time. Each message contains a Cyclic Redundancy Check (CRC) which specifies the message size exactly. If the message detected conflicts with the CRC, the ECU receiving it will determine the message to be an error and consider that communication has not been possible.

Communication problems that affect the whole bus, as a result of opens and terminal push outs are more likely to occur on data busses that operate at a high speed than a data bus that operates at a lower speed. When an open circuit or terminal push out occurs one or more ECUs can become isolated from the remainder of the bus. The isolated ECU will attempt to communicate, but will not be able to receive messages or determine arbitration from other ECUs. Each time the isolated ECU attempts to communicate it alters the bus voltage on the intact bus circuit. Without functioning arbitration the isolated ECU alters the bus voltage while other bus messages are being sent.