Theory Of Operation Can-ETHERNET

1. CHARACTERISTICS OF AUTOMOTIVE ETHERNET 
   1. The Automotive Ethernet network is being used for communications between critical time sensitive and video related ECUs. Only two ECUs are ever connected to a Automotive Ethernet network. The Automotive Ethernet network use a twisted pair of insulated copper wires for the data bus. One wire is designated as Ethernet (+), the other is Ethernet (-). Connectors house the network circuit terminals in adjacent cavities and connect them to a module. The connectors also connect certain branches of wiring in-line along the wiring harness. A gateway module is used to link A.E. to other communication networks. A.E. messaging from multiple ECUs can be mirrored/reflected using an ethernet switch, that are internal to the Security GateWay (SGW) Module and Radio at the Diagnostic Port/Data Link Connector (DLC). The internal ethernet switch joins multiple physical connections and can independently forward messages from one A.E. network to another A.E. network.
2. THEORY AND OPERATION OF AUTOMOTIVE ETHERNET 
   1. The Automotive Ethernet voltages represent symbols that an ECU sender encodes based on a desired bit stream and an ECU receiver uses the resulting symbol stream to decode. There are several methods used for encoding and decoding the different voltages generated. Information can be transferred bidirectionally on Automotive Ethernet at 100Mbit/s. Two interconnected ECUs can send and receive simultaneously (full duplex). As a sender, an ECU adds its own differential voltage to the two wires; while as a receiver, it subtracts its own voltage from the applied total voltage. The result of the subtraction corresponds to the voltage that was sent by the opposite ECU. This mechanism is a component of the echo cancellation method that is used in other Ethernet systems. For differential voltages to be added or subtracted, the two ECUs must know when a new symbol begins. This means that both ECUs must be synchronized to the symbol stream. This is done with the help of a Master ECU and Slave ECU. The Master ECU generates a continuous symbol stream to which the Slave ECU is synchronized.

      The Automotive Ethernet uses a twisted pair cable on which symmetrical differential voltages are applied is used for the physical connection. The voltages represent symbols that a sender encodes based on the desired bit stream. A receiver uses the resulting symbol stream in turn to decode the contained bits. Only two ECUs are ever connected to one cable. Thus, only point-to-point connection topology is available. Information is transferred bidirectionally on a wire pair at 100 Mbps. Two interconnected ECUs can send and receive simultaneously (full duplex). As a sender, a ECU adds its own differential voltage to the two wires; while as a receiver, it subtracts its own voltage from the applied total voltage. The result of the subtraction corresponds to the voltage that was sent by the opposite ECU. This mechanism is a component of the echo cancellation method that is used in other Ethernet systems.

      In order for differential voltages to be added or subtracted, the two ECUs must know when a new symbol begins. This means that both ECUs must be synchronized to the symbol stream. This is done with the help of a Sending ECU and Receiving ECU. The Sender generates a continuous symbol stream to which the Receiver is synchronized. The basic software of the microcontroller configures as a Sender or Receiver.

      When an open circuit or terminal push out occurs one or more modules can become isolated from the remainder of the bus. The isolated module will attempt to communicate, but will not be able to receive messages or determine arbitration from other modules. Each time the isolated module attempts to communicate it alters the bus voltage on the intact bus circuit. Without functioning arbitration the isolated module alters the bus voltage while other bus messages are being sent thereby corrupting the messages on the remainder of the bus.

3. TOLERANCE 
   1. Automotive Ethernet has limited fault tolerance. The majority of faults result in a complete loss of communication with the Automotive Ethernet network. One of the first steps in proper diagnosis is identifying whether a Automotive Ethernet fault is present. The symptoms of a Automotive Ethernet communication network fault are not unique. Lost time and improper repairs may result if the concern is not properly verified and identified. Whenever a Automotive Ethernet communication network fault is suspected, the first step is to connect the scan tool and check for network communication and U-codes.
4. FAULT SYMPTOMS AND DIAGNOSIS 
   1. When communication hard faults occur on the Automotive Ethernet bus circuits, such as a short to ground or short to power, typically the vehicle will experience a no crank/no start with the MIL illuminated. U-codes may be found in any of the modules on the bus. Check power, ground, the connector, and both bus circuits to the suspect module before replacing the module. If communication on an entire network or networks is not possible, ensure the scan tool is functioning properly and operating with the latest software level.

      There are several faults that can interrupt normal operation of the Automotive Ethernet communication network. The network wire may chafe, resulting in one or both wires shorting to chassis ground, shorting to power, or shorting to each other. Damage to a connector can result in the same faults. If either CAN (+) or CAN (-) is shorted to ground, voltage, or together at any place in the circuit, communication is not possible. When an open occurs in the circuit, any module downstream of the open will not communicate. It is possible for a module to fail, resulting in loss of communication with that particular module. If internally shorted, a module fault may bring the whole network down, resulting in no communication on the entire Automotive Ethernet network. If this type of concern occurs, typically the network will begin to communicate normally if the faulty module is disconnected. Remember to verify power and ground, as well as connector faults, before replacing any module.

Always perform the PRE-DIAGNOSTIC TROUBLESHOOTING PROCEDURE before proceeding.  Refer to PRE-DIAGNOSTIC TROUBLESHOOTING PROCEDURE .