Automotive ETHERNET Operation

AUTOMOTIVE ETHERNET 

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.