Automotive ETHERNET Operation

WARNING: This page is about a different car, the 2022 RAM ProMaster 3500, 2022 RAM ProMaster 2500, and 2022 RAM ProMaster 1500. However, it is still accessible from the selected car via links, so may be relevant.

AUTOMOTIVE ETHERNET

The Automotive Ethernet uses a twisted pair cable on which symmetrical differential voltages are applied for the physical connection. The voltages represent symbols that a sender encodes based on the desired bitstream. 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 bidirectional on a wire pair at 100 Mbps. Two interconnected ECUs can send and receive simultaneously (full-duplex). As a sender, an ECU adds its differential voltage to the two wires, while as a receiver, it subtracts its 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.

The two ECUs must know when a new symbol begins for differential voltages to be added or subtracted so that both ECUs must be synchronized to the symbol stream. The synchronization is done with the help of a Master (Sender) ECU and Slave (Receiving) ECU. The Master (Sender) generates a continuous symbol stream to which the Slave (Receiver) is synchronized. The software of the microcontroller configures as a Master (Sender) or Slave (Receiver). The Ethernet controller first listens on the physical medium before a message is sent, preventing another ECU in the network from overwriting the current message. When the medium is free, the Ethernet controller can begin its transmission.

The transmission connection is made through TCP (Transmission Control Protocol), known as a connection-oriented transmission. The connection between the two ECUs is identified using an IP address, port number, and a three-way handshake or three-step process before the actual data transmission.

In the first step, the Master (Sender) ECU wanting to establish the connection first transmits a segment containing a set synchronization flag. The synchronization flag signals the Master (Sender) ECUs desire to connect with the Slave (Receiver) ECU. A sequence number is composed of a randomly generated number, the ISN (Initial Sequence Number), and a consecutive number. As a result, the order of all segments is known, and the data stream can be reassembled later by the Slave (Receiver) ECU. The window field gives information about the available memory of the ECU. It ensures that a Master (Sender) ECU does not send more data to the Slave (Receiver) ECU than its available memory allows. Because communication can be bidirectional, both ECUs communicate their available memory to the opposite ECU.
In the second step, the original Slave (Receiver) ECU sends a synchronization acknowledgment segment containing a set synchronization flag and a set acknowledgment flag. As a result, this ECU signals the original Master (Sender) ECU that it has received the request to establish a connection and acknowledges positively. The ECU also sends its sequence number. Once complete, both ECUs know their opposite ECU, and all future exchanged segments can be associated with the corresponding Master (Sender) ECU.
The third and last step for establishing the connection includes an acknowledgment that the original Master (Sender) ECU transmits. It uses this to signal the opposite ECU that the connection has been successfully established, and then data is exchanged between the two ECUs.