Bus Systems

Bus Systems

Bus system analysis
The electronic control units in the vehicle are connected to one another via a network. In this system network, the central gateway module plays a decisive role. The central gateway module is responsible for ensuring that information is transferred from one bus system to another bus system.
The engine control system and chassis control system are linked via the PT-CAN (or PT-CAN2) and the FlexRay bus system to the ZGM. The control units of the general vehicle electrical system are connected via the K-CAN and the K-CAN2.
For most control units in the area of information and communication technology, the MOST is available as an information carrier. The vehicle diagnosis communicates across the D-CAN. The vehicle is programmed / encoded via the Ethernet access. The overall network consists of various bus systems that ensure communication between the individual control units.

Overview of the bus systems
In principle, two groups of bus systems are distinguished:
- Main bus systems: Ethernet, FlexRay, K-CAN, K-CAN2, MOST, PT-CAN and PT-CAN2
- Sub bus systems: BSD, D-CAN (diagnosis CAN), LIN, Local CAN

Body CAN, K-CAN
The K-CAN is responsible for communication of the components with low data transfer rates. The K-CAN is also linked to the other bus systems across the central gateway module. A number of control units in the K-CAN have a LIN bus as sub bus. The K-CAN has a data transfer rate of 100kBit/s and consists of two twisted wires. The K -CAN has the possibility to be operated as a single-wire bus in the event of errors.

Body CAN2, K-CAN2
The K-CAN2 is responsible for communication of the control units with high data transfer rates. The K-CAN2 is also linked to the other bus systems across the central gateway module. A LIN bus is connected as sub bus on all control units in the K-CAN2. The K-CAN2 has a data transfer rate of 500kBit/s and consists of two twisted wires.

Powertrain CAN, PT-CAN
The PT-CAN connects the engine control system with the transmission control unit, but also interconnects systems in the area of safety and driver assistance systems. It is line-based with tap lines to the individual systems. The PT-CAN has a data transfer rate of 500kBit/s and consists of two twisted wires.

Powertrain CAN2, PT-CAN2
The PT-CAN2 forms a redundancy for the PT-CAN in the area of the engine control system and also transfers signals to the fuel pump control. The PT-CAN2 has a data transfer rate of 500 kBit/s and consists of two twisted wires with an additional wake-up line.

Ethernet
Ethernet is a manufacturer-neutral, cable-bound network technology. The protocols TCP/IP (Transmission Control Protocol/ Internet Protocol) and UDP (User Datagram Protocol) are used as transfer protocols.

FlexRay
With a maximum data transfer rate of 10 MBit/s per channel, FlexRay is significantly faster than the data buses employed so far in the areas of body and powertrain/suspension in motor vehicles. The central gateway module sets up the link between the various bus systems and the FlexRay. Depending on the equipment fitted in the vehicle, the ZGM has one or two so-called star couplers, each with four bus drivers. The bus drivers forward the data of the control units across the communication controller to the central gateway module (ZGM). The deterministic data interchange ensures that each message is transferred in the time-controlled section in real time.
Real time means that the transmission takes place in a specified time.

MOST bus system
MOST is a data bus technology for multimedia applications. The MOST bus uses light impulses for data interchange and has a ring structure. Data transfer on the ring bus takes place in one direction only. Only the central gateway module can implement data exchange between the MOST bus and other bus systems. The Car Information Computer functions as master control unit; the gateway to the remaining bus system is the central gateway module.

Possible faults in bus systems
The following fault causes can lead to bus faults:
- Short circuit of a bus line
- Interruption of a bus line
- Fault in a gateway
- Fault in the transmitter or receiver of a control unit