Communication data buses (Description): Description and operation

Description

General

A number of different types of data bus can be incorporated into vehicle wiring harnesses for the transmission of commands and information between ECU's. Some of the buses are installed as a network connecting several ECU's together and some are installed as dedicated links between pairs of ECU's. The bus configuration installed on a particular vehicle depends on the model and equipment level.

Freelander incorporates the following data bus systems: l Controller Area Network (CAN) bus. A network connected between the instrument pack, the ABS and powertrain related ECU's l Diagnostic DS2 bus l ISO 9141 K line

Serial interface circuits in the ECU's transmit and receive commands and information on the buses as digital messages. ECU's connected to the same bus use a common protocol (format) and baud rate (transmission speed) for the messages they transmit. The protocol and baud rate vary from bus to bus.

Not all system ECU's are connected to the CAN bus system but they may require information that is being transmitted on the CAN bus system. The instrument pack acts as a gateway translating CAN bus signals into a format compatible with non CAN ECU's. Some signals are also converted from their existing state into the CAN format and transmitted on the CAN bus system for use by CAN based ECU's, for example the air conditioning pressure sensor reading is converted by the instrument pack and sent via CAN to the respective engine management system.

A twisted pair of wires are used for the CAN bus and single wires are used for the diagnostic buses. Bus wires can be repaired using the recommended connectors and the recommended procedure. The unwound length of CAN bus wires must not exceed 40 mm (1.6 in). If a fault is suspected on the CAN bus system it must be diagnosed and rectified using TestBook/T4.

Data Bus Types

CAN Bus

The CAN bus is a high speed broadcast network where the ECU's automatically transmit information on the bus every few microseconds. The two wires of the bus are identified as CAN Low (L) and CAN High (H), and are twisted together to minimise the electromagnetic interference (noise) produced by the CAN messages. To prevent message errors from electrical reflections, 120 ohm resistors are incorporated into the CAN wire terminals of the instrument pack and the Engine Control Module (ECM).

Messages consist of a signal which is simultaneously transmitted, in opposite phase, on both wires. CAN L switches between 2.5 and 1.5 volts while CAN H switches between 2.5 and 3.5 volts, which causes the potential difference between the two lines to switch between 0 volt (logic 1) and 2 volts (logic 0) to produce the digital signal message.

CAN Bus Switching

3.5v

3.5v

2.5v

1.5v

A

B

A = CAN H; B = CAN L