Toyota
Ford Kuga
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Module Communications Network » Communications Network (System Operation and Component Description)
System Operation

General

In a communications network (data bus system), various modules of different systems are connected to one another via one or several lines.

The data bus system is used exclusively for transmitting data between the connected modules, as well as between the connected modules and the Ford diagnostic unit.

In a data bus system, complete data blocks are transmitted instead of single on/off pulses. In addition to the actual information, these data blocks also contain data regarding the address of the module to be addressed, the size of the data block and information for monitoring the content of each individual data block.

Data bus systems offer various advantages:

  • Simplified data transmission between the modules due to a standardized protocol
  • Fewer sensors and connectors
  • Improved diagnostic options
  • Lower costs

The DLC is connected to the various data bus systems and to the power supply via the standard 16-pin GEM. The signal for the module programming is also transferred via the DLC.

In a data bus system, if there is a break in one or both lines or there is a short to ground or to voltage, then communication between the modules and with the Ford diagnostic unit is disturbed or is no longer possible at all.

In order to be able to establish communication with one another, the modules of the individual systems must use the same language. This language is called a protocol.

At present, Ford uses three different data bus systems. Depending upon model and equipment level, all three data bus systems are used. Each of these data bus systems has its own protocol.

Data bus systems

ISO 9141 bus

The International Organisation for Standardisation ISO 9141 bus. This consists of a single wire and is used exclusively for communication between the modules and the Ford diagnostic unit. The fault memories of the various modules are read out via the ISO 9141 bus.

Local Interconnect Network (LIN) bus

The LIN bus is a standard specifically designed for cost-effective communication between intelligent sensors and actuators in vehicles. The LIN control unit subnet is used in every situation where the bandwidth and versatility of CAN is not needed.

The LIN specification comprises the LIN protocol, a standard format for describing a complete LIN and the interface between an LIN and the application. An LIN comprises a LIN master and one or more LIN slaves. The LIN utilizes the master/slave principle for the purpose of bus access control. This has the significant advantage that few resources (CPU performance, ROM, RAM) are required for bus management in the slave module. The master is implemented in a control module or a gateway which has the necessary resources. All communication is initiated by the master. Consequently, a message always consists of a header, which is generated by the master, and a response from the slave. The data transfer rate is in the region of up to 20 Kbit/s. The LIN master knows the time sequence of all data which are to be transmitted. These data are transmitted by the corresponding LIN slaves (e.g. ultrasonic sensors) if they are requested to do so by the LIN master.

LIN is a single-wire bus, i.e. the data are transmitted in the cable via one wire. Usually the same cable is also used to provide the supply voltage. The ground connection of the supply voltage also acts as the ground connection of the data transmission. An LIN does not use a terminating resistor.

Controller Area Network (CAN) bus

This consists of two twisted wires and operates serially (data is transmitted sequentially). It is used for communication between the modules themselves and between the modules and the Ford diagnostic unit. The modules are connected to the data bus in parallel. New modules can be incorporated easily, without modifying the other wiring or modules. The transmitted data is received by every module connected to the control unit network (CAN). As each data packet has an identifier (label), in which the priority of the message is determined as well as the content identification, each module can detect whether or not the data is relevant for its own information processing. This enables several modules to be addressed with a particular data packet and supplied with data simultaneously. For this purpose, it is ensured that important data (for example from the ABS) is transmitted first. The other modules are only able to submit their data to the data bus after the high-priority messages have been received.

The advantages of the CAN bus are:

  • Minimization of wiring requirements
  • High degree of error protection (fault / fail-proof)
  • Robustness
  • Good extendibility
  • Prioritization of messages
  • Inexpensive
  • Automatic repetition of faulty messages
  • Independent system monitoring and the ability to disconnect faulty modules from the data bus automatically

Due to the increased number of modules and the resulting continued increase in data transfer, two different CAN bus systems are used. Essentially, they only differ in terms of their data transmission rates and application areas.

To be able to distinguish between the individual CAN bus systems, the CAN bus system with the high transfer speed is called the high speed CAN bus (HS-CAN). The data are transmitted at a baud rate of 500 kB/s.

The CAN bus system with the medium transfer speed is called the medium speed CAN bus (MS-CAN) and is mainly used for communication in the comfort electronics or the multimedia system.

The data are transmitted at a baud rate of 125 kB/s.

An interface (gateway) is used to exchange data between the HS-CAN and the MS-CAN. This provides the connection between the three CAN databus systems and is installed in the GEM and in the electronic instrument cluster. The number of modules which are connected to the three databus systems depends on the equipment level of the vehicle.

In order to guarantee a high degree of error protection, two 120 Ohm terminating resistors are installed in the CAN. These are integrated in the first module connected to the CAN and in the last module connected to the CAN respectively and are used for suppression as well as the elimination of voltage peaks. In order to ensure correct functioning of the data bus system, the modules must always be connected with an integral terminating resistor. In order to enable communication between the modules on the HS-CAN and the modules on the MS-CAN, one module is connected to both data bus systems.

The connection of both data bus systems is designated as a gateway. In this gateway, the received data is converted to the transmission rate required for the relevant data bus and is transmitted. This ensures an optimal distribution of information between both data bus systems.

Component Description

ABS

The GEM is integrated in the CJB (central junction box) and cannot be replaced as a separate unit.

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