Principle of Communication

Principle Of Communication:

The bus may be regarded as a two-wire telephone line running between the control modules. With the ignition switched on, communication goes on continuously. With the ignition switched oft, a control module that is operative may if necessary awaken the other control modules. This happens for example when a door is opened and the TWICE, to which the door switches are connected, awakens the others so that the DICE will be able to switch on the interior lighting.

When one of the control module microprocessors speaks, the other control modules listen to it regardless of whether the information is of interest to them or not. The MIU transfers the information from one bus to another. All information sent out on a bus by one control module is then available to all other control modules connected to a bus.

With the bus system, signal leads can be removed and the number of sensors reduced.







On oars with Trionic T5 or Motronic, the MIU has its own coolant temperature sensor.

Cars with the Trionic T7 have only one engine coolant temperature sensor. Since the engine control module is connected to a bus, the temperature of the coolant is continuously available to all systems. That is why only one engine coolant temperature sensor is needed.

On diesel cars the EDC 15 has an engine coolant temperature sensor. Since the engine control module is connected to a bus, the temperature of the coolant is continuously available to all systems. That is why only one engine coolant temperature sensor is needed on cars with a diesel engine.

The coolant temperature is used by the DICE for controlling the radiator fans (see illustration), by the MIU for controlling the temperature gauge, and by the ACC for controlling ventilation fan speed and air distribution when starting from cold. Basically, it could be said that the control modules connected to a bus are like one big control module with a number of sensors and a number of actuators. Bus communication may be regarded as internal control module communication.

This means that an overall view of the car is essential when carrying out fault diagnosis. A malfunction need not necessarily be caused by a fault in the system where it occurs but may very well be caused by a defective sensor in another system.

Sending

Sending:

A control module sends a message by transmitting ones and zeros in a definite order. The control module sends the message only if no other control module is transmitting. What the various combinations of ones and zeros mean is determined in advance so that all control modules interpret the information identically. The message contains a description of what is sent and its value, e.g. "Coolant temperature 100 °C" or "A/C On".

The two bus leads are supplied with about 2.5 V from each control module. This is accomplished by means of three resistors connected in series between 5 V and ground. The middle resistor ensures that the current on bus+ is slightly lower than that on bus- when no messages are being sent.

When both leads are supplied with about 2.5 V for a certain period of time (2 millionths of a second on the P bus), a one will be transmitted. To transmit a zero, the control module pulls bus+ almost to 5 V and bus-almost to ground for an equal period of time. Note that a long sequence of ones is the same as no message being transmitted.

Receiving







When a control module is transmitting, all the other control modules listen to it. They do this by looking at the difference in voltage between bus+ and bus-. (A voltage difference close to 0 V is interpreted as a one and a difference close to 5 V as a zero.) What the various combinations of ones and zeros mean is determined in advance so that all control modules interpret the information identically. The message contains a description of what is sent and its value, e.g. "Coolant temperature 100 °C" or "A/C On".

Electrical interference, such as from a relay, will affect the voltage on both leads in the same direction. The difference in voltage will not be affected, however, and this makes the bus extremely insensitive to interference.

Messages
Below is a summary of the messages which are sent out by all the car's systems that are connected to a bus. It also shows which systems use the transmitted information. The purpose of the summary is to help you gain an understanding of the car's functional relationships, which is a fundamental prerequisite for rapid fault diagnosis.