Bus Communication

Bus Communication








In the Saab 9-5, all the car's control modules are connected to buses with the exception of the ABS and SRS control modules.

The bus is divided into the power train (P bus) and the instrument bus (I bus). Both busses are connected to the main instrument display panel (MIU). The busses are electronically separated.

The P-bus has a data transfer rate ten times higher than that of the I-bus.

All the information that one control module sends is available to all the other control modules connected to the bus. The MIU is responsible for ensuring that all the information on one bus is also available on the other.

The control modules send out information on the bus at regular intervals. The time between two transmissions depends on the information being transmitted and varies between 10 milliseconds (0.010 seconds) and one second. Information is also sent out by the control modules whenever the information changes.

The transfer of information between the control modules is achieved on two leads BUS+ (green lead) and BUS- (white lead). Both leads of the P-bus are twisted to increase the tolerance to electrical interference.

Diagnostics
In a bus system, all the units must be able to communicate with each other. For example, the engine cannot be started if Trionic is unable to receive immobilizer information from TWICE.

Intermittent bus faults
Trionic and TCM check continuously to see that all the control modules they receive information from are communicating correctly. In the event of an intermittent fault occurring in the bus communication, diagnostic trouble codes will be set in Trionic and TCM. The cause of the fault can be that the power supply or one of the bus leads to the control module in question is absent.

Permanent bus faults
All communication between the diagnostics tool and the bus-connected systems goes via DICE. Irrespective of which system is being contacted, the diagnostics tool and DICE will check to see that all the bus-connected systems in the car are activated and communicating. If any of the bus-connected control modules are missing, the diagnostics tool will detect this. This means that all the bus-connected control modules are communicating correctly if there is no warning from the diagnostics tool.

Incorrect values on the bus
TRIONIC and TCM generate a diagnostic trouble code (DTC) when the information they fetch from the bus has an erroneous value. The text in the diagnostic trouble code refers to the system in which the error exists.

ACC uses the following information:














ACC sends the following information: