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Bus communication

Bus communication


P bus and I bus


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 buses consist of a P bus (Powertrain Bus) and an I bus (Instrument Bus). Both buses are connected to the MIU (Main Instrument Unit). The buses are electrically isolated from each other.

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

All information sent out on a bus by a control module is available to all other control modules connected to the bus. The MIU ensures that the information that is available 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.

Information interchange between control modules takes place on two leads, BUS+ (green lead) and BUS- (white lead). The two P-bus leads are twisted to reduce their sensitivity 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.

Permanent bus fault

All communication between Tech 2 and the bus-connected systems goes via DICE.

Irrespective of which system is being contacted, Tech 2 and DICE will check to see that all the bus-connected systems in the car are activated and communicating. If any bus-connected control module is missing, the diagnostic tool will detect this. This means that all the bus-connected control modules are communicating correctly if there is no warning from the diagnostic tool.

Intermittent bus fault

The Trionic control module and the transmission control module (TCM) continuously check that all control modules from which they obtain information are communicating correctly. If intermittent faults occur in bus communication, diagnostic trouble codes will be generated in the Trionic and transmission control modules. The cause of the fault may be that the control module concerned has lost power or one of the bus leads is disconnected.

In the case of an intermittent bus fault, the diagnostic trouble code text will refer to a fault diagnosis procedure described in the ”Bus and diagnostics communication” category.

Incorrect values on the bus

A diagnostic trouble code will be generated in the Trionic and transmission control modules if any information they obtain from the bus is incorrect. The trouble code text indicates the system where the fault has occurred.

For further information on the bus, see Group 3, Bus and Diagnostic Communication .

PMM uses the following information:

Information Sensor system
Memory function (memory store button + memory buttons 1, 2 and 3), driver's seat

PSM

PMM receives information that memory store is being carried out.

Memory buttons 1-3, driver's seat

PSM

The current memory function depending on which button has been pressed.

Reverse gear

DICE

Receives information if reverse gear is engaged. PMM uses the information to tilt the passenger door mirror when the tilt-down button on the switch is pressed.

Steering wheel location

TWICE

PMM receives information on whether the car is LHD or RHD.

Diagnostics

DICE

PMM communicates with the diagnostic tool via DICE



PMM outputs the following information:

Information Used by
Diagnostics

DICE

The diagnostic tool receives information from PMM via DICE.