P-bus and I-bus: Notes

WARNING: This page is about a different car, the 2003 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.

The two electrical buses are connected to the Column Integration Module (CIM), Infotainment Control Module (ICM) and data link connector. The buses are electrically isolated. The CIM and ICM are gateways, transferring information between the buses. The CIM relays information between the electrical buses. The ICM makes sure that information is transferred between the O-bus and the electrical buses.

The P-bus has a data transfer rate of 500 kbits/s and the I-bus of 33 kbits/s. The data rate on the P-bus is high to allow the powertrain systems access to information with the shortest possible delay, for example during air mass compensation when the selector lever is moved from N to D or TCS or ESP modulation.

The control modules send out information on the bus whenever the information changes. As a safety precaution, the information is also sent at regular intervals irrespective of whether or not it is new. The time between two transmissions depends on the information being transmitted and varies between 10 milliseconds (engine torque) and 10 second (VIN). When a control module sends information, the other control modules listen regardless of whether or not they use the information.

A bus system means that it is sufficient with one sensor for each measurement function in the car. The number of separate signal leads can also be reduced. For example, the engine management system has a coolant temperature sensor, which means that coolant temperature is always available to all systems. Therefore, the car only needs one coolant temperature sensor. Coolant temperature is used by the main instrument unit to control the temperature gauge, and the ACC to control ventilation fan speed and air distribution when starting from cold.