P-Bus and I-Bus

A 'bus' is a group of leads through which information is sent in digital and serial forms. Digital means that the voltage difference between the leads has only two values, approximately 0 and 5 V. The information is coded so that different combinations of impulses with values 0 and 5 have different meanings.

Serial means that the information is sent in "packets" transmitted one after the other in rapid succession.

In the Saab 9-3, half of the car's electronic control modules are connected to the bus-system.

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 diagnostics tool is not connected directly to the bus but communicates via the DICE, one of the control modules connected to the I-bus, and therefore has access to all electronic control modules connected to the bus.

The P-bus communicates ten times faster than the I-bus. The reason for this is that the power train systems require information with the least possible delay.

All the information sent from one electronic control module is accessible for all other control modules on the bus. The MIU is responsible for ensuring that information available on one bus is also available on the other bus.

The electronic control modules send out information on the bus at regular intervals. The time between two transmissions depends on the information being sent and varies between 10 milliseconds (0.010 seconds) and 1 second. Information is also sent out by the control modules whenever the information changes.

The transfer of information between electronic control modules is achieved on two leads, bus+ (green lead) and bus- (white lead). The leads are twisted to increase resistance to electrical interference.