P-bus and I-bus, message transmission and reception
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.
A control module sends a message by sending ones and zeros in a predefined order. The control module only sends a message if no other control module is presently sending. When one control module sends a message, all the other control modules listen, interpreting the combination of ones and zeros. The meaning of the combinations of ones and zeros is predefined so that all the control modules can interpret the information in the same way. The message contains information about what is being sent and the value, for example "Coolant temperature 100°C" or "A/C ON".