I-bus, principles of communication

You can think of the bus as a telephone line running between the control modules. The I-bus system does not have a +15 power supply lead, with the exception of for the airbag control module (ACM). With the ignition switched on, communication goes on continuously. The CIM determines the key position from information from the ignition switch module (ISM).

When the key is inserted into the ignition, the CIM wakes the other control modules on the I-bus by briefly sending B+ over the bus. The CIM then sends regular reports over the bus of the key position.

Fig 1: Identifying I-Bus Communication Principles (1 Of 3)

Courtesy of SAAB-SCANIA OF AMERICA, INC.

Communication can still be started even if the key has been removed from the ignition switch. For example, if you press the open button on the remote control, the CIM, which receives the signal, wakes the other control modules so that the body control module (BCM) can turn on the interior lighting and the driver door module (DDM), for example, can unlock the door. The control modules go back to sleep when no messages concerning them are sent on the bus.

Fig 2: Identifying I-Bus Communication Principles (2 Of 3)

Courtesy of SAAB-SCANIA OF AMERICA, INC.

The bus lead is a green cable that is internally grounded in each control module via a 9 kO or a 4 kO resistor. MIU, BCM, DDM, PDM, CIM, RRDM, RLDM and REC are the control modules grounded internally via a 4 kO resistor. To send a zero, the control module raises the bus lead voltage towards 5 V. A potential difference to ground lower than 2.2 V is interpreted as a one, and higher than 2.2 V as a zero. Note that a long series of ones is the same as no message being sent.

Fig 3: Identifying I-Bus Communication Principles (3 Of 3)

Courtesy of SAAB-SCANIA OF AMERICA, INC.