P-bus, principles of communication

The P-bus system has a +15 lead coming directly from the ISM. With the ignition switched on, communication goes on continuously.

Communication stops as soon as the ignition is switched off. The P-bus does not have a wake-up function similar to that of the I-bus.

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

Courtesy of SAAB-SCANIA OF AMERICA, INC.

The two bus leads (a green wire for bus + and a white wire for bus -) are fed with roughly 2.5V by each control module. The voltage is created using three resistors connected in series between 5V and ground. The middle resistor means that the voltage on bus + is slightly higher than that on bus - when no message is being sent.

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

Courtesy of SAAB-SCANIA OF AMERICA, INC.

A one is sent when the voltage on both leads is roughly 2.5 V for a certain period of time (2 millionths of a second). A zero is sent when the control module raise the voltage on bus + towards 5 V and lowers that of bus - to ground for the same period of time. A potential difference lower than 1.5 V is interpreted as a one, and higher than 1.5 V as a zero. Note that a long series of zeros is the same as no message being sent.

The P-bus, which has a high data rate, has two wires to reduce its sensitivity to interference. A possible source of interference, such as a relay, will have the same affect on the voltage of both wires. The difference in voltage between the wires will therefore remain the same, and the message will not be affected. The wires are also twisted to further reduce their sensitivity.

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

Courtesy of SAAB-SCANIA OF AMERICA, INC.