9-3 (9440) L4-2.0L Turbo (2.0 HOT) (2003)
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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.
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 5 V and ground. The middle resistor means that the voltage on bus + is slightly higher than that on bus - when
no message is being sent.
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
