Peripheral Bus (P-Bus)
The P-Bus is a single wire serial communications bus that is used exclusively on vehicle that are equipped with ZKE III. These vehicles are the E38, E39 and E53.
The P-Bus provides the Central Body Electronics system with a low speed bus for use by the General Module (GM) to control various functions. These functions are carried out by various peripheral modules. The peripheral modules are located in areas of the vehicle close to sensors or actuators where wiring the components separately would create an excessively large wiring harness. In some cases (e.g. Sunroof module) these peripheral modules are integrated into an actuator or switch to create one unit.
The P-Bus is only used within the ZKE system and is very similar in communication protocol and speed to the I/K-Bus. The P-Bus is not designed for a rapid exchange of continuous information. Instead, the messages on the P-Bus are short control commands. This limited message flow allows for fast reaction time by the Peripheral module. (e.g. a door lock request).
In comparison with previous electronic systems, bus networks provide a simple method to operate various body electronic systems. Using the example of a power window circuit, the previous methods to operate a window were inefficient. The power window circuit carried a large amount of current which required larger gauge wires and heavy duty switches. Window switches were subject to wear from arcing contacts and the wiring size did not allow much flexibility when passing through bulkheads and door jamb areas.
A bus network needs less high current circuits and uses a smaller amount of heavy gauge wire. The switches are only used to signal the modules and they do not carry high current. The switches are used to provide a low current ground input signal which increases the life of the switch and improves reliability considerably.
The drivers side window switch is a control unit on the P-Bus. If the driver needs to open the front passenger side window, a signal is sent from the driver's side switch block module to the passenger side door module. The passenger side door module contains the load circuits for switching the window motor. The passenger side door module will respond to the "window open" telegram from the drivers door switchblock by actuating the switching circuit for the window motor.
(Early E36)
In addition to simplifying the power window circuit, the bus network also allows functions that were not possible with a conventional power window circuit. For example, the remote operation of the power windows from the key transceiver (convenience open feature).
The convenience open feature on the E38 operates by a radio frequency signal from the key transceiver. The "open request" signal is received by the FBZV module. The FBZV module sends a digital signal to the General Module (GM III). The GM then sends an "open windows and sunroof" telegram over the P-Bus and all 4 windows and sunroof will open. This type of feature is much too complex for a conventional window circuit. The bus network allows new features like this to be possible.