Detailed description: O-bus: Notes
The LEDs in the transmitters, though not laser diodes, produce high-intensity light. The light is therefore classed as laser class 1. Do not look directly at the light source (detached fibre of connection). Visual inspection to establish that light is emitted from a fibre is used during fault diagnosis. In such cases, do not point the fibre towards the eyes but shine the light onto a piece of paper or other light object and look at the reflection.
The optical bus consists of an orange 2.3 mm thick plastic cable that looks similar to a standard electrical cable. The optical cable contains a 1.0 mm diameter, transparent, plastic, optical fibre and two protective plastic layers. Each end of the cable has a plastic or brass cable terminal. The purpose of the terminal is to secure and center the cable on the control module. Each control module is connected to two cables; one for incoming data and one for outgoing data. The bus is a ring bus, which means that messages are sent around the ring by the various control modules.
The messages are digital and consist of red light pulses. Each control module converts received messages from optical signals into electrical signals and then back into optical signals for transmission to the next control module.
The control module that is the source of a specific type of information modifies the information as necessary each time it passes. The other control modules are free to receive and use any relevant messages.
The O-bus has a data rate of 25 Mbit/s. This large capacity means that audio data can be sent in real-time between a playback device or radio receiver and the amplifiers. The P-bus and I-bus are both connected to the ICM. The ICM relays information between the electrical buses and the O-bus.
None of the modules on the O-bus have a +15 lead. All the modules wake up when they receive a light pulse and remain awake so long as a module is communicating. Pressing a button on the ICM is the most reliable way to wake the bus immediately.
In the most basic model of the car, there are only optical cables behind the dashboard between the ICM and EHU. In other car variants, the optical cables are routed to a CD player or changer in the dashboard, an amplifier under the driver's seat and to the luggage compartment, which can contain other units.
The cable terminals of the cables can be unplugged from the control module connector. It is very important that the two cable in the connector are not confused. Pin 1 receives information and pin 2 sends it. Sending and receiving diodes are located inside the control module connector. Additional control modules can be connected. Each control module must be connected to a specific place in the ring as compensation has been made for the short delays in communication.
Cables cannot be joined but are supplied in ready lengths. The plastic cable terminals are laser welded to the cable and the brass terminals are pressed on. The cable must not be bent to a radius of less than 25 mm or be subjected to temperatures greater than 85° C. The ends of the cable must not be dirty, but exceptional cleanliness is not necessary. The cable is resistant to small amounts of oil, acid, brake fluid, alcohol and glycol. If the cable is subjected to an impact, the transparent plastic fibre will whiten reducing the light intensity and could cause a break in communication. The cable must not be pulled tight over sharp edges. Although this will not cause permanent damage it will reduce the light intensity. Insufficient light intensity can be experienced as a rattling from the audio system, similar to atmospheric interference.