Found this to be interesting:


Mercedes-Benz Optical-Fiber Data Bus

The next time you feel married to a car, you may actually get a "ring" on that Mercedes-Benz. On the newer Mercedes models you'll find a Domestic Digital Bus (D2B), which is an optical data bus system connecting the audio and telephone components by optical fibers in a ring structure. Since the light pulses are transmitted in one direction only, the optical information must pass through all previous components before reaching the final component in the ring.

The components must be "awakened" from a powered-down sleep mode and sequentially brought on-line. The system sequencing begins with an ignition-on, or radio-on, electrical wake-up pulse from the radio ("head unit"). This electrical pulse is send to all the components at once in a star configuration. At the same time, the head unit issues a series of "ring start" optical commands.

This sequencing continues as follows:

(1) All the components "wake up."
(2) The CD changer receives a "ring start" command from the head unit.
(3) The CD changer passes the ring start command and the message "changer is here" to the telephone.
(4) The telephone passes the ring start command and the message "changer is here" with its own "telephone is here" message to the voice activation module.
(5) The Voice Activation Module passes the ring start command and the messages "changer is here" and "telephone is here" along with its own "Voice Activation Module is here" message back to the head unit.
(6) When the head unit recognizes the valid return of the "ring start" command and the "I'm here" component messages, a synchronous data ring lock is established. A continuous red glow observed in any of the optical connector cavities is the easiest way to identify that a ring lock has been established.
When handling and replacing the fiber optic cable, the minimum bend radius is 25mm to insure the concentrated light reflection within the cable does not break down the internal fluorinated polymer reflective cladding. Externally, the cables are an orange polymid 12 material with a clear terminal on one end and a red terminal on the other. The red end of the cable must always be located on the chamfered side of the connectors and the clear end must be located on the squared side of the connectors.

Fiber optic cable is lightweight, impervious to electro-magnetic interference, and is able to handle data speeds of 20MB per second, so it is an excellent choice for inter-communication and control of today's automotive audio and telephone systems.

Written by Mike Muellerleile, Euro Specialist. Mike is a certified ASE Master and L1 with 27 years of diagnostic repair experience.

 

benzomonterey, very good info on the MB fiber_optic ring. Now it makes everyone understand how the data flowing inside the fiber-optic ring.

 

I thought that the FBI broke up the Fiber Optic Ring years ago!

 

have you guys noticed how all the electrical "things" seem to be possibly centralized? I've come to the conclusion that the most of the switches for the various items don't operate those components directly, but go through some sort of central computer. The reason I say this is it seems there's a slight delay when activating stuff, including the horn (very minimal delay), headlights, even the backup lights (when I shift my 6-spd into reverse, there's a delay -- don't know about the auto trans). Anyway, just a thought.

 

Mark, you're right, but its a different network called CAN, and its distributed rather than centralised. Ive posted some stuff in the General forum about it yesterday - I couldnt post this response here yesterday - normal hangup issue !