“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 next time you feel married to a car, you should at least hold out for a ring! On the newer Mercedes-Benz, Models 202 and 210, 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 sent 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. CD changer receives a "ring start" command from the head unit.
3. CD changer passes the "ring start" command and the message "changer is here" to the telephone.
4. 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. Voice activation module passes the "ring start" command and the message's "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 ensure the concentrated light reflection within the cable does not break down the internal fluorinated polymer reflective cladding. Externally, the cables are an orange-colored polymid 12 material with a clear terminal on one end and a red terminal on the other end. 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 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.

Mike Muellerleile is an IDENTIFIX European specialist. He is an ASE master and L1 certified technician with 27 years of diagnostic repair experience.

This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time. For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time. www.identifix.com. © 2008 IDENTIFIX. All Rights Reserved. © 2002 IDENTIFIX. All Rights Reserved.

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