Auto Service World
Feature   October 1, 2014   by By Michael Klaas, Identifix European Specialist Certified: ASE Master, L1

To Be or Not D2B

One of the most annoying things about a vehicle can often be the failure of a simple creature comfort such as not having the radio work. Engineers have been making these ‘simple’ radios more and more complex, which can be very...

One of the most annoying things about a vehicle can often be the failure of a simple creature comfort such as not having the radio work. Engineers have been making these ‘simple’ radios more and more complex, which can be very frustrating and sometimes hard to diagnose. But with some understanding of the basic fundamentals of fiber optics and networking, these radios can actually be simple to diagnose.

I recently had a call on a 2009 Mercedes E350 with no sound from the audio system. Knowing a little bit of the evolution of fiber optics on Mercedes also helps in a diagnosis. So what is fiber optics? Fiber optics pass light signals from one component to another. These light signals get converted to electrical signals and either use the information to operate or pass it further along to the next component. Each component has two fiber optic cables, an “IN” and an “OUT.” The first version of fiber optics was proprietary to Mercedes and used as early as 1998. This fiber optic system was called Domestic Digital Bus (D2B), which was later upgraded to Media Oriented System Transport (MOST) starting around 2003 with the introduction of the new model E-Class 211. Though both operate the same way, MOST fiber optics proved to have quicker communication (24.8 megabytes per second), are more durable (minimum bend radius of 25mm on D2B; MOST minimum bend radius states don’t kink), cost less and be able to handle more devices (up to 64) on the network.

Now that we know what fiber optics is and what it does, it is necessary to know how this particular vehicle is networked. A 2009 E350 has the COMAND radio as the head unit or gateway MOST master and MOST slave components: Sound System Amplifier, Satellite Radio and Media Interface Control Units in its ring configuration (See Figure 1).

The MOST master issues an optical and an electrical “wake-up” signal to everyone in the system simultaneously, and is also responsible for diagnostic fault DTCs that provide the location of the electrical or optical fault. Luckily the technician I spoke with had a compatible scan tool that would communicate with the “telematics” modules to attain these important faults. The technician found a fault D016: “MOST communication has a malfunction (position of ring break cannot be identified)” and D018: “MOST communication has a malfunction (ring break at position 1) in the COMAND unit.”

Now here is where information of how the system is networked will pinpoint where the problem lies. The components on the ring are listed in a clockwise format but the positions are listed in a counter-clockwise format. So when looking at the ring configuration (See Figure 1), “ring break at position 1” is anything between the Media Interface Control Unit and the MOST Master (COMAND unit). This fault can be caused by: Fiber Optic Transceiver (FOT) IN at position 1, FOT OUT at position 2, module at position 2 not having power/ground, fiber optic damage, or having fiber optic lines reversed at position 2.

We are narrowing the playing field! It is important to understand at this point that, based upon the complaint of no sound, this is a fiber optic issue and not an electrical wake-up line issue. An open or short in the electrical wake-up line will normally cause the system to either have issues turning on, turning off or turning on/off on its own. Because of this, we can physically check the fiber optic cables to make sure that the Media Interface Control Unit is receiving the optical wake-up of 9 red flashes but not passing it on to the MOST master.

Remember, each component has an “IN” optical cable and an “OUT” optical cable.

On MOST systems, the IN cable will have the flashing red light with an arrow on the connector pointing toward the component.

The OUT cable will have no light when unplugged from the component with an arrow pointing away from the component. It will pass along the light to the next component once the connector is plugged back in (see Figure 2).

With this knowledge, the technician pulled the fiber optic connector out of the COMAND unit and separated the fiber optic cables coming back into it. When he plugged the OUT cable back into the COMAND and turned the on the power button, there was no visual optical light coming back to the COMAND. This led us to believe that the Media Interface Control Unit was faulty (assuming the power/ground/wake-up line to that component and fiber optics were OK).

To achieve the same conclusion, we could have performed an electrical wake-up line test (if available in the scan tool) without having to touch the vehicle. The MOST Master issues an electrical wake-up line signal much like a roll call to each component and in return, the component then issues an optical confirmation that it received the signal. If the MOST Master does not receive the optical return message back, it then flags this as a fault in the “Check of Wake-Up Line to the MOST Components” report.

Before replacement of the component, it is always a good idea to confirm that the actual and specified coding of the components match each other and that the coding is in the correct order for the vehicle. Each model has a specific order of how the modules are to be configured. If the MOST master pulls the electrical wake-up line down in a different order than what is programmed, functionality and errors can occur. After confirming the version coding was correct for the vehicle, the replacement of the Media Interface Control Unit re-established ring lock and the audio/entertainment system started working again.

Print this page


1 Comment » for To Be or Not D2B
  1. Diogo Cerdeira says:


    I saw your post, it seems that i have a problem like yours.

    Mine Command NTG4.5, from a Mercedes C220 W204 2011, sometimes blocks the screen, sometimes is ok, e check the DTC’s in Engeneering Mode and i verify the DTCd017, Ring Pos.0.
    Did you think i can repair the unit by myself, replace some part, or i need to change the all unit?

    Thanks in advance.

Have your say:

Your email address will not be published. Required fields are marked *