The [Optical Bus Diagnostics]  function found in the Applications Screen is used on models with a MOST bus to narrow down the location of a break in the network. Many control modules will be completely inaccessible via conventional diagnostics if there's a break in the MOST bus.  

The above example shows a 4F Audi A6 with a break in the MOST bus. In this case, it's clear that the modules are all electrically OK, but the Optical bus is broken between the Sound System module and the Voice Control module.

You may change the Diagnostic Type from "Standard" to "3dB Attenuation" if your results are not conclusive. The attenuation means all modules are sending with less output (initiated by the MOST master), but the individual MOST participating control modules can attenuate individually to check if individual modules are responsible for problems.

VW/Audi sells a handy loop of fiber (part number 4E0 973 802 ) that can be used in place of each of the components in the system. This can be used to check whether a component is responsible for breaking the loop.

For CAN Gateways that use UDS, a HEX-NET or HEX-V2 is required to use this function.

Click on [Start] to begin checking the controllers on the bus..

[Copy] will copy the results to Windows Clipboard. Once you have pressed the Copy Codes button, you may paste the results into the application of your choice, such as MS Word or Notepad.

[Save] will save a plain-text report to your PC, typically in this directory: C:\Ross-Tech\VCDS\Logs

Use [Done, Go Back] to return to the Applications screen.

This video explains the theory of operation of different networks found in VW/Audi cars along with an example of the Optical Bus Diagnostics function in action: