Ring Break Diagnostics

General

This function is important when fault-tracing the MOST network. Ring break diagnostics are used when there is an open-circuit in the MOST network and diagnostic trouble code (DTC) ICM-DC02 has been stored in the infotainment control module (ICM).

A ring break diagnosis means that all control modules send a message to the infotainment control module (ICM) in reverse order to their locations in the MOST network. This means that the integrated audio module (IAM) transmits first, not last. The control module which does not work will not transmit a response to the infotainment control module (ICM). This also means that all control modules connected before the broken control module will not be able to transmit a message to the infotainment control module (ICM).

The same applies in the event of an open-circuit in the fiber optic cable in the MOST network. None of the control modules before the open-circuit will be able to send their message to the infotainment control module (ICM). The infotainment control module (ICM) is then able to calculate the position of the open-circuit from the incoming messages.

HINT: It is important to know which control modules are connected and in what order to identify the control module which sends each message.

Fig 1: MOST Network Control Module Diagram

Courtesy of VOLVO CARS CORPORATION

Example 1

A MOST network contains all 8 control modules. There is an open-circuit in the cable between the traffic message channel module (TMC) and the Subwoofer module (SUB). See the illustration.

During the ring break diagnostic, the infotainment control module (ICM) receives responses in reverse order to the order of the control modules on the MOST network. Those that are before the Traffic message channel module (TMC) cannot respond to the infotainment control module (ICM). The infotainment control module (ICM) receives responses up to position four and presents the value 4.

If there was no open-circuit, the infotainment control module (ICM) would have responded "OK".

Example 2

A MOST network to which the Phone module (PHM) is not connected, however the other seven control modules are connected.

There is an open-circuit in the cable between the traffic message channel module (TMC) and the audio module (AUD).

During the ring break diagnostic, the response value from all the control modules before the Phone module (PHM) will be 1 less than when all control modules are connected. The infotainment control module (ICM) receives responses up to position three and presents the value 3.

If there was no open-circuit, the infotainment control module (ICM) would have responded "OK".