Ring Break Diagnostics

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).

During a ring break diagnostic, all control modules transmit a message to the infotainment control module (ICM) in reverse order to their position on the MOST network. This means that the Integrated Audio Module (IAM) transmits first, not last.

The control module which is not working will not transmit a response to the infotainment control module (ICM). This also means that the control modules which are connected before the defective control module will be unable 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: Identifying Ring Break Diagnostics

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 Remote Digital Audio Receiver (RDAR) and the Audio module (AUD). 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 audio module (AUD) cannot respond to the infotainment control module (ICM). The infotainment control module (ICM) receives responses up to position five and presents the value 5.

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

Example 2

A MOST network to which the subwoofer module (SUB) is not connected, however the other eight control modules are connected.

There is an open-circuit in the cable between the Remote Digital Audio Receiver (RDAR) and the audio module (AUD).

During the ring break diagnostic, the response value from all the control modules before the subwoofer module (SUB) will be one less than example 1 when all control modules are connected. 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".