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).

During ring break diagnostics, 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 Accessory USB unit (AUU) 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 with an open circuit on the optical cable in the MOST network. No control module before the open circuit can send messages to the Infotainment control module (ICM).

By means of the incoming messages, the Infotainment control module (ICM) can determine where the open circuit has taken place.

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 7 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 (applies to the S40/V50).

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 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 six control modules are connected.

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

Using ring break diagnostics, all control modules before the Phone module (PHM) will have minus one on its response compared to example 1 where all control modules were connected.

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.