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 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.
Example
A MOST network with all control modules.
An open circuit occurs on the cable between the Audio module (AUD) and the Accessory USB unit (AUU), see figure.
The infotainment control module (ICM) receives responses up to and including the Accessory USB unit (AUU) and presents the value 3.
If there was no open-circuit, the infotainment control module (ICM) would have responded "OK.