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-U1A2400 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 to open-circuits in the optical cable in the MOST network. No control module before the open-circuit will transmit a message to the infotainment control module (ICM). By using the incoming messages the infotainment control module (ICM) can present at which control module the open-circuit has occurred.
HINT: It is important to know which control modules are connected and in which order they are connected in order to know which control modules lie before and after the indicated control module.
Example for model year 2008 structure week 2007 20-
A MOST-network contains all 8 control modules. There is an open-circuit in the cable between the integrated audio module (IAM) and the Phone module (PHM), as shown.
In a ring break diagnostic the infotainment control module (ICM) receives replies from the control modules on the MOST network. Those before the Phone module (PHM), i. e. is the integrated audio module (IAM), cannot transmit their reply to the infotainment control module (ICM).
The infotainment control module (ICM) now receives a response from all control modules except the integrated audio module (IAM), and therefore presents the control module which is first in the loop and which responded, in this case the Phone module (PHM).
If there was no open-circuit, the infotainment control module (ICM) would have responded "OK.