Fault Management On The MOST Network

Fault management on the MOST network

General
The MOST network is monitored by the infotainment control module (ICM). If it detects a fault in the MOST network, a diagnostic trouble code (DTC) is stored in the infotainment control module (ICM). There are different types of diagnostic trouble codes (DTCs) depending on the type of fault. A general rule is that if the light pulse in the MOST network disappears, the entire network stops working.
Fault types which are processed are:
- no communication from a control module
- faulty communication on the MOST network.

No communication from a control module
The infotainment control module (ICM) knows which control modules are connected to the MOST network. If any control module on the MOST network ceases to communicate, a diagnostic trouble code (DTC) is stored in the infotainment control module (ICM). There is a diagnostic trouble code (DTC) for every control module on the MOST network for this fault.
Missing communication may be due to an internal fault in the control module or because the function is incorrectly implemented in the control module.

Faulty communication
For faults when messages cannot be sent on the MOST network, diagnostic trouble code (DTC) ICM-U1A2400 is stored.
Causes of this fault may be that:
- a control module, except the infotainment control module (ICM), has a defective optical connector
- a control module has an internal fault which means that the optical connection stops working
- a control module on the MOST network is not powered. If there is no power supply, the optical connector stops transmitting light
- the optical cable is damaged or there is an open-circuit. This fault can occur if the bend radius of the cable is too small or if the cable is kinked or trapped
- there is dirt or oil on the optical connectors which impedes the light
- the connection to a control module has come loose
- the fiber optic terminals in the connector are cross-connected
- there is a loose connection because the fiber optical wiring is incorrectly installed or the fiber optic terminal pin has been pressed back in the connector
- a bridging connector has come loose or is damaged.
Ring break diagnostics are used to remedy diagnostic trouble code (DTC) ICM-U1A2400. If a break is discovered in the MOST network after running a ring break diagnostic, diagnostic trouble code (DTC) ICM-U1A1587 will be stored in the infotainment control module (ICM). If this is the case, which control module the open-circuit is located in can be read off from the infotainment control module (ICM).

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-U1A2400 has been stored in the infotainment control module (ICM).
Ring break diagnosis means that all control modules send a message to Infotainment Control Module (ICM) in reverse order to how they are located in the MOST-net. This means that Accessory USB unit (AUU) sends first and not last (applies to S80/V70/XC70, for XC60 Remote digital audio receiver (RDAR)/Digital Audio Broadcast Module (DABM).
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 S80/V70/XC70
A MOST-network contains all 8 control modules. There is an open circuit in the cable between the Phone Module (PHM) and the audio module (AUD), see the illustration.
In a ring break diagnostic the infotainment control module (ICM) receives replies from the control modules on the MOST network. Those located before the Audio module (AUD), i.e., the Phone Module (PHM), 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 Phone Module (PHM), and therefore presents the control module which is first in the loop and which responded, in this case the Audio module (AUD).
If there was no open circuit, the infotainment control module (ICM) would have responded "OK".