Due to the fact that MOST is based on a ring topology, there are several possibilities of faults. For the optical transmission, typical ones are the Sudden Signal Off (SSO), the Critical Unlock (CU), and the ring break. In case of a ring break, no communication is possible anymore and SSO and CU can seriously disturb the signal transmission. If no reliable MOST communication is possible, tests like e.g. Ring Break Diagnosis (RBD) need to be triggered and the results have to be collected. With the first generation of MOST25, three general concepts for RBD have already emerged as possible solutions to analyze a broken ring:
1. Concept: Run RBD on every start-up. In this case every device in the ring starts in RBD mode on every start-up of the system. Advantage: For this solution no additional hardware is required, only the software has to be configured. As a result, this solution does not require a lot of effort. Disadvantages: The whole system starts in RBD mode every time. The result can only provide one error at the time. If there are more defects in the system, the complete test procedure has to be repeated after fixing the detected error, until communication is stable again. In addition, this solution provides no information whether a device is not working at all, e.g. through an electrical defect, or if the optical fiber between two devices is damaged.
2. Concept: Run RBD on first switch-on. For this solution the hardware has to be able to detect whether the power supply has been removed for more than 30 seconds. Only if this is the case, the system starts in RBD mode like in the first solution. Advantage: No loss of time on regular start-up compared to solution 1. Disadvantages: Like in solution 1, only one defect is discoverable on each test-run. Again a conclusion about the electrical functionality of a device is not possible. Additional logic needs to be integrated into hardware, to detect first switch-on. To trigger the test, the power supply has to be removed and reattached for all MOST devices at the same time, e.g. by disconnecting and re-connecting the battery.
3. Concept: Run RBD through an additional electrical line. The RBD is triggered through a signal on a so-called ECL. In addition to triggering the test it is possible to collect the results on this line. Advantages: Availability of the test result for every single device on one test run. In addition to the optical test result, information about the electrical functionality is also received. Beyond that, this line can be used for waking up the system or other tests and diagnosis as well. Disadvantages: The additional hardware and software can be quite complex according to the requirements and due to restrictive timing requirements. Also, testing and integrating the whole system with all devices is an additional challenge.