Circuit/System Description

The Media Oriented Systems Transport (MOST) Infotainment network is a dedicated high speed multimedia streaming data bus independent from GMLAN. The MOST bus is configured in a physical hardwired (series) loop with each segment between devices receiving and sending cumulative data in one direction only. Each device on the MOST bus will be required to have 2 pairs of twisted pair copper wire circuits (outputting from the device's transmit TX, and inputting to the device's receive RX), as well as 1 electronic control line (12 V which is toggled to ground) to support wake up and "ring break diagnostics". The electronic control line voltage will be supplied at each MOST device. Note that the MOST bus circuits will not have continuity through each MOST device.

The A11 Radio is the MOST Master and will monitor the bus for vehicle configuration, Infotainment data messages and errors on the bus. The MOST initialization consists of a short 100 ms low voltage pulse on the electronic control line connected to all devices contained on the MOST ring. When the MOST devices receive this wakeup message, they will respond with a generic device response. Once these initial responses on the MOST bus are reported successfully without error to the A11 Radio, the next responses will report the MOST device addresses and their functionality. The A11 Radio will learn this information and also record the address node sequence of the MOST bus at this point. This node address list will now be stored within the A11 Radio as the MOST bus configuration (called Last Working MOST ID of Node 1-9 on scan tool data display).

When MOST receive, transmit, or control line faults are detected, transmit/receive messages are not received as expected from the wakeup request. The A11 Radio and the K74 Human Machine Interface Control Module will then perform diagnostics to isolate these MOST faults.

• If the MOST electronic control line is shorted low to 0 V for excess amount of time, the A11 Radio will set a U2098 DTC and the K74 Human Machine Interface Control Module will set a U0029 02 DTC. At this point the MOST bus will be unable to communicate until the shorted MOST control line is repaired.
• Once the shorted MOST control line diagnostics pass, the A11 Radio will attempt to resend the initial short pulse attempts up to 3 times on the MOST control line. If the expected responses are not received due to a fault on the transmit, receive, electronic control lines, or an internal device, the A11 Radio continues into a failure mode setting a U0028 DTC. The K74 Human Machine Interface Control Module may also set a U0029 DTC. The A11 Radio will continue on to send 400 ms long pulses on the electronic control line while DTC U0028 is current, which commands all transmitting devices to send "Surrogate MOST Master" location in this MOST fault/diagnostic mode. When the A11 Radio receives the new MOST Master identity, the Surrogate MOST Master device can be identified based on scan tool data parameter Surrogate MOST Master Node Upstream Position. The scan tool, vehicle build and schematics will be used to determine the MOST bus configuration and direction by utilizing the Last Working MOST ID of Node 1 - 9 parameters from the A11 Radio data display. This newly enabled Surrogate MOST Master Node Upstream Position will assist in determining the location of the MOST device/bus/control fault. Because the Surrogate MOST Master device is able to communicate to the A11 Radio, the transmit TX portion is working correctly. Therefore, the suspected areas for further diagnosis include:
  1. The MOST device and MOST circuits immediately upstream from the Surrogate MOST Master device.
  2. The transmit TX function, power, ground, and the electronic control line connected to the MOST device immediately upstream from the Surrogate MOST Master device.
  3. The Surrogate MOST Master device receive RX function.