MOST Infotainment System Concerns

Martin Instructor Burnaby, British Columbia Posted   Latest   Edited  
Network Communications
2019 GMC Yukon SLT 6.2L (J L86) 6-spd (6L80)

MOST Infotainment System Problems? - Who Ya Gonna Call? MOSTbusters!

Well, perhaps not quite, but exorcising the demons in modern technology "infotainment" (INFOrmation and enterTAINMENT) systems, can be a step away from the "comfort zone" for many technicians in this day and age of advanced systems technologies. 

Depending on the infotainment system, it can be a daunting task just to verify and determine whether a concern expressed by a customer, is a normal or abnormal condition associated with the system, or an issue associated with a customer device. 

A concern could be software or firmware related, a failure of the system or "heavens forbid" an issue related to incompatibility of an affected device such as a cell phone, brought into the vehicle by the customer. The latter concern could be a "hard pill to swallow" for the owner of that nice shiny new vehicle, if they learn that their device is not compatible with their new vehicle system!

Setting brought in media related concerns aside, whatever the type of infotainment system, a solid understanding of how the system is connected and communicates, will be very important towards achieving a successful and efficient resolution when diagnosing a system concern. 

Introduced in this post is MOST Bus (Media Oriented Systems Transport), namely MOST 50 and is the only MOST system utilized on General Motors vehicles.

The focus will be on the wired MOST 50 as used since 2014 on GM trucks with Next Generation Infotainment (NGI) systems and available on other models with NGI infotainment system families.

MOST 50 bus is a "ring" type data bus using a twisted pair serial data bus beginning at the radio and ending at the radio. As with many networks where the bus connects to multiple modules (nodes), there are also a number of other inputs and outputs to these nodes on the bus, required for it to function.

So, let's build a MOST bus. To reduce complexity, we will assume that each module receives power and ground. Also, Local Interconnect Network (LIN) bus circuits will not be shown. These circuits direct user inputs to control the system using knobs, touch screen and steering wheel inputs. 

Composite and Low Voltage Differential Sampling (LVDS) cables that transmit the audio and video signals will also be omitted. The intent here is to explore the MOST bus along with modules that have all of the supporting circuits (ingredients) that allow them to function as nodes on the MOST bus.

We will begin by adding some modules Figure 1. followed by twisted pair conductors Figure 2. It will be important to note that all MOST bus data flows in the direction shown in Figure 3. As with other networks used on GM vehicles, the MOST Bus utilizes power, ground and a communication enable or wake-up circuit known as the Electronic Control Line (ECL), which is employed for wake-up and Ring Break Diagnostics (RBD). Figure 4. depicts ECL connection to all nodes, with the radio being the bus master.

What differentiates the ECL from other signals, is its function during system wake-up to initialize the network and also during failure modes where Ring Break Diagnosis (RBD) is encountered. An important consideration during a MOST bus failure, is whether the fault is associated with the MOST bus or the ECL. If the ECL fails or a module fails to communicate on the bus, we may be required to follow a different diagnostic path depending on DTCs.

To initialize the MOST bus, the radio provides a ground path to pull the ECL circuit low for 100 milliseconds, serving as a wake-up signal to each of the connected nodes.

Each node subsequently responds on the MOST data line. If the system is functioning without faults, a subsequent request for data on the ECL results in nodes transmitting their unique MOST device addresses and additional information.

Each node is then assigned a node ID # that can be viewed in GDS 2 radio data list. Node IDs are used to determine the last working configuration of the nodes on the bus.

The MOST bus has no direct connection to Low Speed GMLAN or High Speed GMLAN data buses. However, some of the modules on the MOST bus are connected via HS or LS GMLAN and this can be dependent on system Regular Production Option (RPO). 

The radio (component code A11) is the MOST bus master and various conditions, such as DTCs and other faults are provided via the radio connection to the LS GMLAN. The Media Disc Player (component code A33) may or may not be connected on the LS LAN, while service information may not identify Media Disc Player RPO U42 as being on LS LAN.

An amplifier (component code T3) if used and the Instrument Panel Cluster (IPC component code P16) are connected via LS GMLAN. Figure 5. The Human Machine Interface (HMI component code K74) is connected to the HS LAN and is used for programming events. 

The HMI also plays a supporting role in diagnostics. Figure 6. So, in the event that modules on the bus are not communicating, that avenue should be followed before going down the MOST diagnostic path. Note: It is important to follow the Diagnostic System Check - Vehicle (DCS-V) and perform a Vehicle DTC check if directed.

To diagnose MOST bus system conditions effectively, it is necessary to utilize information that may be vehicle build related (RPO) and will depend on system schematics and an understanding of the description and operation of the system.

MOST Bus is configurable as a "plug and play" network and while as many as 64 nodes can be supported, 4 or 5 is a typical number. In the data list, an "Unrecognized" status is usually associated with possible nodes that could be connected, but are not configured on the bus.

During initialization, each node is identified and assigned a number by the A11 radio. The node identification numbers as shown in Figure 7. will be useful for reference during Ring Break Diagnosis (RBD). Node identification of components on the MOST bus, begins with the A11 radio as node 1, ascending in number through each subsequent node in the ring en route back the radio. 

Each node receives data at the Receive (RX) INIC and sends data to the next node via the Transmit (TX) INIC. Service information schematic diagrams may not accurately reflect that there is no actual physical connection through each node.

Data flows uni-directionally from the A11 radio around the ring in ascending order of assigned nodes, ending back at the radio. Three categories of data can be transmitted around the ring simultaneously, courtesy of Time Division Multiple Access (TDMA) providing for dedicated channels.

Connectivity of the two wire conductors that form the MOST bus to each module, begins and ends at the MOST bus terminals of each connector. 

Internal to each node is an Intelligent Network Interface Controller (INIC) to receive and transmit data on the Bus, but there is no physical hard wire connected test pathway through the INICs as shown in Figure 8. and Figure 9. respectively. In other words, continuity or other physical measurements cannot be conducted through the modules using conventional test equipment.

It is important to note that node assignment is based on the order of connection to the ring and not by any particular module, except the A11 radio which is always number 1. To clarify this, see Figure 10. for some comparisons of node assignment. Control data is asynchronous and is associated with management of messages, packages and various commands. 

MOST packet data is also asynchronous and provides movement of graphics and large files. Streaming data is synchronous and comprises digital audio video files. Examining this data requires advanced equipment not normally associated with diagnostics at the automotive service level.

In diagnosis, many of the basic tools and familiar testing procedures such as utilizing a Digital Multi-Meter to perform checks for open circuits, shorts to ground, shorts to voltage and circuits shorted together. As with many other network diagnostics, the Diagnostic System Check-Vehicle (DSC-V) is a useful path to follow. Accurate recording of the RPO codes and knowledge of the radio family, will be of the essence. 

RPO codes provide the diagnostic technician with identification of the infotainment system on the vehicle, while having an understanding of the radio families will benefit the technician. However, outside of training and product familiarization, it seems that there is little available information to identify any given radio, as belonging to a particular family of radios.

So, let's take a look at Figure 11. to view radio data for a MOST bus that is functioning normally with no breaks. The upper arrow identifies that there is no Surrogate MOST Master Upstream by assigning a value of None. The lower arrow identifies that there are no communication breaks with a value of 0. This is a counter that increments up each time a break occurs and can be particularly useful when assessing the type of failure, such as intermittent conditions. 

Figure 12. shows the normal path followed in GDS 2 when diagnosing a MOST bus concern. Unless there is a MOST bus associated DTC, this path is not normally followed, but in this case the objective is to show the resulting message "Incomplete or Missing Data for Test" that will be displayed when no fault is present. Following the Diagnostic System Check -Vehicle (DSC-V) when a fault occurs, will lead the technician to perform a Vehicle DTC Check, where results may be shown as in Figure 13. 

It is important to begin with the Vehicle DTC Check rather than heading straight to Module checks, for reasons known to those of us who have followed the wrong path and been led astray! Figure 13. shows the modules communicating and some with DTCs assigned. Modules with red circles and strike throughs, can be identified as not being fitted to this vehicle by verifying the vehicle RPO codes.

For this reason, it is important to gather information from more than one source. The vehicle build in Investigate Vehicle History (dealership internal), RPO list in SI and on the Service Parts IDentification (SPID) label will be useful, as will schematics and the system Description of Operation documents.

Let's re-organize the module lists for better viewing by clicking on the "DLC" pin button as shown in Figure 14. and Figure 15. This sorts the modules into lists as desired. In this case we will view modules connected on the LS GMLAN terminal DLC 1.

From the Vehicle DTC check, DTCs U0028 and U0029 are the DTCs to follow. Following SI, U0028 is the DTC to diagnose. There is another DTC, but that can be ignored as it is related to the state of disassembly of the vehicle. Since we now have associated DTCs, following the same path as previously used, we arrive at the MOST Bus Diagnostic Starting Point Figure 16. will now yield results when selected. 

Figure 17. and Figure 18. provide us with key information about the area of the system that has the issue. Figure 17. shows that the break is between nodes 1 and 2, with node 1 being the first and node 2 being the second identified as possible locations and anything in between. Figure 18. shows a "Surrogate MOST Master Node Upstream Position 4". 

Prior to the update to GDS 2 to incorporate the MOST Bus Diagnostic Starting Point" function, using the Surrogate path was the only method of predicting the fault location. However, it still serves to verify that the A11 radio has identified a ring break in a specific area of the bus.

Now, you may recall that when there was no break in the bus, that a position of "None" was displayed. Let's take a look at Figure 18. to narrow down the area of focus. Counting backwards from the assigned nodes identifies that position "4" is the Media Disc Player. What does this all mean? 

Figure 19. shows counting back in clockwise direction (opposite to data flow), beginning with the radio as position "0", that "4" does identify the Media Disc Player. Consider that all data packets flow from the radio in order of nodes, picking up from each node and resulting in the radio receiving information back from all nodes. In this particular instance, the last packets of information that were received came from the transmit INICs of the Media Disc Player. 

The concern could be an internal module fault with the Media Disc Player receive INICs, connections back to the radio and the radio transmit INICs. This was the area of fault assigned by the radio as Node Locations of the Bus Communication Break 1-2. Review Figure 7. to count node locations from the A11 Radio as node 1 and Media Disc Player as node 2. 

The next step in diagnosis is to locate the first location identified as being the first node location with the break, which is node 1 the A11 Radio. If we inspect the wiring to the radio, all is fine and if the radio is disconnected to be bypassed the U0028 DTC counter will increment up, so bypassing the radio is not overly useful at this point. 

Wiggling the wiring at the radio connect does not increment the break counter. So, next we perform the same wiggle tests at the Media Disc Player with no change. We then disconnect the Media Disc Player and use the correct adapter from the MOST Bus Diagnostic Tool Kit Figure 21. and install the jumper to the harness connector. Keep in mind that the objectives here were to introduce the MOST bus and identify how the data and DTC strategies are portrayed when issues are directly due to failures on the bus.

Other system failures may well result in generation of bus DTCs such as U0028, while the cause may be attributed to modules, wiring and controls. It is important to analyze all DTCs to determine which is the correct diagnostic path to take.

At this point, audio returns, confirming that the Multimedia Disc Player is at fault. Since we have the MOST circuits bypassed and audio returned, it is best practice to verify all circuits to the unit before condemning it.

With the MOST jumper in place, we can tap into the ECL to measure voltage as shown in Figure 22. At key on a momentary pulse of 100 milliseconds can be observed, followed by a steady voltage signal when there are no breaks in the circuit. This image was captured with no Ring Breaks, since the offending module had been bypassed with a jumper from the kit.

What is shown in the brief video clip, is the result of a ring break that was a result of the HMI MOST bus connector being disconnected. The A11 Radio immediately initiated Ring Break Diagnostics (RBD)

When a MOST ring break occurs, the A11 Radio RBD function will result in pulses of 300 milliseconds as shown in the video clip. Note: Ignore the door chime. Notice the logic probe light and the DMM analog scale during RBD. 

The focus here has been on the bus and connected modules, along with use of GDS 2 and MDI to review data and perform bus related diagnostics using the MOST jumper kit and ECL. No special tools are required to complete this diagnosis other than a scan tool. The MOST jumpers make it easier to bypass the modules, but jumpers can be fabricated using 0.64mm terminals and tapping into the ECL circuit is easy enough using the schematic for reference.

There are other causes than breaks on the MOST ring that will generate DTCs that may at first appear to be related to the MOST bus, such as loss of communication with a module on the bus. So, it will be important to gather as much information as possible and follow a structured diagnostic path to resolve issues efficiently.

Depending exactly what, when and where the system failed, the customer may drop off the vehicle airing a concern only of a loss of audio. However, once the vehicle has been powered down for a while, a loss of display (black screen) may be what the diagnostic technician encounters along with the loss of audio.

Depending on the model year and platform, when clearing DTCs , additional steps may be required to clear the break counter. On 2014 model C/K trucks for example, the radio fuse had to be removed for ~60 seconds for the counter to reset to 0. On … Yukons and trucks, the counter usually resets to 0 with a DTC clear. 

A side note. On occasion during diagnostics, the system may become confused and generate counters when no breaks are present, or not generate counters when breaks are present. A "global reset" (battery disconnect) restores normal function. Also consider that during diagnostics when the jumpers are being installed, the break counter will increment up 1 every time the bus is opened.

Chris Technician
Bryans Road, Maryland

Great write up! Thanks for the info Chris

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Thomas Diagnostician
Saint Petersburg, Florida

excellent, thank you!

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Brian Owner
Parma, Ohio

Thanks for the excellent information, working on a MOST network and this information is helping it all make sense! I was working on a '17 Impala , Display is inoperative shop tried 2 "used"displays, (I did not recommend this, and I am aware of the many problems of used modules on the global vehicles) then they tried a brand new radio and had it programmed. Still no display, this is when I got a

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Caleb Technician
Mishawaka, Indiana

Cool write up ! I work at a GM Dealer an work on a lot of the infotainment an entertainment systems. I'm curious if there is a way to scope the MOST bus. I've tried on my Modis Ultra on a known good 2015 Tahoe backprobed at the radio. However couldn't seem to get any sort of signal. Then I read that the MOST bus is 100 times faster than CAN so maybe my scope is too slow?? Any insite on this

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Martin Instructor
Burnaby, British Columbia

Hi Caleb. Thanks. According to GM training info, traditional DSOs are not considered to beuseful in scoping MOST bus signals. IOW, even if you could capture a signal, it isn't easily dissected to serve any useful diagnostic purpose. However, MOST bus is quite easily diagnosed with a decent DMM and the Diagnostic Jumper Kit EL 51578. Without the jumper kit, 0.64 terminals can be used to make very

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