Introduction to the MOST bus:
In audio systems, the MOST bus is being used more and more frequently nowadays. MOST stands for: Media Oriented Systems Transport. The MOST bus is a ring-shaped network that uses optical transmission. Each control unit in the network uses its own LED to pass on optical signals via a fiber optic cable. The frequency at which the LED flashes forms a signal that ultimately leads to a message that can be received by other control units. In the image below you can see a connector with an incoming light signal.
Because of the combination of LEDs and fiber optic cables, the MOST bus is extremely fast. Via MOST, both audio and video material can be sent and received. Below is a MOST bus network that is used in a car’s audio system. The radio, the display, the amplifier, the entertainment computer, the DVD changer and the instrument cluster are all part of this network.
Via the entertainment computer, for example, a phone can be connected to stream music via Bluetooth. Music or films can also be played via the DVD changer. The data is transmitted via the fiber optic cable to the other control units in the network. The images will appear on the display, and the audio will be played through the speakers by the amplifier.
The data reaches the control unit, after which it is passed on to the next control unit. When one control unit is disconnected, the light beam is interrupted and data transfer is no longer possible.
On the right-hand image you can see a fiber optic cable. Because the cable transmits light, it is important that the cable is not kinked. If it is kinked, for example because it has become pinched behind the glove box, the light beam may be interrupted or weakened. Components in the MOST ring will then no longer be controlled correctly. It is the technician’s task to locate the cause of the fault. This can take a lot of diagnostic time.
The light signals are reflected inside the cable. If the cable is bent too sharply, the signal can no longer be reflected cleanly inside the cable. This is shown in the images below. When installing this optical cable, you therefore need to take into account that the cable must be laid with large loops where it has to bend. The bend radius must be more than 25 mm.
Communication protocols:
There are different generations of the MOST bus. The first generation was MOST25, where the data rate is 25 Mbit/s. MOST50 (50 Mbit/s) was the successor to MOST25. The latest variant is MOST150 (150 Mbit/s), where the data rate is 6 times higher than the first generation. Not only the data rate differs per generation, but also the content and structure of the data frames. The differences are described in the following paragraphs.
MOST25:
With MOST25, the data rate is 25 Mbit/s (megabit per second). This means that 25 x 1,000,000 bits, so 25,000,000 bits per second, are transmitted. Another characteristic of MOST25 is that messages are sent in data blocks, each consisting of 16 frames. Each frame consists of 512 bits (0 to 511). That equals 64 bytes. In the image below you can see an example of a data block.
- Preamble: The preamble (which means opening or introduction) synchronizes the transceivers before the message.
- Boundary descriptor: Indicates how many of the available 60 bytes are allocated to the synchronous and asynchronous bytes.
- Stream data: continuous data transfer (therefore also called synchronous data) that requires a high bandwidth. Used for the continuous transfer of video and audio data.
- Packet data: data transfer by means of packets (therefore also called asynchronous data) in which large data blocks are transmitted. These data blocks mainly consist of information for calculation purposes and messages in the form of asynchronous data, such as GPS information. Packet data also requires high bandwidth.
- Control data: This data maintains communication between the different network ports.
- Frame control: Monitors the respective data frame.
- : Parity bit: used to detect bit errors.
MOST50:
The MOST50 generation uses a data rate of 50 Mbit/sec. This is a doubling of the data rate of MOST25. Not only the data rate is different from MOST25, but also the length and structure of the frames are different. A MOST50 frame consists of 128 bytes, which equals 1024 bits. This too is a doubling compared to MOST25.
The layout of a data frame is also much simpler; a frame consists of:
- Header: Contains the administration, the boundary descriptor, 4 bytes of Control Data.
- Stream data: same as MOST25.
- Package data: same as MOST25.
MOST150:
The MOST150 frames have a structure similar to that of MOST50 frames. The speed is 150 Mbit/sec and they contain 384 bytes (3072 bits), which is three times as much as MOST50 frames.
Locating faults:
When the MOST bus is interrupted, the light signal can no longer be passed on to other control units. As a result, the display will remain black, no sound can be heard or other functions stop working. This may have the following two causes:
- Fiber optic cable is interrupted.
- Control unit is defective.
A broken fiber optic cable may have been caused by incorrect installation, which caused it to become pinched (for example between interior parts such as the glove box), but also because it has been kinked. The kink causes a weakening of the light signal. The fiber optic cable must therefore be checked for damage, kinks or breaks. Because the fiber optic cable sometimes runs throughout the entire car, it can be an extensive search. Often a damaged fiber optic cable is the result of an unprofessional action when installing a radio or dashboard parts. It is therefore advisable to start your search there.
If a light signal does enter the control unit but does not come out again, the control unit may be defective. It is possible that no faults are stored in the fault memory in such a case. The location of the defect can be checked by excluding this control unit from the MOST ring. An example of this can be seen in the image below.
In this case, the DVD changer is no longer part of the MOST ring. The fiber optic cables that were connected to the control unit are now connected to each other with a connector. The light signal is now transferred from one cable to the other. If communication resumes in this situation, it could be that the DVD changer is interrupting the light signal. Of course, this can also be tried with other control units, as they use the same type of connectors.
Faults can also be traced with diagnostic equipment. This is usually called a “ring break diagnosis”. More on this later…
The MOST bus is connected to the gateway. Via the gateway, communication with other types of networks, such as the CAN or LIN bus, is possible.