Below is information about the Design and Function of the MOST network on Volvo (P1, C30/V50/S40/C70) vehicles. Some of this information is specific to Volvo, while some parts are applicable to other cars as well.
Introduction
MOST (Media Oriented Systems Transport) is a standardized network communication system for multimedia applications. The MOST protocol is optimized for fiber optic communication. This means that signals are sent as light pulses.
MOST Data
The MOST protocol determines how light pulses or their absence should be interpreted. Each pulse or absent pulse is called a “bit”. The number of “bits” transmitted on the MOST network is 24.8 million per second (24.8 Mbit/s). A bit can be:
- Binary 0, i.e. no light pulse
- Binary 1, i.e. light pulse.
The light pulses are transmitted in data frames. A data frame consists of different types of data. The size of a data frame is 64 bytes, which is 64 x 8 = 512 bits. The data frame has a frequency of 48 kHz. This means that 48000 data frames are sent per second on the MOST network. The data types which are included in a data frame sent on the MOST network are:
- control data
- synchronous data
- asynchronous data
- administrative data.
Control Data
Control data is used to check and to allocate the functions to the control modules and the functions in the MOST network. The control data message is transmitted from, for example, the infotainment control module (ICM) to another specific control module. The control data message automatically checks if the receiver has received the signal and that the entire message has been correctly transmitted between the transmitting and receiving control modules. The size of the control data is 2 bytes per data frame. A block, which consists of 16 data frames, is required for a control message to be sent. This means that the total size of the control message is 32 bytes. A control message is the minimum size of a message to activate or deactivate functions on the MOST network. Examples of MOST messages are track changes or stop commands on the CD player. Up to 1000 control data messages can be sent per second.
Synchronous data
Synchronous data is used to transmit data which requires considerable quantities of data in real time, for example to play back sound from the Audio Module (AUD). The length of the synchronous data message which can be sent may be up to 60 bytes per data frame, although never less than 24 bytes. This gives a transfer rate of up to 23 Mbits per second for data transmitted as synchronous data. Synchronous data shares the data frame with asynchronous data. The infotainment control module (ICM) controls which control modules transmit synchronous data and which are receivers.
Asynchronous data
Asynchronous data is used to send large quantities of data, such as TCP/IP and digital images, but not in real time. The length of the message can be up to 36 bytes (synchronous data minimum 24 bytes + asynchronous data 36 bytes = 60 bytes). This gives a maximum transfer rate of 14 Mbits per second. When the message has been sent, a check is run to ensure that is has been correctly received.
Asynchronous data shares the data frame with synchronous data. The infotainment control module
(ICM) controls which control modules transmit asynchronous data and which are receivers.
Administrative data
Administrative data consists of two parts. The first part is at the start of the data frame. This describes how much of the transmitted data is synchronous and how much is asynchronous. The second part is at the end of the data frame. This part checks that the control modules on the MOST network are working correctly. The size of administrative data is 2 bytes.
MOST data frame
A data frame consists of different parts. This contains for example:
- control data
- synchronous data
- asynchronous data
- administrative data.
A data frame on the MOST network is 64 bytes. The data frame consists of the following:
- synchronization of the clock on the MOST network, i.e. the frequency at which data is transmitted so that the data is sent at the correct intervals
- administrative data, indicates the size of the asynchronous and synchronous data
- synchronous data
- asynchronous data
- control data
- administrative data
16 data frames are required to form a block. One block is the minimum size of a MOST message.
MOST Protocol
All activations on the MOST network are control messages or MOST messages. The majority of the MOST messages are initiated by the infotainment control module (ICM). The infotainment control module (ICM) informs certain control modules if a function in the control module must be activated. The infotainment control module (ICM) then allocates space to transmit data on the MOST network for those activations which have been initiated in the control modules. This is either synchronous or asynchronous data. The structure of a message is as follows:
DeviceID.FblockID.InstID.FktID.OPType
This means:
- DeviceID - The ID number of one of more control modules in the network
- FblockID - the ID number of a function unit, for example the CD function in the Integrated Audio Module (IAM)
- InstID - ID number to differentiate two identical function units
- FktID - The ID number of the function to be activated, for example play or stop
- OPType - Specifies how the function is to be used, because a function can be used in different ways.
The message may appear as follows:
IAM.CD.01.Track, Length.Set(10)
This means: play song number 10 on the CD player in the Integrated Audio Module (IAM).
The infotainment control module (ICM) knows which control modules are in the MOST network. It can then assign specific ID numbers to all control modules and the included functions. If control modules are removed or added, the ID numbers for the control modules are re-assigned.
Calculating the transfer rate
The number of “ones” and “zeros” transmitted per second determines the transfer rate. The number of data frames multiplied by the clock frequency of a frame gives the transfer rate.
Example
Bit = a binary 1 or 0
Byte = 8 bit
Maximum asynchronous data = 36 byte
This gives: 36 x 8 = 288 bit
The clock frequency for a data frame = 48 kHz
The maximum number of bits of asynchronous data x clock frequency for a frame: 288 x 48000 = 14 Mbit/s.
Software downloads on the MOST network
The concept for downloading software is the same as for the CAN network. The CAN uses a PBL (primary boot loader) which is permanently installed in the control module, and a secondary program loading file (SBL – secondary boot loader) which is downloaded into a RAM while the control module is being programmed. When downloading software for the MOST control modules, a third type of download module is used, GBL (Gateway boot loader). For downloads to the MOST control modules to work, the GBL must be loaded into the infotainment control module (ICM). SBL is used to program the infotainment control module (ICM), in the same way for all other CAN control modules.
GBL Downloading
A PROG command is transmitted on the CAN network to all control modules including the infotainment control module (ICM). This command sets all the control modules to reset mode. Before the infotainment control module (ICM) switches to reset mode, it switches off the light to all MOST control modules.
The GBL is then downloaded to RAM memory in the infotainment control module (ICM). The GBL is then activated and the infotainment control module (ICM) transmits light pulses on the MOST network. The GBL now functions as a connection between the CAN and MOST networks. The GBL sets the MOST control modules to programming mode. All the control modules on the MOST and CAN networks can then be addressed in the same way to download software.
Security on MOST
This function is used to make sure that all control modules on the MOST network can be used. When the MOST network starts up, ignition position I, II or III, the infotainment control module (ICM) checks that all control modules on the MOST network have approved serial numbers. If the infotainment control module (ICM) detects that one or more control modules do not have approved serial numbers, these modules will be set to disable mode. This means that the control module functions cannot be used in the MOST network. The control module only acts as a conduit for the light pulses. It takes 30 seconds for the infotainment control module (ICM) to check the control modules. This means that non-approved control modules will be disabled after 30 seconds.
